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Text Truncated. Only the first 1MB is shown below. Download the file for the complete contents. Advanced MS-DOS Programming ════════════════════════════════════════════════════════════════════════════ Advanced MS-DOS Programming The Microsoft(R) Guide for Assembly Language and C Programmers By Ray Duncan ════════════════════════════════════════════════════════════════════════════ PUBLISHED BY Microsoft Press A Division of Microsoft Corporation 16011 NE 36th Way, Box 97017, Redmond, Washington 98073-9717 Copyright (C) 1986, 1988 by Ray Duncan Published 1986. Second edition 1988. All rights reserved. No part of the contents of this book may be reproduced or transmitted in any form or by any means without the written permission of the publisher. Library of Congress Cataloging in Publication Data Duncan, Ray, 1952- Advanced MS-DOS programming. Rev. ed. of: Advanced MS-DOS. (C)1986. Includes index. 1. MS-DOS (Computer operating system) 2. Assembler language (Computer program language) 3. C (Computer program language) I. Duncan, Ray, 1952- Advanced MS-DOS. II. Title. QA76.76.063D858 1988 005.4'46 88-1251 ISBN 1-55615-157-8 Printed and bound in the United States of America. 1 2 3 4 5 6 7 8 9 FGFG 3 2 1 0 9 8 Distributed to the book trade in the United States by Harper & Row. Distributed to the book trade in Canada by General Publishing Company, Ltd. Penguin Books Ltd., Harmondworth, Middlesex, England Penguin Books Australia Ltd., Ringwood, Victoria, Australia Penguin Books N.Z. Ltd., 182-190 Wairu Road, Auckland 10, New Zealand British Cataloging in Publication Data available IBM(R), PC/AT(R), and PS/2(R) are registered trademarks of International Business Machines Corporation. CodeView(R), Microsoft(R), MS-DOS(R), and XENIX(R) are registered trademarks and InPort TM is a trademark of Microsoft Corporation. ────────────────────────────────────────────────────────────────────────── Technical Editor: Mike Halvorson Production Editor: Mary Ann Jones ────────────────────────────────────────────────────────────────────────── Dedication For Carolyn ──────────────────────────────────────────────────────────────────────────── Contents Road Map to Figures and Tables Acknowledgments Introduction SECTION 1 PROGRAMMING FOR MS-DOS Chapter 1 Genealogy of MS-DOS Chapter 2 MS-DOS in Operation Chapter 3 Structure of MS-DOS Application Programs Chapter 4 MS-DOS Programming Tools Chapter 5 Keyboard and Mouse Input Chapter 6 Video Display Chapter 7 Printer and Serial Port Chapter 8 File Management Chapter 9 Volumes and Directories Chapter 10 Disk Internals Chapter 11 Memory Management Chapter 12 The EXEC Function Chapter 13 Interrupt Handlers Chapter 14 Installable Device Drivers Chapter 15 Filters Chapter 16 Compatibility and Portability SECTION 2 MS-DOS FUNCTIONS REFERENCE SECTION 3 IBM ROM BIOS AND MOUSE FUNCTIONS REFERENCE SECTION 4 LOTUS/INTEL/MICROSOFT EMS FUNCTIONS REFERENCE Index ──────────────────────────────────────────────────────────────────────────── Road Map to Figures and Tables MS-DOS versions and release dates MS-DOS memory map Structure of program segment prefix (PSP) Structure of .EXE load module Register conditions at program entry Segments, groups, and classes Macro Assembler switches C Compiler switches Linker switches MAKE switches ANSI escape sequences Video attributes Structure of normal file control block (FCB) Structure of extended file control block MS-DOS error codes Structure of boot sector Structure of directory entry Structure of fixed-disk master block LIM EMS error codes Intel 80x86 internal interrupts (faults) Intel 80x86, MS-DOS, and ROM BIOS interrupts Device-driver attribute word Device-driver command codes Structure of BIOS parameter block (BPB) Media descriptor byte ──────────────────────────────────────────────────────────────────────────── Acknowledgments My renewed thanks to the outstanding editors and production staff at Microsoft Press, who make beautiful books happen, and to the talented Microsoft developers, who create great programs to write books about. Special thanks to Mike Halvorson, Jeff Hinsch, Mary Ann Jones, Claudette Moore, Dori Shattuck, and Mark Zbikowski; if this book has anything unique to offer, these people deserve most of the credit. ──────────────────────────────────────────────────────────────────────────── Introduction Advanced MS-DOS Programming is written for the experienced C or assembly-language programmer. It provides all the information you need to write robust, high-performance applications under the MS-DOS operating system. Because I believe that working, well-documented programs are unbeatable learning tools, I have included detailed programming examples throughout──including complete utility programs that you can adapt to your own needs. This book is both a tutorial and a reference and is divided into four sections, so that you can find information more easily. Section 1 discusses MS-DOS capabilities and services by functional group in the context of common programming issues, such as user input, control of the display, memory management, and file handling. Special classes of programs, such as interrupt handlers, device drivers, and filters, have their own chapters. Section 2 provides a complete reference guide to MS-DOS function calls, organized so that you can see the calling sequence, results, and version dependencies of each function at a glance. I have also included notes, where relevant, about quirks and special uses of functions as well as cross-references to related functions. An assembly-language example is included for each entry in Section 2. Sections 3 and 4 are references to IBM ROM BIOS, Microsoft Mouse driver, and Lotus/Intel/Microsoft Expanded Memory Specification functions. The entries in these two sections have the same form as in Section 2, except that individual programming examples have been omitted. The programs in this book were written with the marvelous Brief editor from Solution Systems and assembled or compiled with Microsoft Macro Assembler version 5.1 and Microsoft C Compiler version 5.1. They have been tested under MS-DOS versions 2.1, 3.1, 3.3, and 4.0 on an 8088-based IBM PC, an 80286-based IBM PC/AT, and an 80386-based IBM PS/2 Model 80. As far as I am aware, they do not contain any software or hardware dependencies that will prevent them from running properly on any IBM PC─compatible machine running MS-DOS version 2.0 or later. Changes from the First Edition Readers who are familiar with the first edition will find many changes in the second edition, but the general structure of the book remains the same. Most of the material comparing MS-DOS to CP/M and UNIX/XENIX has been removed; although these comparisons were helpful a few years ago, MS-DOS has become its own universe and deserves to be considered on its own terms. The previously monolithic chapter on character devices has been broken into three more manageable chapters focusing on the keyboard and mouse, the display, and the serial port and printer. Hardware-dependent video techniques have been de-emphasized; although this topic is more important than ever, it has grown so complex that it requires a book of its own. A new chapter discusses compatibility and portability of MS-DOS applications and also contains a brief introduction to Microsoft OS/2, the new multitasking, protected-mode operating system. A road map to vital figures and tables has been added, following the Table of Contents, to help you quickly locate the layouts of the program segment prefix, file control block, and the like. The reference sections at the back of the book have been extensively updated and enlarged and are now complete through MS-DOS version 4.0, the IBM PS/2 Model 80 ROM BIOS and the VGA video adapter, the Microsoft Mouse driver version 6.0, and the Lotus/Intel/Microsoft Expanded Memory Specification version 4.0. In the two years since Advanced MS-DOS Programming was first published, hundreds of readers have been kind enough to send me their comments, and I have tried to incorporate many of their suggestions in this new edition. As before, please feel free to contact me via MCI Mail (user name LMI), CompuServe (user ID 72406,1577), or BIX (user name rduncan). Ray Duncan Los Angeles, California September 1988 ──────────────────────────────────────────────────────────────────────────── SECTION 1 PROGRAMMING FOR MS-DOS ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Chapter 1 Genealogy of MS-DOS In only seven years, MS-DOS has evolved from a simple program loader into a sophisticated, stable operating system for personal computers that are based on the Intel 8086 family of microprocessors (Figure 1-1). MS-DOS supports networking, graphical user interfaces, and storage devices of every description; it serves as the platform for thousands of application programs; and it has over 10 million licensed users──dwarfing the combined user bases of all of its competitors. The progenitor of MS-DOS was an operating system called 86-DOS, which was written by Tim Paterson for Seattle Computer Products in mid-1980. At that time, Digital Research's CP/M-80 was the operating system most commonly used on microcomputers based on the Intel 8080 and Zilog Z-80 microprocessors, and a wide range of application software (word processors, database managers, and so forth) was available for use with CP/M-80. To ease the process of porting 8-bit CP/M-80 applications into the new 16-bit environment, 86-DOS was originally designed to mimic CP/M-80 in both available functions and style of operation. Consequently, the structures of 86-DOS's file control blocks, program segment prefixes, and executable files were nearly identical to those of CP/M-80. Existing CP/M-80 programs could be converted mechanically (by processing their source-code files through a special translator program) and, after conversion, would run under 86-DOS either immediately or with very little hand editing. Because 86-DOS was marketed as a proprietary operating system for Seattle Computer Products' line of S-100 bus, 8086-based microcomputers, it made very little impact on the microcomputer world in general. Other vendors of 8086-based microcomputers were understandably reluctant to adopt a competitor's operating system and continued to wait impatiently for the release of Digital Research's CP/M-86. In October 1980, IBM approached the major microcomputer-software houses in search of an operating system for the new line of personal computers it was designing. Microsoft had no operating system of its own to offer (other than a stand-alone version of Microsoft BASIC) but paid a fee to Seattle Computer Products for the right to sell Paterson's 86-DOS. (At that time, Seattle Computer Products received a license to use and sell Microsoft's languages and all 8086 versions of Microsoft's operating system.) In July 1981, Microsoft purchased all rights to 86-DOS, made substantial alterations to it, and renamed it MS-DOS. When the first IBM PC was released in the fall of 1981, IBM offered MS-DOS (referred to as PC-DOS 1.0) as its primary operating system. IBM also selected Digital Research's CP/M-86 and Softech's P-system as alternative operating systems for the PC. However, they were both very slow to appear at IBM PC dealers and suffered the additional disadvantages of higher prices and lack of available programming languages. IBM threw its considerable weight behind PC-DOS by releasing all the IBM-logo PC application software and development tools to run under it. Consequently, most third-party software developers targeted their products for PC-DOS from the start, and CP/M-86 and P-system never became significant factors in the IBM PC─compatible market. In spite of some superficial similarities to its ancestor CP/M-80, MS-DOS version 1.0 contained a number of improvements over CP/M-80, including the following: ■ An improved disk-directory structure that included information about a file's attributes (such as whether it was a system or a hidden file), its exact size in bytes, and the date that the file was created or last modified ■ A superior disk-space allocation and management method, allowing extremely fast sequential or random record access and program loading ■ An expanded set of operating-system services, including hardware-independent function calls to set or read the date and time, a filename parser, multiple-block record I/O, and variable record sizes ■ An AUTOEXEC.BAT batch file to perform a user-defined series of commands when the system was started or reset IBM was the only major computer manufacturer (sometimes referred to as OEM, for original equipment manufacturer) to ship MS-DOS version 1.0 (as PC-DOS 1.0) with its products. MS-DOS version 1.25 (equivalent to IBM PC-DOS 1.1) was released in June 1982 to fix a number of bugs and also to support double-sided disks and improved hardware independence in the DOS kernel. This version was shipped by several vendors besides IBM, including Texas Instruments, COMPAQ, and Columbia, who all entered the personal computer market early. Due to rapid decreases in the prices of RAM and fixed disks, MS-DOS version 1 is no longer in common use. MS-DOS version 2.0 (equivalent to PC-DOS 2.0) was first released in March 1983. It was, in retrospect, a new operating system (though great care was taken to maintain compatibility with MS-DOS version 1). It contained many significant innovations and enhanced features, including those listed on the following page. ■ Support for both larger-capacity floppy disks and hard disks ■ Many UNIX/XENIX-like features, including a hierarchical file structure, file handles, I/O redirection, pipes, and filters ■ Background printing (print spooling) ■ Volume labels, plus additional file attributes ■ Installable device drivers ■ A user-customizable system-configuration file that controlled the loading of additional device drivers, the number of system disk buffers, and so forth ■ Maintenance of environment blocks that could be used to pass information between programs ■ An optional ANSI display driver that allowed programs to position the cursor and control display characteristics in a hardware-independent manner ■ Support for the dynamic allocation, modification, and release of memory by application programs ■ Support for customized user command interpreters (shells) ■ System tables to assist application software in modifying its currency, time, and date formats (known as international support) MS-DOS version 2.11 was subsequently released to improve international support (table-driven currency symbols, date formats, decimal-point symbols, currency separators, and so forth), to add support for 16-bit Kanji characters throughout, and to fix a few minor bugs. Version 2.11 rapidly became the base version shipped for 8086/8088-based personal computers by every major OEM, including Hewlett-Packard, Wang, Digital Equipment Corporation, Texas Instruments, COMPAQ, and Tandy. MS-DOS version 2.25, released in October 1985, was distributed in the Far East but was never shipped by OEMs in the United States and Europe. In this version, the international support for Japanese and Korean character sets was extended even further, additional bugs were repaired, and many of the system utilities were made compatible with MS-DOS version 3.0. MS-DOS version 3.0 was introduced by IBM in August 1984 with the release of the 80286-based PC/AT machines. It represented another major rewrite of the entire operating system and included the important new features listed on the following page. ■ Direct control of the print spooler by application software ■ Further expansion of international support for currency formats ■ Extended error reporting, including a code that suggests a recovery strategy to the application program ■ Support for file and record locking and sharing ■ Support for larger fixed disks MS-DOS version 3.1, which was released in November 1984, added support for the sharing of files and printers across a network. Beginning with version 3.1, a new operating-system module called the redirector intercepts an application program's requests for I/O and filters out the requests that are directed to network devices, passing these requests to another machine for processing. Since version 3.1, the changes to MS-DOS have been evolutionary rather than revolutionary. Version 3.2, which appeared in 1986, generalized the definition of device drivers so that new media types (such as 3.5-inch floppy disks) could be supported more easily. Version 3.3 was released in 1987, concurrently with the new IBM line of PS/2 personal computers, and drastically expanded MS-DOS's multilanguage support for keyboard mappings, printer character sets, and display fonts. Version 4.0, delivered in 1988, was enhanced with a visual shell as well as support for very large file systems. While MS-DOS has been evolving, Microsoft has also put intense efforts into the areas of user interfaces and multitasking operating systems. Microsoft Windows, first shipped in 1985, provides a multitasking, graphical user "desktop" for MS-DOS systems. Windows has won widespread support among developers of complex graphics applications such as desktop publishing and computer-aided design because it allows their programs to take full advantage of whatever output devices are available without introducing any hardware dependence. Microsoft Operating System/2 (MS OS/2), released in 1987, represents a new standard for application developers: a protected-mode, multitasking, virtual-memory system specifically designed for applications requiring high-performance graphics, networking, and interprocess communications. Although MS OS/2 is a new product and is not a derivative of MS-DOS, its user interface and file system are compatible with MS-DOS and Microsoft Windows, and it offers the ability to run one real-mode (MS-DOS) application alongside MS OS/2 protected-mode applications. This compatibility allows users to move between the MS-DOS and OS/2 environments with a minimum of difficulty. ┌─────────────┐ │ MS-DOS 1.0 │ 1981: First operating system on IBM PC │ PC-DOS 1.0 │ └──────┬──────┘ │ ┌────────────┐ │ MS-DOS 1.25 │ Double-sided disk support and bug fixes added: │ PC-DOS 1.1 │ widely distributed by OEMs other than IBM └──────┬──────┘ │ ┌────────────┐ 1983: Introduced with IBM PC/XT; │ MS-DOS 2.0 │ support for UNIX/XENIX-like hierarchical │ PC-DOS 2.0 │ file structure and hard disks added └──────┬──────┘ ├──────────────────────────────────────┐ ┌────────────┐ ┌────────────┐ │ MS-DOS 2.01 │ 2.0 with international │ PC-DOS 2.1 │ Introduced with PCjr └──────┬──────┘ support └─────────────┘ 2.0 with bug fixes │ ┌────────────┐ │ MS-DOS 2.11 │ 2.01 with bug fixes └──────┬──────┘ ├──────────────────────────────────────┐ ┌────────────┐ 1984: Introduced with ┌────────────┐ 1985: Far East OEMs; │ MS-DOS 3.0 │ PC/AT; support for │ MS-DOS 2.25 │ support for extended │ PC-DOS 3.0 │ 1.2 MB floppy disk, └─────────────┘ character sets └──────┬──────┘ larger hard disk added │ ┌────────────┐ │ MS-DOS 3.1 │ Support for Microsoft ┌─────────────┐ 1985: Graphical │ PC-DOS 3.1 │ Networks added │ Windows │ user interface └──────┬──────┘ │ 1.0 │ for MS-DOS │ └──────┬──────┘ ┌────────────┐ │ │ MS-DOS 3.2 │ 1986: Support for 3.5- │ │ PC-DOS 3.2 │ inch disks added │ └──────┬──────┘ │ │ ┌────────────┐ 1987: Compatibility ┌────────────┐ 1987: Introduced with │ Windows │ with OS/2 │ MS-DOS 3.3 │ IBM PS/2; generalized │ 2.0 │ Presentation Manager │ PC-DOS 3.3 │ code-page (font) └─────────────┘ └──────┬──────┘ support │ ┌────────────┐ 1988: Support for │ MS-DOS 4.0 │ logical volumes larger │ PC-DOS 4.0 │ than 32 MB; visual shell └─────────────┘ Figure 1-1. The evolution of MS-DOS. What does the future hold for MS-DOS? Only the long-range planning teams at Microsoft and IBM know for sure. But it seems safe to assume that MS-DOS, with its relatively small memory requirements, adaptability to diverse hardware configurations, and enormous base of users, will remain important to programmers and software publishers for years to come. ──────────────────────────────────────────────────────────────────────────── Chapter 2 MS-DOS in Operation It is unlikely that you will ever be called upon to configure the MS-DOS software for a new model of computer. Still, an acquaintance with the general structure of MS-DOS can often be very helpful in understanding the behavior of the system as a whole. In this chapter, we will discuss how MS-DOS is organized and how it is loaded into memory when the computer is turned on. The Structure of MS-DOS MS-DOS is partitioned into several layers that serve to isolate the kernel logic of the operating system, and the user's perception of the system, from the hardware it is running on. These layers are ■ The BIOS (Basic Input/Output System) ■ The DOS kernel ■ The command processor (shell) We'll discuss the functions of each of these layers separately. The BIOS Module The BIOS is specific to the individual computer system and is provided by the manufacturer of the system. It contains the default resident hardware-dependent drivers for the following devices: ■ Console display and keyboard (CON) ■ Line printer (PRN) ■ Auxiliary device (AUX) ■ Date and time (CLOCK$) ■ Boot disk device (block device) The MS-DOS kernel communicates with these device drivers through I/O request packets; the drivers then translate these requests into the proper commands for the various hardware controllers. In many MS-DOS systems, including the IBM PC, the most primitive parts of the hardware drivers are located in read-only memory (ROM) so that they can be used by stand-alone applications, diagnostics, and the system startup program. The terms resident and installable are used to distinguish between the drivers built into the BIOS and the drivers installed during system initialization by DEVICE commands in the CONFIG.SYS file. (Installable drivers will be discussed in more detail later in this chapter and in Chapter 14.) The BIOS is read into random-access memory (RAM) during system initialization as part of a file named IO.SYS. (In PC-DOS, the file is called IBMBIO.COM.) This file is marked with the special attributes hidden and system. The DOS Kernel The DOS kernel implements MS-DOS as it is seen by application programs. The kernel is a proprietary program supplied by Microsoft Corporation and provides a collection of hardware-independent services called system functions. These functions include the following: ■ File and record management ■ Memory management ■ Character-device input/output ■ Spawning of other programs ■ Access to the real-time clock Programs can access system functions by loading registers with function-specific parameters and then transferring to the operating system by means of a software interrupt. The DOS kernel is read into memory during system initialization from the MSDOS.SYS file on the boot disk. (The file is called IBMDOS.COM in PC-DOS.) This file is marked with the attributes hidden and system. The Command Processor The command processor, or shell, is the user's interface to the operating system. It is responsible for parsing and carrying out user commands, including the loading and execution of other programs from a disk or other mass-storage device. The default shell that is provided with MS-DOS is found in a file called COMMAND.COM. Although COMMAND.COM prompts and responses constitute the ordinary user's complete perception of MS-DOS, it is important to realize that COMMAND.COM is not the operating system, but simply a special class of program running under the control of MS-DOS. COMMAND.COM can be replaced with a shell of the programmer's own design by simply adding a SHELL directive to the system-configuration file (CONFIG.SYS) on the system startup disk. The product COMMAND-PLUS from ESP Systems is an example of such an alternative shell. More about COMMAND.COM The default MS-DOS shell, COMMAND.COM, is divided into three parts: ■ A resident portion ■ An initialization section ■ A transient module The resident portion is loaded in lower memory, above the DOS kernel and its buffers and tables. It contains the routines to process Ctrl-C and Ctrl-Break, critical errors, and the termination (final exit) of other transient programs. This part of COMMAND.COM issues error messages and is responsible for the familiar prompt Abort, Retry, Ignore? The resident portion also contains the code required to reload the transient portion of COMMAND.COM when necessary. The initialization section of COMMAND.COM is loaded above the resident portion when the system is started. It processes the AUTOEXEC.BAT batch file (the user's list of commands to execute at system startup), if one is present, and is then discarded. The transient portion of COMMAND.COM is loaded at the high end of memory, and its memory can also be used for other purposes by application programs. The transient module issues the user prompt, reads the commands from the keyboard or batch file, and causes them to be executed. When an application program terminates, the resident portion of COMMAND.COM does a checksum of the transient module to determine whether it has been destroyed and fetches a fresh copy from the disk if necessary. The user commands that are accepted by COMMAND.COM fall into three categories: ■ Internal commands ■ External commands ■ Batch files Internal commands, sometimes called intrinsic commands, are those carried out by code embedded in COMMAND.COM itself. Commands in this category include COPY, REN(AME), DIR(ECTORY), and DEL(ETE). The routines for the internal commands are included in the transient part of COMMAND.COM. External commands, sometimes called extrinsic commands or transient programs, are the names of programs stored in disk files. Before these programs can be executed, they must be loaded from the disk into the transient program area (TPA) of memory. (See "How MS-DOS Is Loaded" in this chapter.) Familiar examples of external commands are CHKDSK, BACKUP, and RESTORE. As soon as an external command has completed its work, it is discarded from memory; hence, it must be reloaded from disk each time it is invoked. Batch files are text files that contain lists of other intrinsic, extrinsic, or batch commands. These files are processed by a special interpreter that is built into the transient portion of COMMAND.COM. The interpreter reads the batch file one line at a time and carries out each of the specified operations in order. In order to interpret a user's command, COMMAND.COM first looks to see if the user typed the name of a built-in (intrinsic) command that it can carry out directly. If not, it searches for an external command (executable program file) or batch file by the same name. The search is carried out first in the current directory of the current disk drive and then in each of the directories specified in the most recent PATH command. In each directory inspected, COMMAND.COM first tries to find a file with the extension .COM, then .EXE, and finally .BAT. If the search fails for all three file types in all of the possible locations, COMMAND.COM displays the familiar message Bad command or file name If a .COM file or a .EXE file is found, COMMAND.COM uses the MS-DOS EXEC function to load and execute it. The EXEC function builds a special data structure called a program segment prefix (PSP) above the resident portion of COMMAND.COM in the transient program area. The PSP contains various linkages and pointers needed by the application program. Next, the EXEC function loads the program itself, just above the PSP, and performs any relocation that may be necessary. Finally, it sets up the registers appropriately and transfers control to the entry point for the program. (Both the PSP and the EXEC function will be discussed in more detail in Chapters 3 and 12.) When the transient program has finished its job, it calls a special MS-DOS termination function that releases the transient program's memory and returns control to the program that caused the transient program to be loaded (COMMAND.COM, in this case). A transient program has nearly complete control of the system's resources while it is executing. The only other tasks that are accomplished are those performed by interrupt handlers (such as the keyboard input driver and the real-time clock) and operations that the transient program requests from the operating system. MS-DOS does not support sharing of the central processor among several tasks executing concurrently, nor can it wrest control away from a program when it crashes or executes for too long. Such capabilities are the province of MS OS/2, which is a protected-mode system with preemptive multitasking (time-slicing). How MS-DOS Is Loaded When the system is started or reset, program execution begins at address 0FFFF0H. This is a feature of the 8086/8088 family of microprocessors and has nothing to do with MS-DOS. Systems based on these processors are designed so that address 0FFFF0H lies within an area of ROM and contains a jump machine instruction to transfer control to system test code and the ROM bootstrap routine (Figure 2-1). The ROM bootstrap routine reads the disk bootstrap routine from the first sector of the system startup disk (the boot sector) into memory at some arbitrary address and then transfers control to it (Figure 2-2). (The boot sector also contains a table of information about the disk format.) The disk bootstrap routine checks to see if the disk contains a copy of MS-DOS. It does this by reading the first sector of the root directory and determining whether the first two files are IO.SYS and MSDOS.SYS (or IBMBIO.COM and IBMDOS.COM), in that order. If these files are not present, the user is prompted to change disks and strike any key to try again. ┌───────────────────────────────────────────────┐ │ ROM bootstrap routine │ ├───────────────────────────────────────────────┤ │ │ ├───────────────────────────────────────────────┤ Top of RAM │ │ │ │ └──────────────────────┐ │ ┌────────────────────┐ └────────────────────────┘ │ └──────────────────────────┐ │ │ │ │ │ │ 00400H ├───────────────────────────────────────────────┤ │ Interrupt vectors │ 00000H └───────────────────────────────────────────────┘ Figure 2-1. A typical 8086/8088-based computer system immediately after system startup or reset. Execution begins at location 0FFFF0H, which contains a jump instruction that directs program control to the ROM bootstrap routine. ┌───────────────────────────────────────────────┐ │ ROM bootstrap routine │ ├───────────────────────────────────────────────┤ │ │ ├───────────────────────────────────────────────┤ Top of RAM │ │ ├───────────────────────────────────────────────┤ │ Disk bootstrap routine │ ├───────────────────────────────────────────────┤ Arbitrary │ │ load location │ │ └──────────────────────┐ │ ┌────────────────────┐ └────────────────────────┘ │ └──────────────────────────┐ │ │ │ │ 00400H ├───────────────────────────────────────────────┤ │ Interrupt vectors │ 00000H └───────────────────────────────────────────────┘ Figure 2-2. The ROM bootstrap routine loads the disk bootstrap routine into memory from the first sector of the system startup disk and then transfers control to it. If the two system files are found, the disk bootstrap reads them into memory and transfers control to the initial entry point of IO.SYS (Figure 2-3). (In some implementations, the disk bootstrap reads only IO.SYS into memory, and IO.SYS in turn loads the MSDOS.SYS file.) The IO.SYS file that is loaded from the disk actually consists of two separate modules. The first is the BIOS, which contains the linked set of resident device drivers for the console, auxiliary port, printer, block, and clock devices, plus some hardware-specific initialization code that is run only at system startup. The second module, SYSINIT, is supplied by Microsoft and linked into the IO.SYS file, along with the BIOS, by the computer manufacturer. SYSINIT is called by the manufacturer's BIOS initialization code. It determines the amount of contiguous memory present in the system and then relocates itself to high memory. Then it moves the DOS kernel, MSDOS.SYS, from its original load location to its final memory location, overlaying the original SYSINIT code and any other expendable initialization code that was contained in the IO.SYS file (Figure 2-4). Next, SYSINIT calls the initialization code in MSDOS.SYS. The DOS kernel initializes its internal tables and work areas, sets up the interrupt vectors 20H through 2FH, and traces through the linked list of resident device drivers, calling the initialization function for each. (See Chapter 14.) ┌───────────────────────────────────────────────┐ │ ROM bootstrap routine │ ├───────────────────────────────────────────────┤ │ │ ├───────────────────────────────────────────────┤ Top of RAM │ │ ├───────────────────────────────────────────────┤ │ Disk bootstrap routine │ ├───────────────────────────────────────────────┤ │ │ └──────────────────────┐ │ ┌────────────────────┐ └────────────────────────┘ │ └──────────────────────────┐ │ │ ├───────────────────────────────────────────────┤ │ DOS kernel (from MSDOS.SYS) │ ├───────────────────────────────────────────────┤ In temporary │ SYSINIT (from IO.SYS) │ location ├───────────────────────────────────────────────┤ │ BIOS (from IO.SYS) │ ├───────────────────────────────────────────────┤ │ │ 00400H ├───────────────────────────────────────────────┤ │ Interrupt vectors │ 00000H └───────────────────────────────────────────────┘ Figure 2-3. The disk bootstrap reads the file IO.SYS into memory. This file contains the MS-DOS BIOS (resident device drivers) and the SYSINIT module. Either the disk bootstrap or the BIOS (depending upon the manufacturer's implementation) then reads the DOS kernel into memory from the MSDOS.SYS file. These driver functions determine the equipment status, perform any necessary hardware initialization, and set up the vectors for any external hardware interrupts the drivers will service. As part of the initialization sequence, the DOS kernel examines the disk-parameter blocks returned by the resident block-device drivers, determines the largest sector size that will be used in the system, builds some drive-parameter blocks, and allocates a disk sector buffer. Control then returns to SYSINIT. When the DOS kernel has been initialized and all resident device drivers are available, SYSINIT can call on the normal MS-DOS file services to open the CONFIG.SYS file. This optional file can contain a variety of commands that enable the user to customize the MS-DOS environment. For instance, the user can specify additional hardware device drivers, the number of disk buffers, the maximum number of files that can be open at one time, and the filename of the command processor (shell). If it is found, the entire CONFIG.SYS file is loaded into memory for processing. All lowercase characters are converted to uppercase, and the file is interpreted one line at a time to process the commands. Memory is allocated for the disk buffer cache and the internal file control blocks used by the handle file and record system functions. (See Chapter 8.) Any device drivers indicated in the CONFIG.SYS file are sequentially loaded into memory, initialized by calls to their init modules, and linked into the device-driver list. The init function of each driver tells SYSINIT how much memory to reserve for that driver. ┌───────────────────────────────────────────────┐ │ ROM bootstrap routine │ ├───────────────────────────────────────────────┤ │ │ ├───────────────────────────────────────────────┤ Top of RAM │ SYSINIT module │ ├───────────────────────────────────────────────┤ │ │ └──────────────────────┐ │ ┌────────────────────┐ └────────────────────────┘ │ └──────────────────────────┐ │ │ ├───────────────────────────────────────────────┤ │ Installable drivers │ ├───────────────────────────────────────────────┤ │ File control blocks │ ├───────────────────────────────────────────────┤ │ Disk buffer cache │ ├───────────────────────────────────────────────┤ │ DOS kernel │ ├───────────────────────────────────────────────┤ In final │ BIOS │ location ├───────────────────────────────────────────────┤ │ │ ├───────────────────────────────────────────────┤ 00400H ├───────────────────────────────────────────────┤ │ Interrupt vectors │ 00000H └───────────────────────────────────────────────┘ Figure 2-4. SYSINIT moves itself to high memory and relocates the DOS kernel, MSDOS.SYS, downward to its final address. The MS-DOS disk buffer cache and file control block areas are allocated, and then the installable device drivers specified in the CONFIG.SYS file are loaded and linked into the system. After all installable device drivers have been loaded, SYSINIT closes all file handles and reopens the console (CON), printer (PRN), and auxiliary (AUX) devices as the standard input, standard output, standard error, standard list, and standard auxiliary devices. This allows a user-installed character-device driver to override the BIOS's resident drivers for the standard devices. Finally, SYSINIT calls the MS-DOS EXEC function to load the command interpreter, or shell. (The default shell is COMMAND.COM, but another shell can be substituted by means of the CONFIG.SYS file.) Once the shell is loaded, it displays a prompt and waits for the user to enter a command. MS-DOS is now ready for business, and the SYSINIT module is discarded (Figure 2-5). ┌───────────────────────────────────────────────┐ │ ROM bootstrap routine │ ├───────────────────────────────────────────────┤ │ │ ├───────────────────────────────────────────────┤ Top of RAM │ Transient part of COMMAND.COM │ ├───────────────────────────────────────────────┤ └──────────────────────┐ │ ┌────────────────────┐ └────────────────────────┘ │ └──────────────────────────┐ │ Transient program area │ ├───────────────────────────────────────────────┤ │ Resident part of COMMAND.COM │ ├───────────────────────────────────────────────┤ │ Installable drivers │ ├───────────────────────────────────────────────┤ │ File control blocks │ ├───────────────────────────────────────────────┤ │ Disk buffer cache │ ├───────────────────────────────────────────────┤ │ DOS kernel │ ├───────────────────────────────────────────────┤ │ BIOS │ ├───────────────────────────────────────────────┤ │ │ 00400H ├───────────────────────────────────────────────┤ │ Interrupt vectors │ 00000H └───────────────────────────────────────────────┘ Figure 2-5. The final result of the MS-DOS startup process for a typical system. The resident portion of COMMAND.COM lies in low memory, above the DOS kernel. The transient portion containing the batch-file interpreter and intrinsic commands is placed in high memory, where it can be overlaid by extrinsic commands and application programs running in the transient program area. ──────────────────────────────────────────────────────────────────────────── Chapter 3 Structure of MS-DOS Application Programs Programs that run under MS-DOS come in two basic flavors: .COM programs, which have a maximum size of approximately 64 KB, and .EXE programs, which can be as large as available memory. In Intel 8086 parlance, .COM programs fit the tiny model, in which all segment registers contain the same value; that is, the code and data are mixed together. In contrast, .EXE programs fit the small, medium, or large model, in which the segment registers contain different values; that is, the code, data, and stack reside in separate segments. .EXE programs can have multiple code and data segments, which are respectively addressed by long calls and by manipulation of the data segment (DS) register. A .COM-type program resides on the disk as an absolute memory image, in a file with the extension .COM. The file does not have a header or any other internal identifying information. A .EXE program, on the other hand, resides on the disk in a special type of file with a unique header, a relocation map, a checksum, and other information that is (or can be) used by MS-DOS. Both .COM and .EXE programs are brought into memory for execution by the same mechanism: the EXEC function, which constitutes the MS-DOS loader. EXEC can be called with the filename of a program to be loaded by COMMAND.COM (the normal MS-DOS command interpreter), by other shells or user interfaces, or by another program that was previously loaded by EXEC. If there is sufficient free memory in the transient program area, EXEC allocates a block of memory to hold the new program, builds the program segment prefix (PSP) at its base, and then reads the program into memory immediately above the PSP. Finally, EXEC sets up the segment registers and the stack and transfers control to the program. When it is invoked, EXEC can be given the addresses of additional information, such as a command tail, file control blocks, and an environment block; if supplied, this information will be passed on to the new program. (The exact procedure for using the EXEC function in your own programs is discussed, with examples, in Chapter 12.) .COM and .EXE programs are often referred to as transient programs. A transient program "owns" the memory block it has been allocated and has nearly total control of the system's resources while it is executing. When the program terminates, either because it is aborted by the operating system or because it has completed its work and systematically performed a final exit back to MS-DOS, the memory block is then freed (hence the term transient) and can be used by the next program in line to be loaded. The Program Segment Prefix A thorough understanding of the program segment prefix is vital to successful programming under MS-DOS. It is a reserved area, 256 bytes long, that is set up by MS-DOS at the base of the memory block allocated to a transient program. The PSP contains some linkages to MS-DOS that can be used by the transient program, some information MS-DOS saves for its own purposes, and some information MS-DOS passes to the transient program──to be used or not, as the program requires (Figure 3-1). Offset 0000H ┌────────────────────────────────────────────────────────┐ │ Int 20H │ 0002H ├────────────────────────────────────────────────────────┤ │ Segment, end of allocation block │ 0004H ├────────────────────────────────────────────────────────┤ │ Reserved │ 0005H ├────────────────────────────────────────────────────────┤ │ Long call to MS-DOS function dispatcher │ 000AH ├────────────────────────────────────────────────────────┤ │ Previous contents of termination handler │ │ interrupt vector (Int 22H) │ 000EH ├────────────────────────────────────────────────────────┤ │ Previous contents of Ctrl-C interrupt vector (Int 23H) │ 0012H ├────────────────────────────────────────────────────────┤ │ Previous contents of critical-error handler │ │ interrupt vector (Int 24H) │ 0016H ├────────────────────────────────────────────────────────┤ │ Reserved │ 002CH ├────────────────────────────────────────────────────────┤ │ Segment address of environment block │ 002EH ├────────────────────────────────────────────────────────┤ │ Reserved │ 005CH ├────────────────────────────────────────────────────────┤ │ Default file control block #1 │ 006CH ├────────────────────────────────────────────────────────┤ │ Default file control block #2 │ │ (overlaid if FCB #1 opened) │ 008OH ├────────────────────────────────────────────────────────┤ └──────────────────────────┐ │ ┌────────────────────────┐ └─────────────────────────────┘ │ └───────────────────────────────┐ │ Command tail and default disk transfer area (buffer) │ OOFFH └────────────────────────────────────────────────────────┘ Figure 3-1. The structure of the program segment prefix. In the first versions of MS-DOS, the PSP was designed to be compatible with a control area that was built beneath transient programs under Digital Research's venerable CP/M operating system, so that programs could be ported to MS-DOS without extensive logical changes. Although MS-DOS has evolved considerably since those early days, the structure of the PSP is still recognizably similar to its CP/M equivalent. For example, offset 0000H in the PSP contains a linkage to the MS-DOS process-termination handler, which cleans up after the program has finished its job and performs a final exit. Similarly, offset 0005H in the PSP contains a linkage to the MS-DOS function dispatcher, which performs disk operations, console input/output, and other such services at the request of the transient program. Thus, calls to PSP:0000 and PSP:0005 have the same effect as CALL 0000 and CALL 0005 under CP/M. (These linkages are not the "approved" means of obtaining these services, however.) The word at offset 0002H in the PSP contains the segment address of the top of the transient program's allocated memory block. The program can use this value to determine whether it should request more memory to do its job or whether it has extra memory that it can release for use by other processes. Offsets 000AH through 0015H in the PSP contain the previous contents of the interrupt vectors for the termination, Ctrl-C, and critical-error handlers. If the transient program alters these vectors for its own purposes, MS-DOS restores the original values saved in the PSP when the program terminates. The word at PSP offset 002CH holds the segment address of the environment block, which contains a series of ASCIIZ strings (sequences of ASCII characters terminated by a null, or zero, byte). The environment block is inherited from the program that called the EXEC function to load the currently executing program. It contains such information as the current search path used by COMMAND.COM to find executable programs, the location on the disk of COMMAND.COM itself, and the format of the user prompt used by COMMAND.COM. The command tail──the remainder of the command line that invoked the transient program, after the program's name──is copied into the PSP starting at offset 0081H. The length of the command tail, not including the return character at its end, is placed in the byte at offset 0080H. Redirection or piping parameters and their associated filenames do not appear in the portion of the command line (the command tail) that is passed to the transient program, because redirection is transparent to applications. To provide compatibility with CP/M, MS-DOS parses the first two parameters in the command tail into two default file control blocks (FCBs) at PSP:005CH and PSP:006CH, under the assumption that they may be filenames. However, if the parameters are filenames that include a path specification, only the drive code will be valid in these default FCBs, because FCB-type file- and record-access functions do not support hierarchical file structures. Although the default FCBs were an aid in earlier years, when compatibility with CP/M was more of a concern, they are essentially useless in modern MS-DOS application programs that must provide full path support. (File control blocks are discussed in detail in Chapter 8 and hierarchical file structures are discussed in Chapter 9.) The 128-byte area from 0080H through 00FFH in the PSP also serves as the default disk transfer area (DTA), which is set by MS-DOS before passing control to the transient program. If the program does not explicitly change the DTA, any file read or write operations requested with the FCB group of function calls automatically use this area as a data buffer. This is rarely useful and is another facet of MS-DOS's handling of the PSP that is present only for compatibility with CP/M. ────────────────────────────────────────────────────────────────────────── WARNING Programs must not alter any part of the PSP below offset 005CH. ────────────────────────────────────────────────────────────────────────── Introduction to .COM Programs Programs of the .COM persuasion are stored in disk files that hold an absolute image of the machine instructions to be executed. Because the files contain no relocation information, they are more compact, and are loaded for execution slightly faster, than equivalent .EXE files. Note that MS-DOS does not attempt to ascertain whether a .COM file actually contains executable code (there is no signature or checksum, as in the case of a .EXE file); it simply brings any file with the .COM extension into memory and jumps to it. Because .COM programs are loaded immediately above the program segment prefix and do not have a header that can specify another entry point, they must always have an origin of 0100H, which is the length of the PSP. Location 0100H must contain an executable instruction. The maximum length of a .COM program is 65,536 bytes, minus the length of the PSP (256 bytes) and a mandatory word of stack (2 bytes). When control is transferred to the .COM program from MS-DOS, all of the segment registers point to the PSP (Figure 3-2). The stack pointer register contains 0FFFEH if memory allows; otherwise, it is set as high as possible in memory minus 2 bytes. (MS-DOS pushes a zero word on the stack before entry.) SS:SP ┌────────────────────────────────────────────────────────┐ │ │ │ Stack grows downward from top of segment │ │ │ │ │ │ │ │ │ │ │ │ Program code and data │ │ │ CS:0100H ├────────────────────────────────────────────────────────┤ │ Program segment prefix │ CS:0000H └────────────────────────────────────────────────────────┘ DS:0000H ES:0000H SS:0000H Figure 3-2. A memory image of a typical .COM-type program after loading. The contents of the .COM file are brought into memory just above the program segment prefix. Program, code, and data are mixed together in the same segment, and all segment registers contain the same value. Although the size of an executable .COM file can't exceed 64 KB, the current versions of MS-DOS allocate all of the transient program area to .COM programs when they are loaded. Because many such programs date from the early days of MS-DOS and are not necessarily "well-behaved" in their approach to memory management, the operating system simply makes the worst-case assumption and gives .COM programs everything that is available. If a .COM program wants to use the EXEC function to invoke another process, it must first shrink down its memory allocation to the minimum memory it needs in order to continue, taking care to protect its stack. (This is discussed in more detail in Chapter 12.) When a .COM program finishes executing, it can return control to MS-DOS by several means. The preferred method is Int 21H Function 4CH, which allows the program to pass a return code back to the program, shell, or batch file that invoked it. However, if the program is running under MS-DOS version 1, it must exit by means of Int 20H, Int 21H Function 0, or a NEAR RETURN. (Because a word of zero was pushed onto the stack at entry, a NEAR RETURN causes a transfer to PSP:0000, which contains an Int 20H instruction.) A .COM-type application can be linked together from many separate object modules. All of the modules must use the same code-segment name and class name, and the module with the entry point at offset 0100H within the segment must be linked first. In addition, all of the procedures within a .COM program should have the NEAR attribute, because all executable code resides in one segment. When linking a .COM program, the linker will display the message Warning: no stack segment This message can be ignored. The linker output is a .EXE file, which must be converted into a .COM file with the MS-DOS EXE2BIN utility before execution. You can then delete the .EXE file. (An example of this process is provided in Chapter 4.) An Example .COM Program The HELLO.COM program listed in Figure 3-3 demonstrates the structure of a simple assembly-language program that is destined to become a .COM file. (You may find it helpful to compare this listing with the HELLO.EXE program later in this chapter.) Because this program is so short and simple, a relatively high proportion of the source code is actually assembler directives that do not result in any executable code. The NAME statement simply provides a module name for use during the linkage process. This aids understanding of the map that the linker produces. In MASM versions 5.0 and later, the module name is always the same as the filename, and the NAME statement is ignored. The PAGE command, when used with two operands, as in line 2, defines the length and width of the page. These default respectively to 66 lines and 80 characters. If you use the PAGE command without any operands, a formfeed is sent to the printer and a heading is printed. In larger programs, use the PAGE command liberally to place each of your subroutines on separate pages for easy reading. The TITLE command, in line 3, specifies the text string (limited to 60 characters) that is to be printed at the upper left corner of each page. The TITLE command is optional and cannot be used more than once in each assembly-language source file. ────────────────────────────────────────────────────────────────────────── 1: name hello 2: page 55,132 3: title HELLO.COM--print hello on terminal 4: 5: ; 6: ; HELLO.COM: demonstrates various components 7: ; of a functional .COM-type assembly- 8: ; language program, and an MS-DOS 9: ; function call. 10: ; 11: ; Ray Duncan, May 1988 12: ; 13: 14: stdin equ 0 ; standard input handle 15: stdout equ 1 ; standard output handle 16: stderr equ 2 ; standard error handle 17: 18: cr equ 0dh ; ASCII carriage return 19: lf equ 0ah ; ASCII linefeed 20: 21: 22: _TEXT segment word public 'CODE' 23: 24: org 100h ; .COM files always have 25: ; an origin of 100h 26: 27: assume cs:_TEXT,ds:_TEXT,es:_TEXT,ss:_TEXT 28: 29: print proc near ; entry point from MS-DOS 30: 31: mov ah,40h ; function 40h = write 32: mov bx,stdout ; handle for standard output 33: mov cx,msg_len ; length of message 34: mov dx,offset msg ; address of message 35: int 21h ; transfer to MS-DOS 36: 37: mov ax,4c00h ; exit, return code = 0 38: int 21h ; transfer to MS-DOS 39: 40: print endp 41: 42: 43: msg db cr,lf ; message to display 44: db 'Hello World!',cr,lf 45: 46: msg_len equ $-msg ; length of message 47: 48: 49: _TEXT ends 50: 51: end print ; defines entry point ────────────────────────────────────────────────────────────────────────── Figure 3-3. The HELLO.COM program listing. Dropping down past a few comments and EQU statements, we come to a declaration of a code segment that begins in line 22 with a SEGMENT command and ends in line 49 with an ENDS command. The label in the leftmost field of line 22 gives the code segment the name _TEXT. The operand fields at the right end of the line give the segment the attributes WORD, PUBLIC, and `CODE'. (You might find it helpful to read the Microsoft Macro Assembler manual for detailed explanations of each possible segment attribute.) Because this program is going to be converted into a .COM file, all of its executable code and data areas must lie within one code segment. The program must also have its origin at offset 0100H (immediately above the program segment prefix), which is taken care of by the ORG statement in line 24. Following the ORG instruction, we encounter an ASSUME statement on line 27. The concept of ASSUME often baffles new assembly-language programmers. In a way, ASSUME doesn't "do" anything; it simply tells the assembler which segment registers you are going to use to point to the various segments of your program, so that the assembler can provide segment overrides when they are necessary. It's important to notice that the ASSUME statement doesn't take care of loading the segment registers with the proper values; it merely notifies the assembler of your intent to do that within the program. (Remember that, in the case of a .COM program, MS-DOS initializes all the segment registers before entry to point to the PSP.) Within the code segment, we come to another type of block declaration that begins with the PROC command on line 29 and closes with ENDP on line 40. These two instructions declare the beginning and end of a procedure, a block of executable code that performs a single distinct function. The label in the leftmost field of the PROC statement (in this case, print) gives the procedure a name. The operand field gives it an attribute. If the procedure carries the NEAR attribute, only other code in the same segment can call it, whereas if it carries the FAR attribute, code located anywhere in the CPU's memory-addressing space can call it. In .COM programs, all procedures carry the NEAR attribute. For the purposes of this example program, I have kept the print procedure ridiculously simple. It calls MS-DOS Int 21H Function 40H to send the message Hello World! to the video screen, and calls Int 21H Function 4CH to terminate the program. The END statement in line 51 tells the assembler that it has reached the end of the source file and also specifies the entry point for the program. If the entry point is not a label located at offset 0100H, the .EXE file resulting from the assembly and linkage of this source program cannot be converted into a .COM file. Introduction to .EXE Programs We have just discussed a program that was written in such a way that it could be assembled into a .COM file. Such a program is simple in structure, so a programmer who needs to put together this kind of quick utility can concentrate on the program logic and do a minimum amount of worrying about control of the assembler. However, .COM-type programs have some definite disadvantages, and so most serious assembly-language efforts for MS-DOS are written to be converted into .EXE files. Although .COM programs are effectively restricted to a total size of 64 KB for machine code, data, and stack combined, .EXE programs can be practically unlimited in size (up to the limit of the computer's available memory). .EXE programs also place the code, data, and stack in separate parts of the file. Although the normal MS-DOS program loader does not take advantage of this feature of .EXE files, the ability to load different parts of large programs into several separate memory fragments, as well as the opportunity to designate a "pure" code portion of your program that can be shared by several tasks, is very significant in multitasking environments such as Microsoft Windows. The MS-DOS loader always brings a .EXE program into memory immediately above the program segment prefix, although the order of the code, data, and stack segments may vary (Figure 3-4). The .EXE file has a header, or block of control information, with a characteristic format (Figures 3-5 and 3-6). The size of this header varies according to the number of program instructions that need to be relocated at load time, but it is always a multiple of 512 bytes. Before MS-DOS transfers control to the program, the initial values of the code segment (CS) register and instruction pointer (IP) register are calculated from the entry-point information in the .EXE file header and the program's load address. This information derives from an END statement in the source code for one of the program's modules. The data segment (DS) and extra segment (ES) registers are made to point to the PSP so that the program can access the environment-block pointer, command tail, and other useful information contained there. SS:SP ┌────────────────────────────────────────────────────────┐ │ │ │ Stack segment: │ │ stack grows downward from top of segment │ │ │ │ │ │ SS:0000H ├────────────────────────────────────────────────────────┤ │ Data segment │ ├────────────────────────────────────────────────────────┤ │ Program code │ CS:0000H ├────────────────────────────────────────────────────────┤ │ Program segment prefix │ DS:0000H └────────────────────────────────────────────────────────┘ ES:0000H Figure 3-4. A memory image of a typical .EXE-type program immediately after loading. The contents of the .EXE file are relocated and brought into memory above the program segment prefix. Code, data, and stack reside in separate segments and need not be in the order shown here. The entry point can be anywhere in the code segment and is specified by the END statement in the main module of the program. When the program receives control, the DS (data segment) and ES (extra segment) registers point to the program segment prefix; the program usually saves this value and then resets the DS and ES registers to point to its data area. The initial contents of the stack segment (SS) and stack pointer (SP) registers come from the header. This information derives from the declaration of a segment with the attribute STACK somewhere in the program's source code. The memory space allocated for the stack may be initialized or uninitialized, depending on the stack-segment definition; many programmers like to initialize the stack memory with a recognizable data pattern so that they can inspect memory dumps and determine how much stack space is actually used by the program. When a .EXE program finishes processing, it should return control to MS-DOS through Int 21H Function 4CH. Other methods are available, but they offer no advantages and are considerably less convenient (because they usually require the CS register to point to the PSP). Byte offset 0000H ┌────────────────────────────────────────────────────────┐ │ First of .EXE file signature (4DH) │ 0001H ├────────────────────────────────────────────────────────┤ │ Second part of .EXE file signature (5AH) │ 0002H ├────────────────────────────────────────────────────────┤ │ Length of file MOD 512 │ 0004H ├────────────────────────────────────────────────────────┤ │ Size of file in 512-byte pages, including header │ 0006H ├────────────────────────────────────────────────────────┤ │ Number of relocation-table items │ 0008H ├────────────────────────────────────────────────────────┤ │ Size of header in paragraphs (16-byte units) │ 000AH ├────────────────────────────────────────────────────────┤ │ Minimum number of paragraphs needed above program │ 000CH ├────────────────────────────────────────────────────────┤ │ Maximum number of paragraphs desired above program │ 000EH ├────────────────────────────────────────────────────────┤ │ Segment displacement of stack module │ 0010H ├────────────────────────────────────────────────────────┤ │ Contents of SP register at entry │ 0012H ├────────────────────────────────────────────────────────┤ │ Word checksum │ 0014H ├────────────────────────────────────────────────────────┤ │ Contents of IP register at entry │ 0016H ├────────────────────────────────────────────────────────┤ │ Segment displacement of code module │ 0018H ├────────────────────────────────────────────────────────┤ │ Offset of first relocation item in file │ 001AH ├────────────────────────────────────────────────────────┤ │ Overlay number (0 for resident part of program) │ 001BH ├────────────────────────────────────────────────────────┤ │ Variable reserved space │ ├────────────────────────────────────────────────────────┤ │ Relocation table │ ├────────────────────────────────────────────────────────┤ │ Variable reserved space │ ├────────────────────────────────────────────────────────┤ │ Program and data segments │ ├────────────────────────────────────────────────────────┤ │ Stack segment │ └────────────────────────────────────────────────────────┘ Figure 3-5. The format of a .EXE load module. The input to the linker for a .EXE-type program can be many separate object modules. Each module can use a unique code-segment name, and the procedures can carry either the NEAR or the FAR attribute, depending on naming conventions and the size of the executable code. The programmer must take care that the modules linked together contain only one segment with the STACK attribute and only one entry point defined with an END assembler directive. The output from the linker is a file with a .EXE extension. This file can be executed immediately. ────────────────────────────────────────────────────────────────────────── C>DUMP HELLO.EXE 0 1 2 3 4 5 6 7 8 9 A B C D E F 0000 4D 5A 28 00 02 00 01 00 20 00 09 00 FF FF 03 00 MZ(..... ....... 0010 80 00 20 05 00 00 00 00 1E 00 00 00 01 00 01 00 .. ............. 0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ . . . 0200 B8 01 00 8E D8 B4 40 BB 01 00 B9 10 00 90 BA 08 ......@......... 0210 00 CD 21 B8 00 4C CD 21 0D 0A 48 65 6C 6C 6F 20 ..!..L.!..Hello 0220 57 6F 72 6C 64 21 0D 0A World!.. ────────────────────────────────────────────────────────────────────────── Figure 3-6. A hex dump of the HELLO.EXE program, demonstrating the contents of a simple .EXE load module. Note the following interesting values: the .EXE signature in bytes 0000H and 0001H, the number of relocation-table items in bytes 0006H and 0007H, the minimum extra memory allocation (MIN_ALLOC) in bytes 000AH and 000BH, the maximum extra memory allocation (MAX_ALLOC) in bytes 000CH and 000DH, and the initial IP (instruction pointer) register value in bytes 0014H and 0015H. See also Figure 3-5. An Example .EXE Program The HELLO.EXE program in Figure 3-7 demonstrates the fundamental structure of an assembly-language program that is destined to become a .EXE file. At minimum, it should have a module name, a code segment, a stack segment, and a primary procedure that receives control of the computer from MS-DOS after the program is loaded. The HELLO.EXE program also contains a data segment to provide a more complete example. The NAME, TITLE, and PAGE directives were covered in the HELLO.COM example program and are used in the same manner here, so we'll move to the first new item of interest. After a few comments and EQU statements, we come to a declaration of a code segment that begins on line 21 with a SEGMENT command and ends on line 41 with an ENDS command. As in the HELLO.COM example program, the label in the leftmost field of the line gives the code segment the name _TEXT. The operand fields at the right end of the line give the attributes WORD, PUBLIC, and `CODE'. Following the code-segment instruction, we find an ASSUME statement on line 23. Notice that, unlike the equivalent statement in the HELLO.COM program, the ASSUME statement in this program specifies several different segment names. Again, remember that this statement has no direct effect on the contents of the segment registers but affects only the operation of the assembler itself. ────────────────────────────────────────────────────────────────────────── 1: name hello 2: page 55,132 3: title HELLO.EXE--print Hello on terminal 4: ; 5: ; HELLO.EXE: demonstrates various components 6: ; of a functional .EXE-type assembly- 7: ; language program, use of segments, 8: ; and an MS-DOS function call. 9: ; 10: ; Ray Duncan, May 1988 11: ; 12: 13: stdin equ 0 ; standard input handle 14: stdout equ 1 ; standard output handle 15: stderr equ 2 ; standard error handle 16: 17: cr equ 0dh ; ASCII carriage return 18: lf equ 0ah ; ASCII linefeed 19: 20: 21: _TEXT segment word public 'CODE' 22: 23: assume cs:_TEXT,ds:_DATA,ss:STACK 24: 25: print proc far ; entry point from MS-DOS 26: 27: mov ax,_DATA ; make our data segment 28: mov ds,ax ; addressable... 29: 30: mov ah,40h ; function 40h = write 31: mov bx,stdout ; standard output handle 32: mov cx,msg_len ; length of message 33: mov dx,offset msg ; address of message 34: int 21h ; transfer to MS-DOS 35: 36: mov ax,4c00h ; exit, return code = 0 37: int 21h ; transfer to MS-DOS 38: 39: print endp 40: 41: _TEXT ends 42: 43: 44: _DATA segment word public 'DATA' 45: 46: msg db cr,lf ; message to display 47: db 'Hello World!',cr,lf 48: 49: msg_len equ $-msg ; length of message 50: 51: _DATA ends 52: 53: 54: STACK segment para stack `STACK' 55: 56: db 128 dup (?) 57: 58: STACK ends 59: 60: end print ; defines entry point ────────────────────────────────────────────────────────────────────────── Figure 3-7. The HELLO.EXE program listing. Within the code segment, the main print procedure is declared by the PROC command on line 25 and closed with ENDP on line 39. Because the procedure resides in a .EXE file, we have given it the FAR attribute as an example, but the attribute is really irrelevant because the program is so small and the procedure is not called by anything else in the same program. The print procedure first initializes the DS register, as indicated in the earlier ASSUME statement, loading it with a value that causes it to point to the base of the data area. (MS-DOS automatically sets up the CS and SS registers.) Next, the procedure uses MS-DOS Int 21H Function 40H to display the message Hello World! on the screen, just as in the HELLO.COM program. Finally, the procedure exits back to MS-DOS with an Int 21H Function 4CH on lines 36 and 37, passing a return code of zero (which by convention means a success). Lines 44 through 51 declare a data segment named _DATA, which contains the variables and constants the program will use. If the various modules of a program contain multiple data segments with the same name, the linker will collect them and place them in the same physical memory segment. Lines 54 through 58 establish a stack segment; PUSH and POP instructions will access this area of scratch memory. Before MS-DOS transfers control to a .EXE program, it sets up the SS and SP registers according to the declared size and location of the stack segment. Be sure to allow enough room for the maximum stack depth that can occur at runtime, plus a safe number of extra words for registers pushed onto the stack during an MS-DOS service call. If the stack overflows, it may damage your other code and data segments and cause your program to behave strangely or even to crash altogether! The END statement on line 60 winds up our brief HELLO.EXE program, telling the assembler that it has reached the end of the source file and providing the label of the program's point of entry from MS-DOS. The differences between .COM and .EXE programs are summarized in Figure 3-8. ╓┌─┌──────────────────┌──────────────────────────┌───────────────────────────╖ .COM program .EXE program ────────────────────────────────────────────────────────────────────────── Maximum size 65,536 bytes minus 256 No limit bytes for PSP and 2 bytes for stack Entry point PSP:0100H Defined by END statement AL at entry 00H if default FCB #1 has Same valid drive, 0FFH if invalid drive .COM program .EXE program ────────────────────────────────────────────────────────────────────────── AH at entry 00H if default FCB #2 has Same valid drive, 0FFH if invalid drive CS at entry PSP Segment containing module with entry point IP at entry 0100H Offset of entry point within its segment DS at entry PSP PSP ES at entry PSP PSP SS at entry PSP Segment with STACK attribute SP at entry 0FFFEH or top word in Size of segment defined with available memory, STACK attribute .COM program .EXE program ────────────────────────────────────────────────────────────────────────── available memory, STACK attribute whichever is lower Stack at entry Zero word Initialized or uninitialized Stack size 65,536 bytes minus 256 Defined in segment with bytes for PSP and size of STACK attribute executable code and data Subroutine calls Usually NEAR NEAR or FAR Exit method Int 21H Function 4CH Int 21H Function 4CH preferred, NEAR RET if preferred MS-DOS version 1 Size of file Exact size of program Size of program plus header (multiple of 512 bytes) ────────────────────────────────────────────────────────────────────────── .COM program .EXE program ────────────────────────────────────────────────────────────────────────── Figure 3-8. Summary of the differences between .COM and .EXE programs, including their entry conditions. More About Assembly-Language Programs Now that we've looked at working examples of .COM and .EXE assembly-language programs, let's backtrack and discuss their elements a little more formally. The following discussion is based on the Microsoft Macro Assembler, hereafter referred to as MASM. If you are familiar with MASM and are an experienced assembly-language programmer, you may want to skip this section. MASM programs can be thought of as having three structural levels: ■ The module level ■ The segment level ■ The procedure level Modules are simply chunks of source code that can be independently maintained and assembled. Segments are physical groupings of like items (machine code or data) within a program and a corresponding segregation of dissimilar items. Procedures are functional subdivisions of an executable program──routines that carry out a particular task. Program Modules Under MS-DOS, the module-level structure consists of files containing the source code for individual routines. Each source file is translated by the assembler into a relocatable object module. An object module can reside alone in an individual file or with many other object modules in an object-module library of frequently used or related routines. The Microsoft Object Linker (LINK) combines object-module files, often with additional object modules extracted from libraries, into an executable program file. Using modules and object-module libraries reduces the size of your application source files (and vastly increases your productivity), because these files need not contain the source code for routines they have in common with other programs. This technique also allows you to maintain the routines more easily, because you need to alter only one copy of their source code stored in one place, instead of many copies stored in different applications. When you improve (or fix) one of these routines, you can simply reassemble it, put its object module back into the library, relink all of the programs that use the routine, and voilga: instant upgrade. Program Segments The term segments refers to two discrete programming concepts: physical segments and logical segments. Physical segments are 64 KB blocks of memory. The Intel 8086/8088 and 80286 microprocessors have four segment registers, which are essentially used as pointers to these blocks. (The 80386 has six segment registers, which are a superset of those found on the 8086/8088 and 80286.) Each segment register can point to the bottom of a different 64 KB area of memory. Thus, a program can address any location in memory by appropriate manipulation of the segment registers, but the maximum amount of memory that it can address simultaneously is 256 KB. As we discussed earlier in the chapter, .COM programs assume that all four segment registers always point to the same place──the bottom of the program. Thus, they are limited to a maximum size of 64 KB. .EXE programs, on the other hand, can address many different physical segments and can reset the segment registers to point to each segment as it is needed. Consequently, the only practical limit on the size of a .EXE program is the amount of available memory. The example programs throughout the remainder of this book focus on .EXE programs. Logical segments are the program components. A minimum of three logical segments must be declared in any .EXE program: a code segment, a data segment, and a stack segment. Programs with more than 64 KB of code or data have more than one code or data segment. The routines or data that are used most frequently are put into the primary code and data segments for speed, and routines or data that are used less frequently are put into secondary code and data segments. Segments are declared with the SEGMENT and ENDS directives in the following form: name SEGMENT attributes . . . name ENDS The attributes of a segment include its align type (BYTE, WORD, or PARA), combine type (PUBLIC, PRIVATE, COMMON, or STACK), and class type. The segment attributes are used by the linker when it is combining logical segments to create the physical segments of an executable program. Most of the time, you can get by just fine using a small selection of attributes in a rather stereotypical way. However, if you want to use the full range of attributes, you might want to read the detailed explanation in the MASM manual. Programs are classified into one memory model or another based on the number of their code and data segments. The most commonly used memory model for assembly-language programs is the small model, which has one code and one data segment, but you can also use the medium, compact, and large models (Figure 3-9). (Two additional models exist with which we will not be concerning ourselves further: the tiny model, which consists of intermixed code and data in a single segment── for example, a .COM file under MS-DOS; and the huge model, which is supported by the Microsoft C Optimizing Compiler and which allows use of data structures larger than 64 KB.) Model Code segments Data segments ────────────────────────────────────────────────────────────────────────── Small One One Medium Multiple One Compact One Multiple Large Multiple Multiple ────────────────────────────────────────────────────────────────────────── Figure 3-9. Memory models commonly used in assembly-language and C programs. For each memory model, Microsoft has established certain segment and class names that are used by all its high-level-language compilers (Figure 3-10). Because segment names are arbitrary, you may as well adopt the Microsoft conventions. Their use will make it easier for you to integrate your assembly-language routines into programs written in languages such as C, or to use routines from high-level-language libraries in your assembly-language programs. Another important Microsoft high-level-language convention is to use the GROUP directive to name the near data segment (the segment the program expects to address with offsets from the DS register) and the stack segment as members of DGROUP (the automatic data group), a special name recognized by the linker and also by the program loaders in Microsoft Windows and Microsoft OS/2. The GROUP directive causes logical segments with different names to be combined into a single physical segment so that they can be addressed using the same segment base address. In C programs, DGROUP also contains the local heap, which is used by the C runtime library for dynamic allocation of small amounts of memory. ╓┌─┌───────────┌────────────┌───────────┌───────────┌────────────┌───────────╖ Memory Segment Align Combine Class Group model name type type type ────────────────────────────────────────────────────────────────────────── Memory Segment Align Combine Class Group model name type type type ────────────────────────────────────────────────────────────────────────── Small _TEXT WORD PUBLIC CODE _DATA WORD PUBLIC DATA DGROUP STACK PARA STACK STACK DGROUP Medium module_TEXT WORD PUBLIC CODE . WORD PUBLIC DATA DGROUP . . _DATA STACK PARA STACK STACK DGROUP Compact _TEXT WORD PUBLIC CODE data PARA PRIVATE FAR_DATA . WORD PUBLIC DATA DGROUP . . _DATA STACK PARA STACK STACK DGROUP Memory Segment Align Combine Class Group model name type type type ────────────────────────────────────────────────────────────────────────── STACK PARA STACK STACK DGROUP Large module_TEXT WORD PUBLIC CODE . . . data PARA PRIVATE FAR_DATA . . . _DATA WORD PUBLIC DATA DGROUP STACK PARA STACK STACK DGROUP ────────────────────────────────────────────────────────────────────────── Figure 3-10. Segments, groups, and classes for the standard memory models as used with assembly-language programs. The Microsoft C Optimizing Compiler and other high-level-language compilers use a superset of these segments and classes. For pure assembly-language programs that will run under MS-DOS, you can ignore DGROUP. However, if you plan to integrate assembly-language routines and programs written in high-level languages, you'll want to follow the Microsoft DGROUP convention. For example, if you are planning to link routines from a C library into an assembly-language program, you should include the line DGROUP group _DATA,STACK near the beginning of the program. The final Microsoft convention of interest in creating .EXE programs is segment order. The high-level compilers assume that code segments always come first, followed by far data segments, followed by the near data segment, with the stack and heap last. This order won't concern you much until you begin integrating assembly-language code with routines from high-level-language libraries, but it is easiest to learn to use the convention right from the start. Program Procedures The procedure level of program structure is partly real and partly conceptual. Procedures are basically just a fancy guise for subroutines. Procedures within a program are declared with the PROC and ENDP directives in the following form: name PROC attribute . . . RET name ENDP The attribute carried by a PROC declaration, which is either NEAR or FAR, tells the assembler what type of call you expect to use to enter the procedure──that is, whether the procedure will be called from other routines in the same segment or from routines in other segments. When the assembler encounters a RET instruction within the procedure, it uses the attribute information to generate the correct opcode for either a near (intra-segment) or far (inter-segment) return. Each program should have a main procedure that receives control from MS-DOS. You specify the entry point for the program by including the name of the main procedure in the END statement in one of the program's source files. The main procedure's attribute (NEAR or FAR) is really not too important, because the program returns control to MS-DOS with a function call rather than a RET instruction. However, by convention, most programmers assign the main procedure the FAR attribute anyway. You should break the remainder of the program into procedures in an orderly way, with each procedure performing a well-defined single function, returning its results to its caller, and avoiding actions that have global effects within the program. Ideally procedures invoke each other only by CALL instructions, have only one entry point and one exit point, and always exit by means of a RET instruction, never by jumping to some other location within the program. For ease of understanding and maintenance, a procedure should not exceed one page (about 60 lines); if it is longer than a page, it is probably too complex and you should delegate some of its function to one or more subsidiary procedures. You should preface the source code for each procedure with a detailed comment that states the procedure's calling sequence, results returned, registers affected, and any data items accessed or modified. The effort invested in making your procedures compact, clean, flexible, and well-documented will be repaid many times over when you reuse the procedures in other programs. ──────────────────────────────────────────────────────────────────────────── Chapter 4 MS-DOS Programming Tools Preparing a new program to run under MS-DOS is an iterative process with four basic steps: ■ Use of a text editor to create or modify an ASCII source-code file ■ Use of an assembler or high-level-language compiler (such as the Microsoft Macro Assembler or the Microsoft C Optimizing Compiler) to translate the source file into relocatable object code ■ Use of a linker to transform the relocatable object code into an executable MS-DOS load module ■ Use of a debugger to methodically test and debug the program Additional utilities the MS-DOS software developer may find necessary or helpful include the following: ■ LIB, which creates and maintains object-module libraries ■ CREF, which generates a cross-reference listing ■ EXE2BIN, which converts .EXE files to .COM files ■ MAKE, which compares dates of files and carries out operations based on the result of the comparison This chapter gives an operational overview of the Microsoft programming tools for MS-DOS, including the assembler, the C compiler, the linker, and the librarian. In general, the information provided here also applies to the IBM programming tools for MS-DOS, which are really the Microsoft products with minor variations and different version numbers. Even if your preferred programming language is not C or assembly language, you will need at least a passing familiarity with these tools because all of the examples in the IBM and Microsoft DOS reference manuals are written in one of these languages. The survey in this chapter, together with the example programs and reference section elsewhere in the book, should provide the experienced programmer with sufficient information to immediately begin writing useful programs. Readers who do not have a background in C, assembly language, or the Intel 80x86 microprocessor architecture should refer to the tutorial and reference works listed at the end of this chapter. File Types The MS-DOS programming tools can create and process many different file types. The following extensions are used by convention for these files: ╓┌─┌──────────┌──────────────────────────────────────────────────────────────╖ Extension File type Extension File type ────────────────────────────────────────────────────────────────────────── .ASM Assembly-language source file .C C source file .COM MS-DOS executable load module that does not require relocation at runtime .CRF Cross-reference information file produced by the assembler for processing by CREF.EXE .DEF Module-definition file describing a program's segment behavior (MS OS/2 and Microsoft Windows programs only; not relevant to normal MS-DOS applications) .EXE MS-DOS executable load module that requires relocation at runtime .H C header file containing C source code for constants, macros, and functions; merged into another C program with the #include Extension File type ────────────────────────────────────────────────────────────────────────── and functions; merged into another C program with the #include directive .INC Include file for assembly-language programs, typically containing macros and/or equates for systemwide values such as error codes .LIB Object-module library file made up of one or more .OBJ files; indexed and manipulated by LIB.EXE .LST Program listing, produced by the assembler, that includes memory locations, machine code, the original program text, and error messages .MAP Listing of symbols and their locations within a load module; produced by the linker .OBJ Relocatable-object-code file produced by an assembler or compiler Extension File type ────────────────────────────────────────────────────────────────────────── compiler .REF Cross-reference listing produced by CREF.EXE from the information in a .CRF file ────────────────────────────────────────────────────────────────────────── The Microsoft Macro Assembler The Microsoft Macro Assembler (MASM) is distributed as the file MASM.EXE. When beginning a program translation, MASM needs the following information: ■ The name of the file containing the source program ■ The filename for the object program to be created ■ The destination of the program listing ■ The filename for the information that is later processed by the cross-reference utility (CREF.EXE) You can invoke MASM in two ways. If you enter the name of the assembler alone, it prompts you for the names of each of the various input and output files. The assembler supplies reasonable defaults for all the responses except the source filename, as shown in the following example: C>MASM <Enter> Microsoft (R) Macro Assembler Version 5.10 Copyright (C) Microsoft Corp 1981, 1988. All rights reserved. Source filename [.ASM]: HELLO <Enter> Object filename [HELLO.OBJ]: <Enter> Source listing [NUL.LST]: <Enter> Cross-reference [NUL.CRF]: <Enter> 49006 Bytes symbol space free 0 Warning Errors 0 Severe Errors C> You can use a logical device name (such as PRN or COM1) at any of the MASM prompts to send that output of the assembler to a character device rather than a file. Note that the default for the listing and cross-reference files is the NUL device──that is, no file is created. If you end any response with a semicolon, MASM assumes that the remaining responses are all to be the default. A more efficient way to use MASM is to supply all parameters in the command line, as follows: MASM [options] source,[object],[listing],[crossref] For example, the following command lines are equivalent to the preceding interactive session: C>MASM HELLO,,NUL,NUL <Enter> or C>MASM HELLO; <Enter> These commands use the file HELLO.ASM as the source, generate the object-code file HELLO.OBJ, and send the listing and cross-reference files to the bit bucket. MASM accepts several optional switches in the command line, to control code generation and output files. Figure 4-1 lists the switches accepted by MASM version 5.1. As shown in the following example, you can put frequently used options in a MASM environment variable, where they will be found automatically by the assembler: C>SET MASM=/T /Zi <Enter> The switches in the environment variable will be overridden by any that you enter in the command line. In other versions of the Microsoft Macro Assembler, additional or fewer switches may be available. For exact instructions, see the manual for the version of MASM that you are using. ╓┌─┌──────────┌──────────────────────────────────────────────────────────────╖ Switch Meaning ────────────────────────────────────────────────────────────────────────── /A Arrange segments in alphabetic order. /Bn Set size of source-file buffer (in KB). /C Force creation of a cross-reference (.CRF) file. /D Produce listing on both passes (to find phase errors). /Dsymbol Define symbol as a null text string (symbol can be referenced by conditional assembly directives in file). /E Assemble for 80x87 numeric coprocessor emulator using IEEE real-number format. /Ipath Set search path for include files. /L Force creation of a program-listing file. /LA Force listing of all generated code. /ML Preserve case sensitivity in all names (uppercase names distinct from their lowercase equivalents). /MX Preserve lowercase in external names only (names defined with PUBLIC or EXTRN directives). Switch Meaning ────────────────────────────────────────────────────────────────────────── PUBLIC or EXTRN directives). /MU Convert all lowercase names to uppercase. /N Suppress generation of tables of macros, structures, records, segments, groups, and symbols at the end of the listing. /P Check for impure code in 80286/80386 protected mode. /S Arrange segments in order of occurrence (default). /T "Terse" mode; suppress all messages unless errors are encountered during the assembly. /V "Verbose" mode; report number of lines and symbols at end of assembly. /Wn Set error display (warning) level; n=0─2. /X Force listing of false conditionals. /Z Display source lines containing errors on the screen. /Zd Include line-number information in .OBJ file. /Zi Include line-number and symbol information in .OBJ file. ────────────────────────────────────────────────────────────────────────── Figure 4-1. Microsoft Macro Assembler version 5.1 switches. MASM allows you to override the default extensions on any file──a feature that can be rather dangerous. For example, if in the preceding example you had responded to the Object filename prompt with HELLO.ASM, the assembler would have accepted the entry without comment and destroyed your source file. This is not too likely to happen in the interactive command mode, but you must be very careful with file extensions when MASM is used in a batch file. The Microsoft C Optimizing Compiler The Microsoft C Optimizing Compiler consists of three executable files── C1.EXE, C2.EXE, and C3.EXE──that implement the C preprocessor, language translator, code generator, and code optimizer. An additional control program, CL.EXE, executes the three compiler files in order, passing each the necessary information about filenames and compilation options. Before using the C compiler and the linker, you need to set up four environment variables: Variable Action ────────────────────────────────────────────────────────────────────────── PATH=path Specifies the location of the three executable C compiler files (C1, C2, and C3) if they are not in the current directory; used by CL.EXE. INCLUDE=path Specifies the location of #include files (default extension .H) that are not found in the current directory. LIB=path Specifies the location(s) for object-code libraries that are not found in the current directory. TMP=path Specifies the location for temporary working files created by the C compiler and linker. ────────────────────────────────────────────────────────────────────────── CL.EXE does not support an interactive mode or response files. You always invoke it with a command line of the following form: CL [options] file [file ...] You may list any number of files──if a file has a .C extension, it will be compiled into a relocatable-object-module (.OBJ) file. Ordinarily, if the compiler encounters no errors, it automatically passes all resulting .OBJ files and any additional .OBJ files specified in the command line to the linker, along with the names of the appropriate runtime libraries. The C compiler has many optional switches controlling its memory models, output files, code generation, and code optimization. These are summarized in Figure 4-2. The C compiler's arcane switch syntax is derived largely from UNIX/XENIX, so don't expect it to make any sense. ╓┌─┌────────────────────────┌────────────────────────────────────────────────╖ Switch Meaning ────────────────────────────────────────────────────────────────────────── /Ax Select memory model: C = compact model H = huge model L = large model M = medium model Switch Meaning ────────────────────────────────────────────────────────────────────────── M = medium model S = small model (default) /c Compile only; do not invoke linker. /C Do not strip comments. /D<name>[=text] Define macro. /E Send preprocessor output to standard output. /EP Send preprocessor output to standard output without line numbers. /F<n> Set stack size (in hexadecimal bytes). /Fa [filename] Generate assembly listing. /Fc [filename] Generate mixed source/object listing. /Fe [filename] Force executable filename. /Fl [filename] Generate object listing. /Fm [filename] Generate map file. /Fo [filename] Force object-module filename. /FPx Select floating-point control: a = calls with alternate math library c = calls with emulator library c87 = calls with 8087 library Switch Meaning ────────────────────────────────────────────────────────────────────────── c87 = calls with 8087 library i = in-line with emulator (default) i87 = in-line with 8087 /Fs [filename] Generate source listing. /Gx Select code generation: 0 = 8086 instructions (default) 1 = 186 instructions 2 = 286 instructions c = Pascal style function calls s = no stack checking t[n] = data size threshold /H<n> Specify external name length. /I<path> Specify additional #include path. /J Specify default char type as unsigned. /link [options] Pass switches and library names to linker. /Ox Select optimization: a = ignore aliasing d = disable optimizations i = enable intrinsic functions Switch Meaning ────────────────────────────────────────────────────────────────────────── i = enable intrinsic functions l = enable loop optimizations n = disable "unsafe" optimizations p = enable precision optimizations r = disable in-line return s = optimize for space /Ox t = optimize for speed (default) w = ignore aliasing except across function calls x = enable maximum optimization (equivalent to /Oailt /Gs) /P Send preprocessor output to file. /Sx Select source-listing control: l<columns> = set line width p<lines> = set page length s<string> = set subtitle string t<string> = set title string /Tc<file> Compile file without .C extension. /u Remove all predefined macros. Switch Meaning ────────────────────────────────────────────────────────────────────────── /u Remove all predefined macros. /U<name> Remove specified predefined macro. /V<string> Set version string. /W<n> Set warning level (0─3). /X Ignore "standard places" for include files. /Zx Select miscellaneous compilation control: a = disable extensions c = make Pascal functions case-insensitive d = include line-number information e = enable extensions (default) g = generate declarations i = include symbolic debugging information l = remove default library info p<n> = pack structures on n-byte boundary s = check syntax only ────────────────────────────────────────────────────────────────────────── Figure 4-2. Microsoft C Optimizing Compiler version 5.1 switches. The Microsoft Object Linker The object module produced by MASM from a source file is in a form that contains relocation information and may also contain unresolved references to external locations or subroutines. It is written in a common format that is also produced by the various high-level compilers (such as FORTRAN and C) that run under MS-DOS. The computer cannot execute object modules without further processing. The Microsoft Object Linker (LINK), distributed as the file LINK.EXE, accepts one or more of these object modules, resolves external references, includes any necessary routines from designated libraries, performs any necessary offset relocations, and writes a file that can be loaded and executed by MS-DOS. The output of LINK is always in .EXE load-module format. (See Chapter 3.) As with MASM, you can give LINK its parameters interactively or by entering all the required information in a single command line. If you enter the name of the linker alone, the following type of dialog ensues: C>LINK <Enter> Microsoft (R) Overlay Linker Version 3.61 Copyright (C) Microsoft Corp 1983-1987. All rights reserved. Object Modules [.OBJ]: HELLO <Enter> Run File [HELLO.EXE]: <Enter> List File [NUL.MAP]: HELLO <Enter> Libraries [.LIB]: <Enter> C> If you are using LINK version 4.0 or later, the linker also asks for the name of a module-definition (.DEF) file. Simply press the Enter key in response to such a prompt. Module-definition files are used when building Microsoft Windows or MS OS/2 "new .EXE" executable files but are not relevant in normal MS-DOS applications. The input file for this example was HELLO.OBJ; the output files were HELLO.EXE (the executable program) and HELLO.MAP (the load map produced by the linker after all references and addresses were resolved). Figure 4-3 shows the load map. ────────────────────────────────────────────────────────────────────────── Start Stop Length Name Class 00000H 00017H 00018H _TEXT CODE 00018H 00027H 00010H _DATA DATA 00030H 000AFH 00080H STACK STACK 000B0H 000BBH 0000CH $$TYPES DEBTYP 000C0H 000D6H 00017H $$SYMBOLS DEBSYM Address Publics by Name Address Publics by Value Program entry point at 0000:0000 ────────────────────────────────────────────────────────────────────────── Figure 4-3. Map produced by the Microsoft Object Linker (LINK) during the generation of the HELLO.EXE program from Chapter 3. The program contains one CODE, one DATA, and one STACK segment. The first instruction to be executed lies in the first byte of the CODE segment. The $$TYPES and $$SYMBOLS segments contain information for the CodeView debugger and are not part of the program; these segments are ignored by the normal MS-DOS loader. You can obtain the same result more quickly by entering all parameters in the command line, in the following form: LINK options objectfile, [exefile], [mapfile], [libraries] Thus, the command-line equivalent to the preceding interactive session is C>LINK HELLO,HELLO,HELLO,, <Enter> or C>LINK HELLO,,HELLO; <Enter> If you enter a semicolon as the last character in the command line, LINK assumes the default values for all further parameters. A third method of commanding LINK is with a response file. A response file contains lines of text that correspond to the responses you would give the linker interactively. You specify the name of the response file in the command line with a leading @ character, as follows: LINK @filename You can also enter the name of a response file at any prompt. If the response file is not complete, LINK will prompt you for the missing information. When entering linker commands, you can specify multiple object files with the + operator or with spaces, as in the following example: C>LINK HELLO+VMODE+DOSINT,MYPROG,,GRAPHICS; <Enter> This command would link the files HELLO.OBJ, VMODE.OBJ, and DOSINT.OBJ, searching the library file GRAPHICS.LIB to resolve any references to symbols not defined in the specified object files, and would produce a file named MYPROG.EXE. LINK uses the current drive and directory when they are not explicitly included in a filename; it will not automatically use the same drive and directory you specified for a previous file in the same command line. By using the + operator or space characters in the libraries field, you can specify up to 32 library files to be searched. Each high-level- language compiler provides default libraries that are searched automatically during the linkage process if the linker can find them (unless they are explicitly excluded with the /NOD switch). LINK looks for libraries first in the current directory of the default disk drive, then along any paths that were provided in the command line, and finally along the path(s) specified by the LIB variable if it is present in the environment. LINK accepts several optional switches as part of the command line or at the end of any interactive prompt. Figure 4-4 lists these switches. The number of switches available and their actions vary among different versions of LINK. See your Microsoft Object Linker instruction manual for detailed information about your particular version. ╓┌─┌────────┌───────────────────────────┌────────────────────────────────────╖ Switch Full form Meaning Switch Full form Meaning ────────────────────────────────────────────────────────────────────────── /A:n /ALIGNMENT:n Set segment sector alignment factor. N must be a power of 2 (default = 512). Not related to logical-segment alignment (BYTE, WORD, PARA, PAGE, and so forth). Relevant to segmented executable files (Microsoft Windows and MS OS/2) only. /B /BATCH Suppress linker prompt if a library cannot be found in the current directory or in the locations specified by the LIB environment variable. /CO /CODEVIEW Include symbolic debugging information in the .EXE file for use by CodeView. /CP /CPARMAXALLOC Set the field in the .EXE file header Switch Full form Meaning ────────────────────────────────────────────────────────────────────────── /CP /CPARMAXALLOC Set the field in the .EXE file header controlling the amount of memory allocated to the program in addition to the memory required for the program's code, stack, and initialized data. /DO /DOSSEG Use standard Microsoft segment naming and ordering conventions. /DS /DSALLOCATE Load data at high end of the data segment. Relevant to real-mode programs only. /E /EXEPACK Pack executable file by removing sequences of repeated bytes and optimizing relocation table. /F /FARCALLTRANSLATION Optimize far calls to labels within Switch Full form Meaning ────────────────────────────────────────────────────────────────────────── /F /FARCALLTRANSLATION Optimize far calls to labels within the same physical segment for speed by replacing them with near calls and NOPs. /HE /HELP Display information about available options. /HI /HIGH Load program as high in memory as possible. /I /INFORMATION Display information about progress of linking, including pass numbers and the names of object files being linked. /INC /INCREMENTAL Force production of .SYM and .ILK files for subsequent use by ILINK (incremental linker). May not be used Switch Full form Meaning ────────────────────────────────────────────────────────────────────────── (incremental linker). May not be used with /EXEPACK. Relevant to segmented executable files (Microsoft Windows and MS OS/2) only. /LI /LINENUMBERS Write address of the first instruction that corresponds to each source-code line to the map file. Has no effect if the compiler does not include line-number information in the object module. Force creation of a map file. /M[:n] /MAP[:n] Force creation of a .MAP file listing all public symbols, sorted by name and by location. The optional value n is the maximum number of symbols that can be sorted (default = 2048); when n is supplied, the alphabetically Switch Full form Meaning ────────────────────────────────────────────────────────────────────────── n is supplied, the alphabetically sorted list is omitted. /NOD /NODEFAULTLIBRARYSEARCH Skip search of any default compiler libraries specified in the .OBJ file. /NOE /NOEXTENDEDDICTSEARCH Ignore extended library dictionary (if it is present). The extended dictionary ordinarily provides the linker with information about inter-module dependencies, to speed up linking. /NOF /NOFARCALLTRANSLATION Disable optimization of far calls to labels within the same segment. /NOG /NOGROUPASSOCIATION Ignore group associations when assigning addresses to data and code items. Switch Full form Meaning ────────────────────────────────────────────────────────────────────────── items. /NOI /NOIGNORECASE Do not ignore case in names during linking. /NON /NONULLSDOSSEG Arrange segments as for /DOSSEG but do not insert 16 null bytes at start of _TEXT segment. /NOP /NOPACKCODE Do not pack contiguous logical code segments into a single physical segment. /O:n /OVERLAYINTERRUPT:n Use interrupt number n with the overlay manager supplied with some Microsoft high-level languages. /PAC[:n] /PACKCODE[:n] Pack contiguous logical code segments into a single physical code segment. Switch Full form Meaning ────────────────────────────────────────────────────────────────────────── into a single physical code segment. The optional value n is the maximum size for each packed physical code segment (default = 65,536 bytes). Segments in different groups are not packed. /PADC:n /PADCODE:n Add n filler bytes to end of each code module so that a larger module can be inserted later with ILINK. Relevant to segmented executable files (Windows and MS OS/2) only. /PADD:n /PADDATA:n Add n filler bytes to end of each data module so that a larger module can be inserted later with ILINK. Relevant to segmented executable files (Microsoft Windows and MS OS/2) only. Switch Full form Meaning ────────────────────────────────────────────────────────────────────────── only. /PAU /PAUSE Pause during linking, allowing a change of disks before .EXE file is written. /SE:n /SEGMENTS:n Set maximum number of segments in linked program (default = 128). /ST:n /STACK:n Set stack size of program in bytes; ignore stack segment size declarations within object modules and definition file. /W /WARNFIXUP Display warning messages for offsets relative to a segment base that is not the same as the group base. Relevant to segmented executable files (Microsoft Windows and MS OS/2) Switch Full form Meaning ────────────────────────────────────────────────────────────────────────── files (Microsoft Windows and MS OS/2) only. ────────────────────────────────────────────────────────────────────────── Figure 4-4. Switches accepted by the Microsoft Object Linker (LINK) version 5.0. Earlier versions use a subset of these switches. Note that any abbreviation for a switch is acceptable as long as it is sufficient to specify the switch uniquely. The EXE2BIN Utility The EXE2BIN utility (EXE2BIN.EXE) transforms a .EXE file created by LINK into an executable .COM file, if the program meets the following prerequisites: ■ It cannot contain more than one declared segment and cannot define a stack. ■ It must be less than 64 KB in length. ■ It must have an origin at 0100H. ■ The first location in the file must be specified as the entry point in the source code's END directive. Although .COM files are somewhat more compact than .EXE files, you should avoid using them. Programs that use separate segments for code, data, and stack are much easier to port to protected-mode environments such as MS OS/2; in addition, .COM files do not support the symbolic debugging information used by CodeView. Another use for the EXE2BIN utility is to convert an installable device driver──after it is assembled and linked into a .EXE file──into a memory-image .BIN or .SYS file with an origin of zero. This conversion is required in MS-DOS version 2, which cannot load device drivers as .EXE files. The process of writing an installable device driver is discussed in more detail in Chapter 14. Unlike most of the other programming utilities, EXE2BIN does not have an interactive mode. It always takes its source and destination filenames, separated by spaces, from the MS-DOS command line, as follows: EXE2BIN sourcefile [destinationfile] If you do not supply the source-file extension, it defaults to .EXE; the destination-file extension defaults to .BIN. If you do not specify a name for the destination file, EXE2BIN gives it the same name as the source file, with a .BIN extension. For example, to convert the file HELLO.EXE into HELLO.COM, you would use the following command line: C>EXE2BIN HELLO.EXE HELLO.COM <Enter> The EXE2BIN program also has other capabilities, such as pure binary conversion with segment fixup for creating program images to be placed in ROM; but because these features are rarely used during MS-DOS application development, they will not be discussed here. The CREF Utility The CREF cross-reference utility CREF.EXE processes a .CRF file produced by MASM, creating an ASCII text file with the default extension .REF. The file contains a cross-reference listing of all symbols declared in the program and the line numbers in which they are referenced. (See Figure 4-5.) Such a listing is very useful when debugging large assembly-language programs with many interdependent procedures and variables. CREF may be supplied with its parameters interactively or in a single command line. If you enter the utility name alone, CREF prompts you for the input and output filenames, as shown in the following example: C>CREF <Enter> Microsoft (R) Cross-Reference Utility Version 5.10 Copyright (C) Microsoft Corp 1981-1985, 1987. All rights reserved. Cross-reference [.CRF]: HELLO <Enter> Listing [HELLO.REF]: 15 Symbols C> ────────────────────────────────────────────────────────────────────────── Microsoft Cross-Reference Version 5.10 Thu May 26 11:09:34 1988 HELLO.EXE --- print Hello on terminal Symbol Cross-Reference (# definition, + modification)Cref-1 @CPU . . . . . . . . . . . . . . 1# @VERSION . . . . . . . . . . . . 1# CODE . . . . . . . . . . . . . . 21 CR . . . . . . . . . . . . . . . 17# 46 47 DATA . . . . . . . . . . . . . . 44 LF . . . . . . . . . . . . . . . 18# 46 47 MSG. . . . . . . . . . . . . . . 33 46# MSG_LEN. . . . . . . . . . . . . 32 49# PRINT. . . . . . . . . . . . . . 25# 39 60 STACK. . . . . . . . . . . . . . 23 54# 54 58 STDERR . . . . . . . . . . . . . 15# STDIN. . . . . . . . . . . . . . 13# STDOUT . . . . . . . . . . . . . 14# 31 _DATA. . . . . . . . . . . . . . 23 27 44# 51 _TEXT. . . . . . . . . . . . . . 21# 23 41 15 Symbols ────────────────────────────────────────────────────────────────────────── Figure 4-5. Cross-reference listing HELLO.REF produced by the CREF utility from the file HELLO.CRF, for the HELLO.EXE program example from Chapter 3. The symbols declared in the program are listed on the left in alphabetic order. To the right of each symbol is a list of all the lines where that symbol is referenced. The number with a # sign after it denotes the line where the symbol is declared. Numbers followed by a + sign indicate that the symbol is modified at the specified line. The line numbers given in the cross-reference listing correspond to the line numbers generated by the assembler in the program-listing (.LST) file, not to any physical line count in the original source file. The parameters may also be entered in the command line in the following form: CREF CRF_file, listing_file For example, the command-line equivalent to the preceding interactive session is: C>CREF HELLO,HELLO <Enter> If CREF cannot find the specified .CRF file, it displays an error message. Otherwise, it leaves the cross-reference listing in the specified file on the disk. You can send the file to the printer with the COPY command, in the following form: COPY listing_file PRN: You can also send the cross-reference listing directly to a character device as it is generated by responding to the Listing prompt with the name of the device. The Microsoft Library Manager Although the object modules that are produced by MASM or by high-level- language compilers can be linked directly into executable load modules, they can also be collected into special files called object-module libraries. The modules in a library are indexed by name and by the public symbols they contain, so that they can be extracted by the linker to satisfy external references in a program. The Microsoft Library Manager (LIB) is distributed as the file LIB.EXE. LIB creates and maintains program libraries, adding, updating, and deleting object files as necessary. LIB can also check a library file for internal consistency or print a table of its contents (Figure 4-6). LIB follows the command conventions of most other Microsoft programming tools. You must supply it with the name of a library file to work on, one or more operations to perform, the name of a listing file or device, and (optionally) the name of the output library. If you do not specify a name for the output library, LIB gives it the same name as the input library and changes the extension of the input library to .BAK. The LIB operations are simply the names of object files, with a prefix character that specifies the action to be taken: Prefix Meaning ────────────────────────────────────────────────────────────────────────── - Delete an object module from the library. * Extract a module and place it in a separate .OBJ file. + Add an object module or the entire contents of another library to the library. ────────────────────────────────────────────────────────────────────────── You can combine command prefixes. For example, -+ replaces a module, and *- extracts a module into a new file and then deletes it from the library. ────────────────────────────────────────────────────────────────────────── _abort............abort _abs..............abs _access...........access _asctime..........asctime _atof.............atof _atoi.............atoi _atol.............atol _bdos.............bdos _brk..............brk _brkctl...........brkctl _bsearch..........bsearch _calloc...........calloc _cgets............cgets _chdir............dir _chmod............chmod _chsize...........chsize . . . _exit Offset: 00000010H Code and data size: 44H __exit _filbuf Offset: 00000160H Code and data size: BBH __filbuf _file Offset: 00000300H Code and data size: CAH __iob __iob2 __lastiob . . . ────────────────────────────────────────────────────────────────────────── Figure 4-6. Extract from the table-of-contents listing produced by the Microsoft Library Manager (LIB) for the Microsoft C library SLIBC.LIB. The first part of the listing is an alphabetic list of all public names declared in all of the modules in the library. Each name is associated with the object module to which it belongs. The second part of the listing is an alphabetic list of the object-module names in the library, each followed by its offset within the library file and the actual size of the module in bytes. The entry for each module is followed by a summary of the public names that are declared within it. When you invoke LIB with its name alone, it requests the other information it needs interactively, as shown in the following example: C>LIB <Enter> Microsoft (R) Library Manager Version 3.08 Copyright (C) Microsoft Corp 1983-1987. All rights reserved. Library name: SLIBC <Enter> Operations: +VIDEO <Enter> List file: SLIBC.LST <Enter> Output library: SLIBC2 <Enter> C> In this example, LIB added the object module VIDEO.OBJ to the library SLIBC.LIB, wrote a library table of contents into the file SLIBC.LST, and named the resulting new library SLIBC2.LIB. The Library Manager can also be run with a command line of the following form: LIB library [commands],[list],[newlibrary] For example, the following command line is equivalent to the preceding interactive session: C>LIB SLIBC +VIDEO,SLIBC.LST,SLIBC2; <Enter> As with the other Microsoft utilities, a semicolon at the end of the command line causes LIB to use the default responses for any parameters that are omitted. Like LINK, LIB can also accept its commands from a response file. The contents of the file are lines of text that correspond exactly to the responses you would give LIB interactively. You specify the name of the response file in the command line with a leading @ character, as follows: LIB @filename LIB has only three switches: /I (/IGNORECASE), /N (/NOIGNORECASE), and /PAGESIZE:number. The /IGNORECASE switch is the default. The /NOIGNORECASE switch causes LIB to regard as distinct any symbols that differ only in the case of their component letters. You should place the /PAGESIZE switch, which defines the size of a unit of allocation space for a given library, immediately after the library filename. The library page size is in bytes and must be a power of 2 between 16 and 32,768 (16, 32, 64, and so forth); the default is 16 bytes. Because the index to a library is always a fixed number of pages, setting a larger page size allows you to store more object modules in that library; on the other hand, it will result in more wasted space within the file. The MAKE Utility The MAKE utility (MAKE.EXE) compares dates of files and carries out commands based on the result of that comparison. Because of this single, rather basic capability, MAKE can be used to maintain complex programs built from many modules. The dates of source, object, and executable files are simply compared in a logical sequence; the assembler, compiler, linker, and other programming tools are invoked as appropriate. The MAKE utility processes a plain ASCII text file called, as you might expect, a make file. You start the utility with a command-line entry in the following form: MAKE makefile [options] By convention, a make file has the same name as the executable file that is being maintained, but without an extension. The available MAKE switches are listed in Figure 4-7. A simple make file contains one or more dependency statements separated by blank lines. Each dependency statement can be followed by a list of MS-DOS commands, in the following form: targetfile : sourcefile ... command command . . . If the date and time of any source file are later than those of the target file, the accompanying list of commands is carried out. You may use comment lines, which begin with a # character, freely in a make file. MAKE can also process inference rules and macro definitions. For further details on these advanced capabilities, see the Microsoft or IBM documentation. Switch Meaning ────────────────────────────────────────────────────────────────────────── /D Display last modification date of each file as it is processed. /I Ignore exit (return) codes returned by commands and programs executed as a result of dependency statements. /N Display commands that would be executed as a result of dependency statements but do not execute those commands. /S Do not display commands as they are executed. /X Direct error messages from MAKE, or any program that MAKE runs, <filename> to the specified file. If filename is a hyphen (-), direct error messages to the standard output. ────────────────────────────────────────────────────────────────────────── Figure 4-7. Switches for the MAKE utility. A Complete Example Let's put together everything we've learned about using the MS-DOS programming tools so far. Figure 4-8 shows a sketch of the overall process of building an executable program. Assume that we have the source code for the HELLO.EXE program from Chapter 3 in the file HELLO.ASM. To assemble the source program into the relocatable object module HELLO.OBJ with symbolic debugging information included, also producing a program listing in the file HELLO.LST and a cross-reference data file HELLO.CRF, we would enter C>MASM /C /L /Zi /T HELLO; <Enter> To convert the cross-reference raw-data file HELLO.CRF into a cross-reference listing in the file HELLO.REF, we would enter C>CREF HELLO,HELLO <Enter> ┌───────────────┐ ┌───────────────┐ │ MASM │ │ C or other │ │ source-code │ │ HLL source- │ │ file │ │ code file │ └───┬───────────┘ └───┬───────────┘ │ ┌─────────────────────┘ Compiler ┌─────────────┐ │ Relocatable │ │ object-module ├────┐ │ file (.OBJ) │ │ └───┬───────────┘ │ │ LIB │ ┌──────────────┐ │ ┌───────────────┐ │ Object-module │ LINK │ Executable │ │ libraries ├───────────── program │ │ (.LIB) │ │ (.EXE) │ └───────────────┘ │ └───┬───────────┘ │ │ EXE2BIN ┌───────────────┐ │ ┌──────────────┐ │ HLL │ │ │ Executable │ │ runtime ├──────┘ │ program │ │ libraries │ │ (.COM) │ └───────────────┘ └───────────────┘ Figure 4-8. Creation of an MS-DOS application program, from source code to executable file. To convert the relocatable object file HELLO.OBJ into the executable file HELLO.EXE, creating a load map in the file HELLO.MAP and appending symbolic debugging information to the executable file, we would enter C>LINK /MAP /CODEVIEW HELLO; <Enter> We could also automate the entire process just described by creating a make file named HELLO (with no extension) and including the following instructions: hello.obj : hello.asm masm /C /L /Zi /T hello; cref hello,hello hello.exe : hello.obj link /MAP /CODEVIEW hello; Then, when we have made some change to HELLO.ASM and want to rebuild the executable HELLO.EXE file, we need only enter C>MAKE HELLO <Enter> Programming Resources and References The literature on IBM PC─compatible personal computers, the Intel 80x86 microprocessor family, and assembly-language and C programming is vast. The list below contains a selection of those books that I have found to be useful and reliable. The list should not be construed as an endorsement by Microsoft Corporation. MASM Tutorials Assembly Language Primer for the IBM PC and XT, by Robert Lafore. New American Library, New York, NY, 1984. ISBN 0-452-25711-5. 8086/8088/80286 Assembly Language, by Leo Scanlon. Brady Books, Simon and Schuster, New York, NY, 1988. ISBN 0-13-246919-7. C Tutorials Microsoft C Programming for the IBM, by Robert Lafore. Howard K. Sams & Co., Indianapolis, IN, 1987. ISBN 0-672-22515-8. Proficient C, by Augie Hansen. Microsoft Press, Redmond, WA, 1987. ISBN 1-55615-007-5. Intel 80x86 Microprocessor References iAPX 88 Book. Intel Corporation, Literature Department SV3-3, 3065 Bowers Ave., Santa Clara, CA 95051. Order no. 210200. iAPX 286 Programmer's Reference Manual. Intel Corporation, Literature Department SV3-3, 3065 Bowers Ave., Santa Clara, CA 95051. Order no. 210498. iAPX 386 Programmer's Reference Manual. Intel Corporation, Literature Department SV3-3, 3065 Bowers Ave., Santa Clara, CA 95051. Order no. 230985. PC, PC/AT, and PS/2 Architecture The IBM Personal Computer from the Inside Out (Revised Edition), by Murray Sargent and Richard L. Shoemaker. Addison-Wesley Publishing Company, Reading, MA, 1986. ISBN 0-201-06918-0. Programmer's Guide to PC & PS/2 Video Systems, by Richard Wilton. Microsoft Press, Redmond, WA, 1987. ISBN 1-55615-103-9. Personal Computer Technical Reference. IBM Corporation, IBM Technical Directory, P. O. Box 2009, Racine, WI 53404. Part no. 6322507. Personal Computer AT Technical Reference. IBM Corporation, IBM Technical Directory, P. O. Box 2009, Racine, WI 53404. Part no. 6280070. Options and Adapters Technical Reference. IBM Corporation, IBM Technical Directory, P. O. Box 2009, Racine, WI 53404. Part no. 6322509. Personal System/2 Model 30 Technical Reference. IBM Corporation, IBM Technical Directory, P. O. Box 2009, Racine, WI 53404. Part no. 68X2201. Personal System/2 Model 50/60 Technical Reference. IBM Corporation, IBM Technical Directory, P. O. Box 2009, Racine, WI 53404. Part no. 68X2224. Personal System/2 Model 80 Technical Reference. IBM Corporation, IBM Technical Directory, P. O. Box 2009, Racine, WI 53404. Part no. 68X2256. ──────────────────────────────────────────────────────────────────────────── Chapter 5 Keyboard and Mouse Input The fundamental means of user input under MS-DOS is the keyboard. This follows naturally from the MS-DOS command-line interface, whose lineage can be traced directly to minicomputer operating systems with Teletype consoles. During the first few years of MS-DOS's existence, when 8088/8086-based machines were the norm, nearly every popular application program used key-driven menus and text-mode displays. However, as high-resolution graphics adapters (and 80286/80386-based machines with enough power to drive them) have become less expensive, programs that support windows and a graphical user interface have steadily grown more popular. Such programs typically rely on a pointing device such as a mouse, stylus, joystick, or light pen to let the user navigate in a "point-and-shoot" manner, reducing keyboard entry to a minimum. As a result, support for pointing devices has become an important consideration for all software developers. Keyboard Input Methods Applications running under MS-DOS on IBM PC─compatible machines can use several methods to obtain keyboard input: ■ MS-DOS handle-oriented functions ■ MS-DOS traditional character functions ■ IBM ROM BIOS keyboard-driver functions These methods offer different degrees of flexibility, portability, and hardware independence. The handle, or stream-oriented, functions are philosophically derived from UNIX/XENIX and were first introduced in MS-DOS version 2.0. A program uses these functions by supplying a handle, or token, for the desired device, plus the address and length of a buffer. When a program begins executing, MS-DOS supplies it with predefined handles for certain commonly used character devices, including the keyboard: Handle Device name Opened to ────────────────────────────────────────────────────────────────────────── 0 Standard input (stdin) CON 1 Standard output (stdout) CON 2 Standard error (stderr) CON 3 Standard auxiliary (stdaux) AUX 4 Standard printer (stdprn) PRN ────────────────────────────────────────────────────────────────────────── These handles can be used for read and write operations without further preliminaries. A program can also obtain a handle for a character device by explicitly opening the device for input or output using its logical name (as though it were a file). The handle functions support I/O redirection, allowing a program to take its input from another device or file instead of the keyboard, for example. Redirection is discussed in detail in Chapter 15. The traditional character-input functions are a superset of the character I/O functions that were present in CP/M. Originally included in MS-DOS simply to facilitate the porting of existing applications from CP/M, they are still widely used. In MS-DOS versions 2.0 and later, most of the traditional functions also support I/O redirection (although not as well as the handle functions do). Use of the IBM ROM BIOS keyboard functions presupposes that the program is running on an IBM PC─compatible machine. The ROM BIOS keyboard driver operates at a much more primitive level than the MS-DOS functions and allows a program to circumvent I/O redirection or MS-DOS's special handling of certain control characters. Programs that use the ROM BIOS keyboard driver are inherently less portable than those that use the MS-DOS functions and may interfere with the proper operation of other programs; many of the popular terminate-and-stay-resident (TSR) utilities fall into this category. Keyboard Input with Handles The principal MS-DOS function for keyboard input using handles is Int 21H Function 3FH (Read File or Device). The parameters for this function are a handle, the segment and offset of a buffer, and the length of the buffer. (For a more detailed explanation of this function, see Section II of this book, "MS-DOS Functions Reference.") As an example, let's use the predefined standard input handle (0) and Int 21H Function 3FH to read a line from the keyboard: ────────────────────────────────────────────────────────────────────────── buffer db 80 dup (?) ; keyboard input buffer . . . mov ah,3fh ; function 3fh = read file or device mov bx,0 ; handle for standard input mov cx,80 ; maximum bytes to read mov dx,seg buffer ; DS:DX = buffer address mov ds,dx mov dx,offset buffer int 21h ; transfer to MS-DOS jc error ; jump if error detected . . . ────────────────────────────────────────────────────────────────────────── When control returns from Int 21H Function 3FH, the carry flag is clear if the function was successful, and AX contains the number of characters read. If there was an error, the carry flag is set and AX contains an error code; however, this should never occur when reading the keyboard. The standard input is redirectable, so the code just shown is not a foolproof way of obtaining input from the keyboard. Depending upon whether a redirection parameter was included in the command line by the user, program input might be coming from the keyboard, a file, another character device, or even the bit bucket (NUL device). To bypass redirection and be absolutely certain where your input is coming from, you can ignore the predefined standard input handle and open the console as though it were a file, using the handle obtained from that open operation to perform your keyboard input, as in the following example: ────────────────────────────────────────────────────────────────────────── buffer db 80 dup (?) ; keyboard input buffer fname db 'CON',0 ; keyboard device name handle dw 0 ; keyboard device handle . . . mov ah,3dh ; function 3dh = open mov al,0 ; mode = read mov dx,seg fname ; DS:DX = device name mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if open failed mov handle,ax ; save handle for CON . . . mov ah,3fh ; function 3fh = read file or device mov bx,handle ; BX = handle for CON mov cx,80 ; maximum bytes to read mov dx,offset buffer ; DS:DX = buffer address int 21h ; transfer to MS-DOS jc error ; jump if error detected . . . ────────────────────────────────────────────────────────────────────────── When a programmer uses Int 21H Function 3FH to read from the keyboard, the exact result depends on whether MS-DOS regards the handle to be in ASCII mode or binary mode (sometimes known as cooked mode and raw mode). ASCII mode is the default, although binary mode can be selected with Int 21H Function 44H (IOCTL) when necessary. In ASCII mode, MS-DOS initially places characters obtained from the keyboard in a 128-byte internal buffer, and the user can edit the input with the Backspace key and the special function keys. MS-DOS automatically echoes the characters to the standard output, expanding tab characters to spaces (although they are left as the ASCII code 09H in the buffer). The Ctrl-C, Ctrl-S, and Ctrl-P key combinations receive special handling, and the Enter key is translated to a carriage return─linefeed pair. When the user presses Enter or Ctrl-Z, MS-DOS copies the requested number of characters (or the actual number of characters entered, if less than the number requested) out of the internal buffer into the calling program's buffer. In binary mode, MS-DOS never echoes input characters. It passes the Ctrl-C, Ctrl-S, Ctrl-P, and Ctrl-Z key combinations and the Enter key through to the application unchanged, and Int 21H Function 3FH does not return control to the application until the exact number of characters requested has been received. Ctrl-C checking is discussed in more detail at the end of this chapter. For now, simply note that the application programmer can substitute a custom handler for the default MS-DOS Ctrl-C handler and thereby avoid having the application program lose control of the machine when the user enters a Ctrl-C or Ctrl-Break. Keyboard Input with Traditional Calls The MS-DOS traditional keyboard functions offer a variety of character and line-oriented services with or without echo and Ctrl-C detection. These functions are summarized on the following page. Int 21H Function Action Ctrl-C checking ────────────────────────────────────────────────────────────────────────── 01H Keyboard input with echo Yes 06H Direct console I/O No 07H Keyboard input without echo No 08H Keyboard input without echo Yes 0AH Buffered keyboard input Yes 0BH Input-status check Yes 0CH Input-buffer reset and input Varies ────────────────────────────────────────────────────────────────────────── In MS-DOS versions 2.0 and later, redirection of the standard input affects all these functions. In other words, they act as though they were special cases of an Int 21H Function 3FH call using the predefined standard input handle (0). The character-input functions (01H, 06H, 07H, and 08H) all return a character in the AL register. For example, the following sequence waits until a key is pressed and then returns it in AL: ────────────────────────────────────────────────────────────────────────── mov ah,1 ; function 01h = read keyboard int 21h ; transfer to MS-DOS ────────────────────────────────────────────────────────────────────────── The character-input functions differ in whether the input is echoed to the screen and whether they are sensitive to Ctrl-C interrupts. Although MS-DOS provides no pure keyboard-status function that is immune to Ctrl-C, a program can read keyboard status (somewhat circuitously) without interference by using Int 21H Function 06H. Extended keys, such as the IBM PC keyboard's special function keys, require two calls to a character-input function. As an alternative to single-character input, a program can use buffered-line input (Int 21H Function 0AH) to obtain an entire line from the keyboard in one operation. MS-DOS builds up buffered lines in an internal buffer and does not pass them to the calling program until the user presses the Enter key. While the line is being entered, all the usual editing keys are active and are handled by the MS-DOS keyboard driver. You use Int 21H Function 0AH as follows: ────────────────────────────────────────────────────────────────────────── buff db 81 ; maximum length of input db 0 ; actual length (from MS-DOS) db 81 dup (0) ; receives keyboard input . . . mov ah,0ah ; function 0ah = read buffered line mov dx,seg buff ; DS:DX = buffer address mov ds,dx mov dx,offset buff int 21h ; transfer to MS-DOS . . . ────────────────────────────────────────────────────────────────────────── Int 21H Function 0AH differs from Int 21H Function 3FH in several important ways. First, the maximum length is passed in the first byte of the buffer, rather than in the CX register. Second, the actual length is returned in the second byte of the structure, rather than in the AX register. Finally, when the user has entered one less than the specified maximum number of characters, MS-DOS ignores all subsequent characters and sounds a warning beep until the Enter key is pressed. For detailed information about each of the traditional keyboard-input functions, see Section II of this book, "MS-DOS Functions Reference." Keyboard Input with ROM BIOS Functions Programmers writing applications for IBM PC compatibles can bypass the MS-DOS keyboard functions and choose from two hardware-dependent techniques for keyboard input. The first method is to call the ROM BIOS keyboard driver using Int 16H. For example, the following sequence reads a single character from the keyboard input buffer and returns it in the AL register: ────────────────────────────────────────────────────────────────────────── mov ah,0 ; function 0=read keyboard int 16h ; transfer to ROM BIOS ────────────────────────────────────────────────────────────────────────── Int 16H Function 00H also returns the keyboard scan code in the AH register, allowing the program to detect key codes that are not ordinarily returned by MS-DOS. Other Int 16H services return the keyboard status (that is, whether a character is waiting) or the keyboard shift state (from the ROM BIOS data area 0000:0417H). For a more detailed explanation of ROM BIOS keyboard functions, see Section III of this book, "IBM ROM BIOS and Mouse Functions Reference." You should consider carefully before building ROM BIOS dependence into an application. Although this technique allows you to bypass any I/O redirection that may be in effect, ways exist to do this without introducing dependence on the ROM BIOS. And there are real disadvantages to calling the ROM BIOS keyboard driver: ■ It always bypasses I/O redirection, which sometimes may not be desirable. ■ It is dependent on IBM PC compatibility and does not work correctly, unchanged, on some older machines such as the Hewlett-Packard TouchScreen or the Wang Professional Computer. ■ It may introduce complicated interactions with TSR utilities. The other and more hardware-dependent method of keyboard input on an IBM PC is to write a new handler for ROM BIOS Int 09H and service the keyboard controller's interrupts directly. This involves translation of scan codes to ASCII characters and maintenance of the type-ahead buffer. In ordinary PC applications, there is no reason to take over keyboard I/O at this level; therefore, I will not discuss this method further here. If you are curious about the techniques that would be required, the best reference is the listing for the ROM BIOS Int 09H handler in the IBM PC or PC/AT technical reference manual. Ctrl-C and Ctrl-Break Handlers In the discussion of keyboard input with the MS-DOS handle and traditional functions, I made some passing references to the fact that Ctrl-C entries can interfere with the expected behavior of those functions. Let's look at this subject in more detail now. During most character I/O operations, MS-DOS checks for a Ctrl-C (ASCII code 03H) waiting at the keyboard and executes an Int 23H if one is detected. If the system break flag is on, MS-DOS also checks for a Ctrl-C entry during certain other operations (such as file reads and writes). Ordinarily, the Int 23H vector points to a routine that simply terminates the currently active process and returns control to the parent process── usually the MS-DOS command interpreter. In other words, if your program is executing and you enter a Ctrl-C, accidentally or intentionally, MS-DOS simply aborts the program. Any files the program has opened using file control blocks will not be closed properly, any interrupt vectors it has altered may not be restored correctly, and if it is performing any direct I/O operations (for example, if it contains an interrupt driver for the serial port), all kinds of unexpected events may occur. Although you can use a number of partially effective methods to defeat Ctrl-C checking, such as performing keyboard input with Int 21H Functions 06H and 07H, placing all character devices into binary mode, or turning off the system break flag with Int 21H Function 33H, none of these is completely foolproof. The simplest and most elegant way to defeat Ctrl-C checking is simply to substitute your own Int 23H handler, which can take some action appropriate to your program. When the program terminates, MS-DOS automatically restores the previous contents of the Int 23H vector from information saved in the program segment prefix. The following example shows how to install your own Ctrl-C handler (which in this case does nothing at all): ────────────────────────────────────────────────────────────────────────── push ds ; save data segment ; set int 23h vector... mov ax,2523h ; function 25h = set interrupt ; int 23h = vector for ; Ctrl-C handler mov dx,seg handler ; DS:DX = handler address mov ds,dx mov dx,offset handler int 21h ; transfer to MS-DOS pop ds ; restore data segment . . . handler: ; a Ctrl-C handler iret ; that does nothing ────────────────────────────────────────────────────────────────────────── The first part of the code (which alters the contents of the Int 23H vector) would be executed in the initialization part of the application. The handler receives control whenever MS-DOS detects a Ctrl-C at the keyboard. (Because this handler consists only of an interrupt return, the Ctrl-C will remain in the keyboard input stream and will be passed to the application when it requests a character from the keyboard, appearing on the screen as ^C.) When an Int 23H handler is called, MS-DOS is in a stable state. Thus, the handler can call any MS-DOS function. It can also reset the segment registers and the stack pointer and transfer control to some other point in the application without ever returning control to MS-DOS with an IRET. On IBM PC compatibles, an additional interrupt handler must be taken into consideration. Whenever the ROM BIOS keyboard driver detects the key combination Ctrl-Break, it calls a handler whose address is stored in the vector for Int 1BH. The default ROM BIOS Int 1BH handler does nothing. MS-DOS alters the Int 1BH vector to point to its own handler, which sets a flag and returns; the net effect is to remap the Ctrl-Break into a Ctrl-C that is forced ahead of any other characters waiting in the keyboard buffer. Taking over the Int 1BH vector in an application is somewhat tricky but extremely useful. Because the keyboard is interrupt driven, a press of Ctrl-Break lets the application regain control under almost any circumstance──often, even if the program has crashed or is in an endless loop. You cannot, in general, use the same handler for Int 1BH that you use for Int 23H. The Int 1BH handler is more limited in what it can do, because it has been called as a result of a hardware interrupt and MS-DOS may have been executing a critical section of code at the time the interrupt was serviced. Thus, all registers except CS:IP are in an unknown state; they may have to be saved and then modified before your interrupt handler can execute. Similarly, the depth of the stack in use when the Int 1BH handler is called is unknown, and if the handler is to perform stack-intensive operations, it may have to save the stack segment and the stack pointer and switch to a new stack that is known to have sufficient depth. In normal application programs, you should probably avoid retaining control in an Int 1BH handler, rather than performing an IRET. Because of subtle differences among non-IBM ROM BIOSes, it is difficult to predict the state of the keyboard controller and the 8259 Programmable Interrupt Controller (PIC) when the Int 1BH handler begins executing. Also, MS-DOS itself may not be in a stable state at the point of interrupt, a situation that can manifest itself in unexpected critical errors during subsequent I/O operations. Finally, MS-DOS versions 3.2 and later allocate a stack from an internal pool for use by the Int 09H handler. If the Int 1BH handler never returns, the Int 09H handler never returns either, and repeated entries of Ctrl-Break will eventually exhaust the stack pool, halting the system. Because Int 1BH is a ROM BIOS interrupt and not an MS-DOS interrupt, MS-DOS does not restore the previous contents of the Int 1BH vector when a program exits. If your program modifies this vector, it must save the original value and restore it before terminating. Otherwise, the vector will be left pointing to some random area in the next program that runs, and the next time the user presses Ctrl-Break a system crash is the best you can hope for. Ctrl-C and Ctrl-Break Handlers and High-Level Languages Capturing the Ctrl-C and Ctrl-Break interrupts is straightforward when you are programming in assembly language. The process is only slightly more difficult with high-level languages, as long as you have enough information about the language's calling conventions that you can link in a small assembly-language routine as part of the program. The BREAK.ASM listing in Figure 5-1 contains source code for a Ctrl-Break handler that can be linked with small-model Microsoft C programs running on an IBM PC compatible. The short C program in Figure 5-2 demonstrates use of the handler. (This code should be readily portable to other C compilers.) ────────────────────────────────────────────────────────────────────────── page 55,132 title Ctrl-C & Ctrl-Break Handlers name break ; ; Ctrl-C and Ctrl-Break handler for Microsoft C ; programs running on IBM PC compatibles ; ; by Ray Duncan ; ; Assemble with: C>MASM /Mx BREAK; ; ; This module allows C programs to retain control ; when the user enters a Ctrl-Break or Ctrl-C. ; It uses Microsoft C parameter-passing conventions ; and assumes the C small memory model. ; ; The procedure _capture is called to install ; a new handler for the Ctrl-C and Ctrl-Break ; interrupts (1bh and 23h). _capture is passed ; the address of a static variable, which will be ; set to true by the handler whenever a Ctrl-C ; or Ctrl-Break is detected. The C syntax is: ; ; static int flag; ; capture(&flag); ; ; The procedure _release is called by the C program ; to restore the original Ctrl-Break and Ctrl-C ; handler. The C syntax is: ; release(); ; ; The procedure ctrlbrk is the actual interrupt ; handler. It receives control when a software ; int 1bh is executed by the ROM BIOS or int 23h ; is executed by MS-DOS. It simply sets the C ; program's variable to true (1) and returns. ; args equ 4 ; stack offset of arguments, ; C small memory model cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII linefeed _TEXT segment word public 'CODE' assume cs:_TEXT public _capture _capture proc near ; take over Ctrl-Break ; and Ctrl-C interrupt vectors push bp ; set up stack frame mov bp,sp push ds ; save registers push di push si ; save address of ; calling program's "flag" mov ax,word ptr [bp+args] mov word ptr cs:flag,ax mov word ptr cs:flag+2,ds ; save address of original mov ax,3523h ; int 23h handler int 21h mov word ptr cs:int23,bx mov word ptr cs:int23+2,es mov ax,351bh ; save address of original int 21h ; int 1bh handler mov word ptr cs:int1b,bx mov word ptr cs:int1b+2,es push cs ; set DS:DX = address pop ds ; of new handler mov dx,offset _TEXT:ctrlbrk mov ax,02523h ; set int 23h vector int 21h mov ax,0251bh ; set int 1bh vector int 21h pop si ; restore registers pop di pop ds pop bp ; discard stack frame ret ; and return to caller _capture endp public _release _release proc near ; restore original Ctrl-C ; and Ctrl-Break handlers push bp ; save registers push ds push di push si lds dx,cs:int1b ; get address of previous ; int 1bh handler mov ax,251bh ; set int 1bh vector int 21h lds dx,cs:int23 ; get address of previous ; int 23h handler mov ax,2523h ; set int 23h vector int 21h pop si ; restore registers pop di ; and return to caller pop ds pop bp ret release endp ctrlbrk proc far ; Ctrl-C and Ctrl-Break ; interrupt handler push bx ; save registers push ds lds bx,cs:flag ; get address of C program's ; "flag variable" ; and set the flag "true" mov word ptr ds:[bx],1 pop ds ; restore registers pop bx iret ; return from handler ctrlbrk endp flag dd 0 ; far pointer to caller's ; Ctrl-Break or Ctrl-C flag int23 dd 0 ; address of original ; Ctrl-C handler int1b dd 0 ; address of original ; Ctrl-Break handler _TEXT ends end ────────────────────────────────────────────────────────────────────────── Figure 5-1. BREAK.ASM: A Ctrl-C and Ctrl-Break interrupt handler that can be linked with Microsoft C programs. ────────────────────────────────────────────────────────────────────────── /* TRYBREAK.C Demo of BREAK.ASM Ctrl-Break and Ctrl-C interrupt handler, by Ray Duncan To create the executable file TRYBREAK.EXE, enter: MASM /Mx BREAK; CL TRYBREAK.C BREAK.OBJ */ #include <stdio.h> main(int argc, char *argv[]) { int hit = 0; /* flag for key press */ int c = 0; /* character from keyboard */ static int flag = 0; /* true if Ctrl-Break or Ctrl-C detected */ puts("\n*** TRYBREAK.C running ***\n"); puts("Press Ctrl-C or Ctrl-Break to test handler,"); puts("Press the Esc key to exit TRYBREAK.\n"); capture(&flag); /* install new Ctrl-C and Ctrl-Break handler and pass address of flag */ puts("TRYBREAK has captured interrupt vectors.\n"); while(1) { hit = kbhit(); /* check for key press */ /* (MS-DOS sees Ctrl-C when keyboard polled) */ if(flag != 0) /* if flag is true, an */ { /* interrupt has occurred */ puts("\nControl-Break detected.\n"); flag = 0; /* reset interrupt flag */ } if(hit != 0) /* if any key waiting */ { c = getch(); /* read key, exit if Esc */ if( (c & 0x7f) == 0x1b) break; putch(c); /* otherwise display it */ } } release(); /* restore original Ctrl-C and Ctrl-Break handlers */ puts("\n\nTRYBREAK has released interrupt vectors."); } ────────────────────────────────────────────────────────────────────────── Figure 5-2. TRYBREAK.C: A simple Microsoft C program that demonstrates use of the interrupt handler BREAK.ASM from Figure 5-1. In the example handler, the procedure named capture is called with the address of an integer variable within the C program. It saves the address of the variable, points the Int 1BH and Int 23H vectors to its own interrupt handler, and then returns. When MS-DOS detects a Ctrl-C or Ctrl-Break, the interrupt handler sets the integer variable within the C program to true (1) and returns. The C program can then poll this variable at its leisure. Of course, to detect more than one Ctrl-C, the program must reset the variable to zero again. The procedure named release simply restores the Int 1BH and Int 23H vectors to their original values, thereby disabling the interrupt handler. Although it is not strictly necessary for release to do anything about Int 23H, this action does give the C program the option of restoring the default handler for Int 23H without terminating. Pointing Devices Device drivers for pointing devices are supplied by the hardware manufacturer and are loaded with a DEVICE statement in the CONFIG.SYS file. Although the hardware characteristics of the available pointing devices differ greatly, nearly all of their drivers present the same software interface to application programs: the Int 33H protocol used by the Microsoft Mouse driver. Version 6 of the Microsoft Mouse driver (which was current as this was written) offers the following functions: ╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖ Function Meaning ────────────────────────────────────────────────────────────────────────── 00H Reset mouse and get status. Function Meaning ────────────────────────────────────────────────────────────────────────── 00H Reset mouse and get status. 01H Show mouse pointer. 02H Hide mouse pointer. 03H Get button status and pointer position. 04H Set pointer position. 05H Get button-press information. 06H Get button-release information. 07H Set horizontal limits for pointer. 08H Set vertical limits for pointer. 09H Set graphics pointer type. 0AH Set text pointer type. 0BH Read mouse-motion counters. 0CH Install interrupt handler for mouse events. 0DH Turn on light pen emulation. 0EH Turn off light pen emulation. 0FH Set mickeys to pixel ratio. 10H Set pointer exclusion area. 13H Set double-speed threshold. 14H Swap mouse-event interrupt routines. Function Meaning ────────────────────────────────────────────────────────────────────────── 14H Swap mouse-event interrupt routines. 15H Get buffer size for mouse-driver state. 16H Save mouse-driver state. 17H Restore mouse-driver state. 18H Install alternate handler for mouse events. 19H Get address of alternate handler. 1AH Set mouse sensitivity. 1BH Get mouse sensitivity. 1CH Set mouse interrupt rate. 1DH Select display page for pointer. 1EH Get display page for pointer. 1FH Disable mouse driver. 20H Enable mouse driver. 21H Reset mouse driver. 22H Set language for mouse-driver messages. 23H Get language number. 24H Get driver version, mouse type, and IRQ number. ────────────────────────────────────────────────────────────────────────── Function Meaning ────────────────────────────────────────────────────────────────────────── Although this list of mouse functions may appear intimidating, the average application will only need a few of them. A program first calls Int 33H Function 00H to initialize the mouse driver for the current display mode and to check its status. At this point, the mouse is "alive" and the application can obtain its state and position; however, the pointer does not become visible until the process calls Int 33H Function 01H. The program can then call Int 33H Functions 03H, 05H, and 06H to monitor the mouse position and the status of the mouse buttons. Alternatively, the program can register an interrupt handler for mouse events, using Int 33H Function 0CH. This latter technique eliminates the need to poll the mouse driver; the driver will notify the program by calling the interrupt handler whenever the mouse is moved or a button is pressed or released. When the application is finished with the mouse, it can call Int 33H Function 02H to hide the mouse pointer. If the program has registered an interrupt handler for mouse events, it should disable further calls to the handler by resetting the mouse driver again with Int 33H Function 00H. For a complete description of the mouse-driver functions, see Section III of this book, "IBM ROM BIOS and Mouse Functions Reference." Figure 5-3 shows a small demonstration program that polls the mouse continually, to display its position and status. ────────────────────────────────────────────────────────────────────────── /* Simple Demo of Int 33H Mouse Driver (C) 1988 Ray Duncan Compile with: CL MOUDEMO.C */ #include <stdio.h> #include <dos.h> union REGS regs; void cls(void); /* function prototypes */ void gotoxy(int, int); main(int argc, char *argv[]) { int x,y,buttons; /* some scratch variables */ /* for the mouse state */ regs.x.ax = 0; /* reset mouse driver */ int86(0x33, ®s, ®s); /* and check status */ if(regs.x.ax == 0) /* exit if no mouse */ { printf("\nMouse not available\n"); exit(1); } cls(); /* clear the screen */ gotoxy(45,0); /* and show help info */ puts("Press Both Mouse Buttons To Exit"); regs.x.ax = 1; /* display mouse cursor */ int86(0x33, ®s, ®s); do { regs.x.ax = 3; /* get mouse position */ int86(0x33, ®s, ®s); /* and button status */ buttons = regs.x.bx & 3; x = regs.x.cx; y = regs.x.dx; gotoxy(0,0); /* display mouse position */ printf("X = %3d Y = %3d", x, y); } while(buttons != 3); /* exit if both buttons down */ regs.x.ax = 2; /* hide mouse cursor */ int86(0x33, ®s, ®s); cls(); /* display message and exit */ gotoxy(0,0); puts("Have a Mice Day!"); } /* Clear the screen */ void cls(void) { regs.x.ax = 0x0600; /* ROM BIOS video driver */ regs.h.bh = 7; /* int 10h function 06h */ regs.x.cx = 0; /* initializes a window */ regs.h.dh = 24; regs.h.dl = 79; int86(0x10, ®s, ®s); } /* Position cursor to (x,y) */ void gotoxy(int x, int y) { regs.h.dl = x; /* ROM BIOS video driver */ regs.h.dh = y; /* int 10h function 02h */ regs.h.bh = 0; /* positions the cursor */ regs.h.ah = 2; int86(0x10, ®s, ®s); } ────────────────────────────────────────────────────────────────────────── Figure 5-3. MOUDEMO.C: A simple Microsoft C program that polls the mouse and continually displays the coordinates of the mouse pointer in the upper left corner of the screen. The program uses the ROM BIOS video driver, which is discussed in Chapter 6, to clear the screen and position the text cursor. ──────────────────────────────────────────────────────────────────────────── Chapter 6 Video Display The visual presentation of an application program is one of its most important elements. Users frequently base their conclusions about a program's performance and "polish" on the speed and attractiveness of its displays. Therefore, a feel for the computer system's display facilities and capabilities at all levels, from MS-DOS down to the bare hardware, is important to you as a programmer. Video Display Adapters The video display adapters found in IBM PC─compatible computers have a hybrid interface to the central processor. The overall display characteristics, such as vertical and horizontal resolution, background color, and palette, are controlled by values written to I/O ports whose addresses are hardwired on the adapter, whereas the appearance of each individual character or graphics pixel on the display is controlled by a specific location within an area of memory called the regen buffer or refresh buffer. Both the CPU and the video controller access this memory; the software updates the display by simply writing character codes or bit patterns directly into the regen buffer. (This is called memory-mapped I/O.) The following adapters are in common use as this book is being written: ■ Monochrome/Printer Display Adapter (MDA). Introduced with the original IBM PC in 1981, this adapter supports 80-by-25 text display on a green (monochrome) screen and has no graphics capabilities at all. ■ Color/Graphics Adapter (CGA). Also introduced by IBM in 1981, this adapter supports 40-by-25 and 80-by-25 text modes and 320-by-200, 4-color or 640-by-200, 2-color graphics (all-points-addressable, or APA) modes on composite or digital RGB monitors. ■ Enhanced Graphics Adapter (EGA). Introduced by IBM in 1985 and upwardly compatible from the CGA, this adapter adds support for 640-by-350, 16-color graphics modes on digital RGB monitors. It also supports an MDA-compatible text mode. ■ Multi-Color Graphics Array (MCGA). Introduced by IBM in 1987 with the Personal System/2 (PS/2) models 25 and 30, this adapter is partially compatible with the CGA and EGA and supports 640-by-480, 2-color or 320-by-200, 256-color graphics on analog RGB monitors. ■ Video Graphics Array (VGA). Introduced by IBM in 1987 with the PS/2 models 50, 60, and 80, this adapter is upwardly compatible from the EGA and supports 640-by-480, 16-color or 320-by-200, 256-color graphics on analog RGB monitors. It also supports an MDA-compatible text mode. ■ Hercules Graphics Card, Graphics CardPlus, and InColor Cards. These are upwardly compatible from the MDA for text display but offer graphics capabilities that are incompatible with all of the IBM adapters. The locations of the regen buffers for the various IBM PC─compatible adapters are shown in Figure 6-1. ┌───────────────────────────────────────────────────────┐ │ ROM BIOS │ FE000H ├───────────────────────────────────────────────────────┤ │ System ROM, Stand-alone BASIC, etc. │ F4000H ├───────────────────────────────────────────────────────┤ │ Reserved for BIOS extensions │ │ (hard-disk controller, etc.) │ C0000H ├───────────────────────────────────────────────────────┤ │ Reserved │ BC000H ├───────────────────────────────────────────────────────┤ │ 16 KB regen buffer for CGA, EGA, MCGA, and VGA │ │ in text modes and 200-line graphics modes │ B8000H ├───────────────────────────────────────────────────────┤ │ Reserved │ B1000H ├───────────────────────────────────────────────────────┤ │ 4 KB Monochrome Adapter regen buffer │ B0000H ├───────────────────────────────────────────────────────┤ │ Regen buffer area for EGA, MCGA, and VGA │ │ in 350-line or 480-line graphics modes │ A0000H ├───────────────────────────────────────────────────────┤ │ Transient part of COMMAND.COM │ ├───────────────────────────────────────────────────────┤ │ Transient program area │ varies ├───────────────────────────────────────────────────────┤ │ MS-DOS and its buffers, │ │ tables, and device drivers │ 00400H ├───────────────────────────────────────────────────────┤ │ Interrupt vectors │ 00000H └───────────────────────────────────────────────────────┘ Figure 6-1. Memory diagram of an IBM PC─compatible personal computer, showing the locations of the regen buffers for various adapters. Support Considerations MS-DOS offers several functions to transfer text to the display. Version 1 supported only Teletype-like output capabilities; version 2 added an optional ANSI console driver to allow the programmer to clear the screen, position the cursor, and select colors and attributes with standard escape sequences embedded in the output. Programs that use only the MS-DOS functions will operate properly on any computer system that runs MS-DOS, regardless of the level of IBM hardware compatibility. On IBM PC─compatible machines, the ROM BIOS contains a video driver that programs can invoke directly, bypassing MS-DOS. The ROM BIOS functions allow a program to write text or individual pixels to the screen or to select display modes, video pages, palette, and foreground and background colors. These functions are relatively efficient (compared with the MS-DOS functions, at least), although the graphics support is primitive. Unfortunately, the display functions of both MS-DOS and the ROM BIOS were designed around the model of a cursor-addressable terminal and therefore do not fully exploit the capabilities of the memory-mapped, high-bandwidth display adapters used on IBM PC─compatible machines. As a result, nearly every popular interactive application with full-screen displays or graphics capability ignores both MS-DOS and the ROM BIOS and writes directly to the video controller's registers and regen buffer. Programs that control the hardware directly are sometimes called "ill-behaved," because they are performing operations that are normally reserved for operating-system device drivers. These programs are a severe management problem in multitasking real-mode environments such as DesqView and Microsoft Windows, and they are the main reason why such environments are not used more widely. It could be argued, however, that the blame for such problematic behavior lies not with the application programs but with the failure of MS-DOS and the ROM BIOS──even six years after the first appearance of the IBM PC──to provide display functions of adequate range and power. MS-DOS Display Functions Under MS-DOS versions 2.0 and later, the preferred method for sending text to the display is to use handle-based Int 21H Function 40H (Write File or Device). When an application program receives control, MS-DOS has already assigned it handles for the standard output (1) and standard error (2) devices, and these handles can be used immediately. For example, the sequence at the top of the following page writes the message hello to the display using the standard output handle. ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message to display msg_len equ $-msg ; length of message . . . mov ah,40h ; function 40h = write file or device mov bx,1 ; BX = standard output handle mov cx,msg_len ; CX = message length mov dx,seg msg ; DS:DX = address of message mov ds,dx mov dx,offset msg int 21h ; transfer to MS-DOS jc error ; jump if error detected . . . ────────────────────────────────────────────────────────────────────────── If there is no error, the function returns the carry flag cleared and the number of characters actually transferred in register AX. Unless a Ctrl-Z is embedded in the text or the standard output is redirected to a disk file and the disk is full, this number should equal the number of characters requested. As in the case of keyboard input, the user's ability to specify command-line redirection parameters that are invisible to the application means that if you use the predefined standard output handle, you can't always be sure where your output is going. However, to ensure that your output actually goes to the display, you can use the predefined standard error handle, which is always opened to the CON (logical console) device and is not redirectable. As an alternative to the standard output and standard error handles, you can bypass any output redirection and open a separate channel to CON, using the handle obtained from that open operation for character output. For example, the following code opens the console display for output and then writes the string hello to it: ────────────────────────────────────────────────────────────────────────── fname db 'CON',0 ; name of CON device handle dw 0 ; handle for CON device msg db 'hello' ; message to display msg_len equ $-msg ; length of message . . . mov ax,3d02h ; AH = function 3dh = open ; AL = mode = read/write mov dx,seg fname ; DS:DX = device name mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if open failed mov handle,ax ; save handle for CON . . . mov ah,40h ; function 40h = write mov cx,msg_len ; CX = message length mov dx,seg msg ; DS:DX = address of message mov ds,dx mov dx,offset msg mov bx,handle ; BX = CON device handle int 21h ; transfer to MS-DOS jc error ; jump if error detected . . . ────────────────────────────────────────────────────────────────────────── As with the keyboard input functions, MS-DOS also supports traditional display functions that are upwardly compatible from the corresponding CP/M output calls: ■ Int 21H Function 02H sends the character in the DL register to the standard output device. It is sensitive to Ctrl-C interrupts, and it handles carriage returns, linefeeds, bell codes, and backspaces appropriately. ■ Int 21H Function 06H transfers the character in the DL register to the standard output device, but it is not sensitive to Ctrl-C interrupts. You must take care when using this function, because it can also be used for input and for status requests. ■ Int 21H Function 09H sends a string to the standard output device. The string is terminated by the $ character. With MS-DOS version 2 or later, these three traditional functions are converted internally to handle-based writes to the standard output and thus are susceptible to output redirection. The sequence at the top of the following page sounds a warning beep by sending an ASCII bell code (07H) to the display driver using the traditional character-output call Int 21H Function 02H. ────────────────────────────────────────────────────────────────────────── . . . mov dl,7 ; 07h = ASCII bell code mov ah,2 ; function 02h = display character int 21h ; transfer to MS-DOS . . . ────────────────────────────────────────────────────────────────────────── The following sequence uses the traditional string-output call Int 21H Function 09H to display a string: ────────────────────────────────────────────────────────────────────────── msg db 'hello$' . . . mov dx,seg msg ; DS:DX = message address mov ds,dx mov dx,offset msg mov ah,9 ; function 09h = write string int 21h ; transfer to MS-DOS . . . ────────────────────────────────────────────────────────────────────────── Note that MS-DOS detects the $ character as a terminator and does not display it on the screen. Screen Control with MS-DOS Functions With version 2.0 or later, if MS-DOS loads the optional device driver ANSI.SYS in response to a DEVICE directive in the CONFIG.SYS file, programs can clear the screen, control the cursor position, and select foreground and background colors by embedding escape sequences in the text output. Escape sequences are so called because they begin with an escape character (1BH), which alerts the driver to intercept and interpret the subsequent characters in the sequence. When the ANSI driver is not loaded, MS-DOS simply passes the escape sequence to the display like any other text, usually resulting in a chaotic screen. The escape sequences that can be used with the ANSI driver for screen control are a subset of those defined in the ANSI 3.64─1979 Standard. These standard sequences are summarized in Figure 6-2. Note that case is significant for the last character in an escape sequence and that numbers must always be represented as ASCII digit strings, not as their binary values. (A separate set of escape sequences supported by ANSI.SYS, but not compatible with the ANSI standard, may be used for reprogramming and remapping the keyboard.) ╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖ Escape sequence Meaning ────────────────────────────────────────────────────────────────────────── Esc[2J Clear screen; place cursor in upper left corner (home position). Esc[K Clear from cursor to end of line. Esc[row;colH Position cursor. (Row is the y coordinate in the range 1─25 and col is the x coordinate in the range 1─80 for 80-by-25 text display modes.) Escape sequences terminated with the letter f instead of H have the same effect. Escape sequence Meaning ────────────────────────────────────────────────────────────────────────── effect. Esc[nA Move cursor up n rows. Esc[nB Move cursor down n rows. Esc[nC Move cursor right n columns. Esc[nD Move cursor left n columns. Esc[s Save current cursor position. Esc[u Restore cursor to saved position. Esc[6n Return current cursor position on the standard input handle in the format Esc[row;colR. Esc[nm Select character attributes: 0 = no special attributes 1 = high intensity 2 = low intensity 3 = italic 4 = underline 5 = blink 6 = rapid blink 7 = reverse video 8 = concealed text (no display) Escape sequence Meaning ────────────────────────────────────────────────────────────────────────── 8 = concealed text (no display) 30 = foreground black 31 = foreground red 32 = foreground green 33 = foreground yellow 34 = foreground blue 35 = foreground magenta 36 = foreground cyan 37 = foreground white 40 = background black 41 = background red 42 = background green 43 = background yellow 44 = background blue 45 = background magenta 46 = background cyan 47 = background white Esc[=nh Select display mode: 0 = 40-by-25, 16-color text (color burst off) Escape sequence Meaning ────────────────────────────────────────────────────────────────────────── 0 = 40-by-25, 16-color text (color burst off) 1 = 40-by-25, 16-color text 2 = 80-by-25, 16-color text (color burst off) 3 = 80-by-25, 16-color text 4 = 320-by-200, 4-color graphics 5 = 320-by-200, 4-color graphics (color burst off) 6 = 620-by-200, 2-color graphics 14 = 640-by-200, 16-color graphics (EGA and VGA, MS-DOS 4.0) 15 = 640-by-350, 2-color graphics (EGA and VGA, MS-DOS 4.0) 16 = 640-by-350, 16-color graphics (EGA and VGA, MS-DOS 4.0) 17 = 640-by-480, 2-color graphics (MCGA and VGA, MS-DOS 4.0) 18 = 640-by-480, 16-color graphics (VGA, MS-DOS 4.0) 19 = 320-by-200, 256-color graphics (MCGA and VGA, MS-DOS 4.0) Escape sequences terminated with l instead of h have Escape sequence Meaning ────────────────────────────────────────────────────────────────────────── Escape sequences terminated with l instead of h have the same effect. Esc[=7h Enable line wrap. Esc[=7l Disable line wrap. ────────────────────────────────────────────────────────────────────────── Figure 6-2. The ANSI escape sequences supported by the MS-DOS ANSI.SYS driver. Programs running under MS-DOS 2.0 or later may use these functions, if ANSI.SYS is loaded, to control the appearance of the display in a hardware-independent manner. The symbol Esc indicates an ASCII escape code──a character with the value 1BH. Note that cursor positions in ANSI escape sequences are one-based, unlike the cursor coordinates used by the IBM ROM BIOS, which are zero-based. Numbers embedded in an escape sequence must always be represented as a string of ASCII digits, not as their binary values. Binary Output Mode Under MS-DOS version 2 or later, you can substantially increase display speeds for well-behaved application programs without sacrificing hardware independence by selecting binary (raw) mode for the standard output. In binary mode, MS-DOS does not check between each character it transfers to the output device for a Ctrl-C waiting at the keyboard, nor does it filter the output string for certain characters such as Ctrl-Z. Bit 5 in the device information word associated with a device handle controls binary mode. Programs access the device information word by using Subfunctions 00H and 01H of the MS-DOS IOCTL function (I/O Control, Int 21H Function 44H). For example, the sequence on the following page places the standard output handle into binary mode. ────────────────────────────────────────────────────────────────────────── ; get device information... mov bx,1 ; standard output handle mov ax,4400h ; function 44h subfunction 00h int 21h ; transfer to MS-DOS mov dh,0 ; set upper byte of DX = 0 or dl,20h ; set binary mode bit in DL ; write device information... ; (BX still has handle) mov ax,4401h ; function 44h subfunction 01h int 21h ; transfer to MS-DOS ────────────────────────────────────────────────────────────────────────── Note that if a program changes the mode of any of the standard handles, it should restore those handles to ASCII (cooked) mode before it exits. Otherwise, subsequent application programs may behave in unexpected ways. For more detailed information on the IOCTL function, see Section II of this book, "MS-DOS Functions Reference." The ROM BIOS Display Functions You can somewhat improve the display performance of programs that are intended for use only on IBM PC─compatible machines by using the ROM BIOS video driver instead of the MS-DOS output functions. Accessed by means of Int 10H, the ROM BIOS driver supports the following functions for all of the currently available IBM display adapters: ╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖ Function Action ────────────────────────────────────────────────────────────────────────── Display mode control 00H Set display mode. 0FH Get display mode. Cursor control 01H Set cursor size. 02H Set cursor position. 03H Get cursor position and size. Writing to the display 09H Write character and attribute at cursor. 0AH Write character-only at cursor. 0EH Write character in teletype mode. Reading from the display 08H Read character and attribute at cursor. Function Action ────────────────────────────────────────────────────────────────────────── Graphics support 0CH Write pixel. 0DH Read pixel. Scroll or clear display 06H Scroll up or initialize window. 07H Scroll down or initialize window. Miscellaneous 04H Read light pen. 05H Select display page. 0BH Select palette/set border color. ────────────────────────────────────────────────────────────────────────── Additional ROM BIOS functions are available on the EGA, MCGA, VGA, and PCjr to support the enhanced features of these adapters, such as programmable palettes and character sets (fonts). Some of the functions are valid only in certain display modes. Each display mode is characterized by the number of colors it can display, its vertical resolution, its horizontal resolution, and whether it supports text or graphics memory mapping. The ROM BIOS identifies it with a unique number. Section III of this book, "IBM ROM BIOS and Mouse Functions Reference," documents all of the ROM BIOS Int 10H functions and display modes. As you can see from the preceding list, the ROM BIOS offers several desirable capabilities that are not available from MS-DOS, including initialization or scrolling of selected screen windows, modification of the cursor shape, and reading back the character being displayed at an arbitrary screen location. These functions can be used to isolate your program from the hardware on any IBM PC─compatible adapter. However, the ROM BIOS functions do not suffice for the needs of a high-performance, interactive, full-screen program such as a word processor. They do not support the rapid display of character strings at an arbitrary screen position, and they do not implement graphics operations at the level normally required by applications (for example, bit-block transfers and rapid drawing of lines, circles, and filled polygons). And, of course, they are of no use whatsoever in non-IBM display modes such as the monochrome graphics mode of the Hercules Graphics Card. Let's look at a simple example of a call to the ROM BIOS video driver. The following sequence writes the string hello to the screen: ────────────────────────────────────────────────────────────────────────── msg db 'hello' msg_len equ $-msg . . . mov si,seg msg ; DS:SI = message address mov ds,si mov si,offset msg mov cx,msg_len ; CX = message length cld next: lodsb ; get AL = next character push si ; save message pointer mov ah,0eh ; int 10h function 0eh = write ; character in teletype mode mov bh,0 ; assume video page 0 mov bl,color ; (use in graphics modes only) int 10h ; transfer to ROM BIOS pop si ; restore message pointer loop next ; loop until message done . . . ────────────────────────────────────────────────────────────────────────── (Note that the SI and DI registers are not necessarily preserved across a call to a ROM BIOS video function.) Memory-mapped Display Techniques Display performance is best when an application program takes over complete control of the video adapter and the refresh buffer. Because the display is memory-mapped, the speed at which characters can be put on the screen is limited only by the CPU's ability to copy bytes from one location in memory to another. The trade-off for this performance is that such programs are highly sensitive to hardware compatibility and do not always function properly on "clones" or even on new models of IBM video adapters. Text Mode Direct programming of the IBM PC─compatible video adapters in their text display modes (sometimes also called alphanumeric display modes) is straightforward. The character set is the same for all, and the cursor home position──(x,y) = (0,0)──is defined to be the upper left corner of the screen (Figure 6-3). The MDA uses 4 KB of memory starting at segment B000H as a regen buffer, and the various adapters with both text and graphics capabilities (CGA, EGA, MCGA, and VGA) use 16 KB of memory starting at segment B800H. (See Figure 6-1.) In the latter case, the 16 KB is divided into "pages" that can be independently updated and displayed. (0,0)┌─────────────────────────────────┐(79,0) │ │ │ │ │ │ │ │ │ │ │ │ │ │ (0,24)└─────────────────────────────────┘(79,24) Figure 6-3. Cursor addressing for 80-by-25 text display modes (IBM ROM BIOS modes 2, 3, and 7). Each character-display position is allotted 2 bytes in the regen buffer. The first byte (even address) contains the ASCII code of the character, which is translated by a special hardware character generator into a dot-matrix pattern for the screen. The second byte (odd address) is the attribute byte. Several bit fields in this byte control such features as blinking, intensity (highlighting), and reverse video, depending on the adapter type and display mode (Figures 6-4 and 6-5). Figure 6-6 shows a hex and ASCII dump of part of the video map for the MDA. Display Background Foreground ────────────────────────────────────────────────────────────────────────── No display (black) 000 000 No display (white) 111 111 Underline 000 001 Normal video 000 111 Reverse video 111 000 ────────────────────────────────────────────────────────────────────────── Figure 6-4. Attribute byte for 80-by-25 monochrome text display mode on the MDA, Hercules cards, EGA, and VGA (IBM ROM BIOS mode 7). Value Color ────────────────────────────────────────────────────────────────────────── 0 Black 1 Blue 2 Green 3 Cyan 4 Red 5 Magenta 6 Brown 7 White 8 Gray 9 Light blue 10 Light green 11 Light cyan 12 Light red 13 Light magenta 14 Yellow 15 Intense white ────────────────────────────────────────────────────────────────────────── Figure 6-5. Attribute byte for the 40-by-25 and 80-by-25 text display modes on the CGA, EGA, MCGA, and VGA (IBM ROM BIOS modes 0─3). The table of color values assumes default palette programming and that the B or I bit controls intensity. ────────────────────────────────────────────────────────────────────────── B000:0000 3e 07 73 07 65 07 6c 07 65 07 63 07 74 07 20 07 B000:0010 74 07 65 07 6d 07 70 07 20 07 20 07 20 07 20 07 B000:0020 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0030 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0040 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0050 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0060 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0070 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0080 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0090 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 ────────────────────────────────────────────────────────────────────────── Figure 6-6. Example dump of the first 160 bytes of the MDA's regen buffer. These bytes correspond to the first visible line on the screen. Note that ASCII character codes are stored in even bytes and their respective character attributes in odd bytes; all the characters in this example line have the attribute normal video. You can calculate the memory offset of any character on the display as the line number (y coordinate) times 80 characters per line times 2 bytes per character, plus the column number (x coordinate) times 2 bytes per character, plus (for the text/graphics adapters) the page number times the size of the page (4 KB per page in 80-by-25 modes; 2 KB per page in 40-by-25 modes). In short, the formula for the offset of the character-attribute pair for a given screen position (x,y) in 80-by-25 text modes is offset = ((y * 50H + x) * 2) + (page * 1000H) In 40-by-25 text modes, the formula is offset = ((y * 50H + x) * 2) + (page * 0800H) Of course, the segment register being used to address the video buffer must be set appropriately, depending on the type of display adapter. As a simple example, assume that the character to be displayed is in the AL register, the desired attribute byte for the character is in the AH register, the x coordinate (column) is in the BX register, and the y coordinate (row) is in the CX register. The following code stores the character and attribute byte into the MDA's video refresh buffer at the proper location: ────────────────────────────────────────────────────────────────────────── push ax ; save char and attribute mov ax,160 mul cx ; DX:AX = Y * 160 shl bx,1 ; multiply X by 2 add bx,ax ; BX = (Y*160) + (X*2) mov ax,0b000h ; ES = segment of monochrome mov es,ax ; adapter refresh buffer pop ax ; restore char and attribute mov es:[bx],ax ; write them to video buffer ────────────────────────────────────────────────────────────────────────── More frequently, we wish to move entire strings into the refresh buffer, starting at a given coordinate. In the next example, assume that the DS:SI registers point to the source string, the ES:DI registers point to the starting position in the video buffer (calculated as shown in the previous example), the AH register contains the attribute byte to be assigned to every character in the string, and the CX register contains the length of the string. The following code moves the entire string into the refresh buffer: ────────────────────────────────────────────────────────────────────────── xfer: lodsb ; fetch next character stosw ; store char + attribute loop xfer ; until all chars moved ────────────────────────────────────────────────────────────────────────── Of course, the video drivers written for actual application programs must take into account many additional factors, such as checking for special control codes (linefeeds, carriage returns, tabs), line wrap, and scrolling. Programs that write characters directly to the CGA regen buffer in text modes must deal with an additional complicating factor──they must examine the video controller's status port and access the refresh buffer only during the horizontal retrace or vertical retrace intervals. (A retrace interval is the period when the electron beam that illuminates the screen phosphors is being repositioned to the start of a new scan line.) Otherwise, the contention for memory between the CPU and the video controller is manifest as unsightly "snow" on the display. (If you are writing programs for any of the other IBM PC─compatible video adapters, such as the MDA, EGA, MCGA, or VGA, you can ignore the retrace intervals; snow is not a problem with these video controllers.) A program can detect the occurrence of a retrace interval by monitoring certain bits in the video controller's status register. For example, assume that the offset for the desired character position has been calculated as in the preceding example and placed in the BX register, the segment for the CGA's refresh buffer is in the ES register, and an ASCII character code to be displayed is in the CL register. The following code waits for the beginning of a new horizontal retrace interval and then writes the character into the buffer: ────────────────────────────────────────────────────────────────────────── mov dx,03dah ; DX = video controller's ; status port address cli ; disable interrupts ; if retrace is already ; in progress, wait for ; it to end... wait1: in al,dx ; read status port and al,1 ; check if retrace bit on jnz wait1 ; yes, wait ; wait for new retrace ; interval to start... wait2: in al,dx ; read status port and al,1 ; retrace bit on yet? jz wait2 ; jump if not yet on mov es:[bx],cl ; write character to ; the regen buffer sti ; enable interrupts again ────────────────────────────────────────────────────────────────────────── The first wait loop "synchronizes" the code to the beginning of a horizontal retrace interval. If only the second wait loop were used (that is, if a character were written when a retrace interval was already in progress), the write would occasionally begin so close to the end of a horizontal retrace "window" that it would partially miss the retrace, resulting in scattered snow at the left edge of the display. Notice that the code also disables interrupts during accesses to the video buffer, so that service of a hardware interrupt won't disrupt the synchronization process. Because of the retrace-interval constraints just outlined, the rate at which you can update the CGA in text modes is severely limited when the updating is done one character at a time. You can obtain better results by calculating all the relevant addresses and setting up the appropriate registers, disabling the video controller by writing to register 3D8H, moving the entire string to the buffer with a REP MOVSW operation, and then reenabling the video controller. If the string is of reasonable length, the user won't even notice a flicker in the display. Of course, this procedure introduces additional hardware dependence into your code because it requires much greater knowledge of the 6845 controller. Luckily, snow is not a problem in CGA graphics modes. Graphics Mode Graphics-mode memory-mapped programming for IBM PC─compatible adapters is considerably more complicated than text-mode programming. Each bit or group of bits in the regen buffer corresponds to an addressable point, or pixel, on the screen. The mapping of bits to pixels differs for each of the available graphics modes, with their differences in resolution and number of supported colors. The newer adapters (EGA, MCGA, and VGA) also use the concept of bit planes, where bits of a pixel are segregated into multiple banks of memory mapped at the same address; you must manipulate these bit planes by a combination of memory-mapped I/O and port addressing. IBM-video-systems graphics programming is a subject large enough for a book of its own, but we can use the 640-by-200, 2-color graphics display mode of the CGA (which is also supported by all subsequent IBM text/graphics adapters) to illustrate a few of the techniques involved. This mode is simple to deal with because each pixel is represented by a single bit. The pixels are assigned (x,y) coordinates in the range (0,0) through (639,199), where x is the horizontal displacement, y is the vertical displacement, and the home position (0,0) is the upper left corner of the display. (See Figure 6-7.) (0,0)┌─────────────────────────────────┐(639,0) │ │ │ │ │ │ │ │ │ │ │ │ │ │ (0,199)└─────────────────────────────────┘(639,199) Figure 6-7. Point addressing for 640-by-200, 2-color graphics modes on the CGA, EGA, MCGA, and VGA (IBM ROM BIOS mode 6). Each successive group of 80 bytes (640 bits) represents one horizontal scan line. Within each byte, the bits map one-for-one onto pixels, with the most significant bit corresponding to the leftmost displayed pixel of a set of eight pixels and the least significant bit corresponding to the rightmost displayed pixel of the set. The memory map is set up so that all the even y coordinates are scanned as a set and all the odd y coordinates are scanned as a set; this mapping is referred to as the memory interlace. To find the regen buffer offset for a particular (x,y) coordinate, you would use the following formula: offset = ((y AND 1) * 2000H) + (y/2 * 50H) + (x/8) The assembly-language implementation of this formula is as follows: ────────────────────────────────────────────────────────────────────────── ; assume AX = Y, BX = X shr bx,1 ; divide X by 8 shr bx,1 shr bx,1 push ax ; save copy of Y shr ax,1 ; find (Y/2) * 50h mov cx,50h ; with product in DX:AX mul cx add bx,ax ; add product to X/8 pop ax ; add (Y AND 1) * 2000h and ax,1 jz label1 add bx,2000h label1: ; now BX = offset into ; video buffer ────────────────────────────────────────────────────────────────────────── After calculating the correct byte address, you can use the following formula to calculate the bit position for a given pixel coordinate: bit = 7 - (x MOD 8) where bit 7 is the most significant bit and bit 0 is the least significant bit. It is easiest to build an 8-byte table, or array of bit masks, and use the operation X AND 7 to extract the appropriate entry from the table: (X AND 7) Bit mask (X AND 7) Bit mask ────────────────────────────────────────────────────────────────────────── 0 80H 4 08H 1 40H 5 04H 2 20H 6 02H 3 10H 7 01H ────────────────────────────────────────────────────────────────────────── The assembly-language implementation of this second calculation is as follows: ────────────────────────────────────────────────────────────────────────── table db 80h ; X AND 7 = offset 0 db 40h ; X AND 7 = offset 1 db 20h ; X AND 7 = offset 2 db 10h ; X AND 7 = offset 3 db 08h ; X AND 7 = offset 4 db 04h ; X AND 7 = offset 5 db 02h ; X AND 7 = offset 6 db 01h ; X AND 7 = offset 7 . . . ; assume BX = X coordinate and bx,7 ; isolate 0─7 offset mov al,[bx+table] ; now AL = mask from table . . . ────────────────────────────────────────────────────────────────────────── The program can then use the mask, together with the byte offset previously calculated, to set or clear the appropriate bit in the video controller's regen buffer. ──────────────────────────────────────────────────────────────────────────── Chapter 7 Printer and Serial Port MS-DOS supports printers, plotters, modems, and other hard-copy output or communication devices with device drivers for parallel ports and serial ports. Parallel ports are so named because they transfer a byte──8 bits── in parallel to the destination device over eight separate physical paths (plus additional status and handshaking signals). The serial port, on the other hand, communicates with the CPU with bytes but sends data to or receives data from its destination device serially──a bit at a time──over a single physical connection. Parallel ports are typically used for high-speed output devices, such as line printers, over relatively short distances (less than 50 feet). They are rarely used for devices that require two-way communication with the computer. Serial ports are used for lower-speed devices, such as modems and terminals, that require two-way communication (although some printers also have serial interfaces). A serial port can drive its device reliably over much greater distances (up to 1000 feet) over as few as three wires── transmit, receive, and ground. The most commonly used type of serial interface follows a standard called RS-232. This standard specifies a 25-wire interface with certain electrical characteristics, the use of various handshaking signals, and a standard DB-25 connector. Other serial-interface standards exist──for example, the RS-422, which is capable of considerably higher speeds than the RS-232── but these are rarely used in personal computers (except for the Apple Macintosh) at this time. MS-DOS has built-in device drivers for three parallel adapters, and for two serial adapters on the PC or PC/AT and three serial adapters on the PS/2. The logical names for these devices are LPT1, LPT2, LPT3, COM1, COM2, and COM3. The standard printer (PRN) and standard auxiliary (AUX) devices are normally aliased to LPT1 and COM1, but you can redirect PRN to one of the serial ports with the MS-DOS MODE command. As with keyboard and video display I/O, you can manage printer and serial-port I/O at several levels that offer different degrees of flexibility and hardware independence: ■ MS-DOS handle-oriented functions ■ MS-DOS traditional character functions ■ IBM ROM BIOS driver functions In the case of the serial port, direct control of the hardware by application programs is also common. I will discuss each of these I/O methods briefly, with examples, in the following pages. Printer Output The preferred method of printer output is to use the handle write function (Int 21H Function 40H) with the predefined standard printer handle (4). For example, you could write the string hello to the printer as follows: ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for printer msg_len equ $-msg ; length of message . . . mov ah,40h ; function 40h = write file or device mov bx,4 ; BX = standard printer handle mov cx,msg_len ; CX = length of string mov dx,seg msg ; DS:DX = string address mov ds,dx mov dx,offset msg int 21h ; transfer to MS-DOS jc error ; jump if error . . . ────────────────────────────────────────────────────────────────────────── If there is no error, the function returns the carry flag cleared and the number of characters actually transferred to the list device in register AX. Under normal circumstances, this number should always be the same as the length requested and the carry flag indicating an error should never be set. However, the output will terminate early if your data contains an end-of-file mark (Ctrl-Z). You can write independently to several list devices (for example, LPT1, LPT2) by issuing a specific open request (Int 21H Function 3DH) for each device and using the handles returned to access the printers individually with Int 21H Function 40H. You have already seen this general approach in Chapters 5 and 6. An alternative method of printer output is to use the traditional Int 21H Function 05H, which transfers the character in the DL register to the printer. (This function is sensitive to Ctrl-C interrupts.) For example, the assembly-language code sequence at the top of the following page would write the the string hello to the line printer. ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for printer msg_len equ $-msg ; length of message . . . mov bx,seg msg ; DS:BX = string address mov ds,bx mov bx,offset msg mov cx,msg_len ; CX = string length next: mov dl,[bx] ; get next character mov ah,5 ; function 05h = printer output int 21h ; transfer to MS-DOS inc bx ; bump string pointer loop next ; loop until string done . . . ────────────────────────────────────────────────────────────────────────── Programs that run on IBM PC─compatible machines can obtain improved printer throughput by bypassing MS-DOS and calling the ROM BIOS printer driver directly by means of Int 17H. Section III of this book, "IBM ROM BIOS and Mouse Functions Reference," documents the Int 17H functions in detail. Use of the ROM BIOS functions also allows your program to test whether the printer is off line or out of paper, a capability that MS-DOS does not offer. For example, the following sequence of instructions calls the ROM BIOS printer driver to send the string hello to the line printer: ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for printer msg_len equ $-msg ; length of message . . . mov bx,seg msg ; DS:BX = string address mov ds,bx mov bx,offset msg mov cx,msg_len ; CX = string length mov dx,0 ; DX = printer number next: mov al,[bx] ; AL = character to print mov ah,0 ; function 00h = printer output int 17h ; transfer to ROM BIOS inc bx ; bump string pointer loop next ; loop until string done . . . ────────────────────────────────────────────────────────────────────────── Note that the printer numbers used by the ROM BIOS are zero-based, whereas the printer numbers in MS-DOS logical-device names are one-based. For example, ROM BIOS printer 0 corresponds to LPT1. Finally, the most hardware-dependent technique of printer output is to access the printer controller directly. Considering the functionality already provided in MS-DOS and the IBM ROM BIOS, as well as the speeds of the devices involved, I cannot see any justification for using direct hardware control in this case. The disadvantage of introducing such extreme hardware dependence for such a low-speed device would far outweigh any small performance gains that might be obtained. The Serial Port MS-DOS support for serial ports (often referred to as the auxiliary device in MS-DOS manuals) is weak compared with its keyboard, video-display, and printer support. This is one area where the application programmer is justified in making programs hardware dependent to extract adequate performance. Programs that restrict themselves to MS-DOS functions to ensure portability can use the handle read and write functions (Int 21H Functions 3FH and 40H), with the predefined standard auxiliary handle (3) to access the serial port. For example, the following code writes the string hello to the serial port that is currently defined as the AUX device: ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for serial port msg_len equ $-msg ; length of message . . . mov ah,40h ; function 40h = write file or device mov bx,3 ; BX = standard aux handle mov cx,msg_len ; CX = string length mov dx,seg msg ; DS:DX = string address mov ds,dx mov dx,offset msg int 21h ; transfer to MS-DOS jc error ; jump if error . . . ────────────────────────────────────────────────────────────────────────── The standard auxiliary handle gives access to only the first serial port (COM1). If you want to read or write COM2 and COM3 using the handle calls, you must issue an open request (Int 21H Function 3DH) for the desired serial port and use the handle returned by that function with Int 21H Functions 3FH and 40H. Some versions of MS-DOS have a bug in character-device handling that manifests itself as follows: If you issue a read request with Int 21H Function 3FH for the exact number of characters that are waiting in the driver's buffer, the length returned in the AX register is the number of characters transferred minus one. You can circumvent this problem by always requesting more characters than you expect to receive or by placing the device handle into binary mode using Int 21H Function 44H. MS-DOS also supports two traditional functions for serial-port I/O. Int 21H Function 03H inputs a character from COM1 and returns it in the AL register; Int 21H Function 04H transmits the character in the DL register to COM1. Like the other traditional calls, these two are direct descendants of the CP/M auxiliary-device functions. For example, the following code sends the string hello to COM1 using the traditional Int 21H Function 04H: ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message for serial port msg_len equ $-msg ; length of message . . . mov bx,seg msg ; DS:BX = string address mov ds,bx mov bx,offset msg mov cx,msg_len ; CX = length of string mov dl,[bx] ; get next character mov ah,4 ; function 04h = aux output int 21h ; transfer to MS-DOS inc bx ; bump pointer to string loop next ; loop until string done . . . ────────────────────────────────────────────────────────────────────────── MS-DOS translates the traditional auxiliary-device functions into calls on the same device driver used by the handle calls. Therefore, it is generally preferable to use the handle functions in the first place, because they allow very long strings to be read or written in one operation, they give access to serial ports other than COM1, and they are symmetrical with the handle video-display, keyboard, printer, and file I/O methods described elsewhere in this book. Although the handle or traditional serial-port functions allow you to write programs that are portable to any machine running MS-DOS, they have a number of disadvantages: ■ The built-in MS-DOS serial-port driver is slow and is not interrupt driven. ■ MS-DOS serial-port I/O is not buffered. ■ Determining the status of the auxiliary device requires a separate call to the IOCTL function (Int 21H Function 44H)──if you request input and no characters are ready, your program will simply hang. ■ MS-DOS offers no standardized function to configure the serial port from within a program. For programs that are going to run on the IBM PC or compatibles, a more flexible technique for serial-port I/O is to call the IBM ROM BIOS serial-port driver by means of Int 14H. You can use this driver to initialize the serial port to a desired configuration and baud rate, examine the status of the controller, and read or write characters. Section III of this book, "IBM ROM BIOS and Mouse Functions Reference," documents the functions available from the ROM BIOS serial-port driver. For example, the following sequence sends the character X to the first serial port (COM1): ────────────────────────────────────────────────────────────────────────── . . . mov ah,1 ; function 01h = send character mov al,'X' ; AL = character to transmit mov dx,0 ; DX = serial-port number int 14h ; transfer to ROM BIOS and ah,80h ; did transmit fail? jnz error ; jump if transmit error . . . ────────────────────────────────────────────────────────────────────────── As with the ROM BIOS printer driver, the serial-port numbers used by the ROM BIOS are zero-based, whereas the serial-port numbers in MS-DOS logical-device names are one-based. In this example, serial port 0 corresponds to COM1. Unfortunately, like the MS-DOS auxiliary-device driver, the ROM BIOS serial-port driver is not interrupt driven. Although it will support higher transfer speeds than the MS-DOS functions, at rates greater than 2400 baud it may still lose characters. Consequently, most programmers writing high-performance applications that use a serial port (such as telecommunications programs) take complete control of the serial-port controller and provide their own interrupt driver. The built-in functions provided by MS-DOS, and by the ROM BIOS in the case of the IBM PC, are simply not adequate. Writing such programs requires a good understanding of the hardware. In the case of the IBM PC, the chips to study are the INS8250 Asynchronous Communications Controller and the Intel 8259A Programmable Interrupt Controller. The IBM technical reference documentation for these chips is a bit disorganized, but most of the necessary information is there if you look for it. The TALK Program The simple terminal-emulator program TALK.ASM (Figure 7-1) is an example of a useful program that performs screen, keyboard, and serial-port I/O. This program recapitulates all of the topics discussed in Chapters 5 through 7. TALK uses the IBM PC's ROM BIOS video driver to put characters on the screen, to clear the display, and to position the cursor; it uses the MS-DOS character-input calls to read the keyboard; and it contains its own interrupt driver for the serial-port controller. ────────────────────────────────────────────────────────────────────────── name talk page 55,132 .lfcond ; List false conditionals too title TALK--Simple terminal emulator ; ; TALK.ASM--Simple IBM PC terminal emulator ; ; Copyright (c) 1988 Ray Duncan ; ; To assemble and link this program into TALK.EXE: ; ; C>MASM TALK; ; C>LINK TALK; ; stdin equ 0 ; standard input handle stdout equ 1 ; standard output handle stderr equ 2 ; standard error handle cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII linefeed bsp equ 08h ; ASCII backspace escape equ 1bh ; ASCII escape code dattr equ 07h ; display attribute to use ; while in emulation mode bufsiz equ 4096 ; size of serial-port buffer echo equ 0 ; 0 = full-duplex, -1 = half-duplex equ -1 false equ 0 com1 equ true ; use COM1 if nonzero com2 equ not com1 ; use COM2 if nonzero pic_mask equ 21h ; 8259 interrupt mask port pic_eoi equ 20h ; 8259 EOI port if com1 com_data equ 03f8h ; port assignments for COM1 com_ier equ 03f9h com_mcr equ 03fch com_sts equ 03fdh com_int equ 0ch ; COM1 interrupt number int_mask equ 10h ; IRQ4 mask for 8259 endif if com2 com_data equ 02f8h ; port assignments for COM2 com_ier equ 02f9h com_mcr equ 02fch com_sts equ 02fdh com_int equ 0bh ; COM2 interrupt number int_mask equ 08h ; IRQ3 mask for 8259 endif _TEXT segment word public 'CODE' assume cs:_TEXT,ds:_DATA,es:_DATA,ss:STACK talk proc far ; entry point from MS-DOS mov ax,_DATA ; make data segment addressable mov ds,ax mov es,ax ; initialize display for ; terminal emulator mode... mov ah,15 ; get display width and int 10h ; current display mode dec ah ; save display width for use mov columns,ah ; by the screen-clear routine cmp al,7 ; enforce text display mode je talk2 ; mode 7 ok, proceed cmp al,3 jbe talk2 ; modes 0-3 ok, proceed mov dx,offset msg1 mov cx,msg1_len jmp talk6 ; print error message and exit talk2: mov bh,dattr ; clear screen and home cursor call cls call asc_enb ; capture serial-port interrupt ; vector and enable interrupts mov dx,offset msg2 ; display message mov cx,msg2_len ; 'terminal emulator running' mov bx,stdout ; BX = standard output handle mov ah,40h ; function 40h = write file or device int 21h ; transfer to MS-DOS talk3: call pc_stat ; keyboard character waiting? jz talk4 ; nothing waiting, jump call pc_in ; read keyboard character cmp al,0 ; is it a function key? jne talk32 ; not function key, jump call pc_in ; function key, discard 2nd ; character of sequence jmp talk5 ; then terminate program talk32: ; keyboard character received if echo push ax ; if half-duplex, echo call pc_out ; character to PC display pop ax endif call com_out ; write char to serial port talk4: call com_stat ; serial-port character waiting? jz talk3 ; nothing waiting, jump call com_in ; read serial-port character cmp al,20h ; is it control code? jae talk45 ; jump if not call ctrl_code ; control code, process it jmp talk3 ; check keyboard again talk45: ; noncontrol char received, call pc_out ; write it to PC display jmp talk4 ; see if any more waiting talk5: ; function key detected, ; prepare to terminate... mov bh,07h ; clear screen and home cursor call cls mov dx,offset msg3 ; display farewell message mov cx,msg3_len talk6: push dx ; save message address push cx ; and message length call asc_dsb ; disable serial-port interrupts ; and release interrupt vector pop cx ; restore message length pop dx ; and address mov bx,stdout ; handle for standard output mov ah,40h ; function 40h = write device int 21h ; transfer to MS-DOS mov ax,4c00h ; terminate program with int 21h ; return code = 0 talk endp com_stat proc near ; check asynch status; returns ; Z = false if character ready ; Z = true if nothing waiting push ax mov ax,asc_in ; compare ring buffer pointers cmp ax,asc_out pop ax ret ; return to caller stat endp com_in proc near ; get character from serial- ; port buffer; returns ; new character in AL push bx ; save register BX com_in1: ; if no char waiting, wait mov bx,asc_out ; until one is received cmp bx,asc_in je com_in1 ; jump, nothing waiting mov al,[bx+asc_buf] ; character is ready, ; extract it from buffer inc bx ; update buffer pointer cmp bx,bufsiz jne com_in2 xor bx,bx ; reset pointer if wrapped com_in2: mov asc_out,bx ; store updated pointer pop bx ; restore register BX ret ; and return to caller com_in endp com_out proc near ; write character in AL ; to serial port push dx ; save register DX push ax ; save character to send mov dx,com_sts ; DX = status port address com_out1: ; check if transmit buffer in al,dx ; is empty (TBE bit = set) and al,20h jz com_out1 ; no, must wait pop ax ; get character to send mov dx,com_data ; DX = data port address out dx,al ; transmit the character pop dx ; restore register DX ret ; and return to caller com_out endp pc_stat proc near ; read keyboard status; returns ; Z = false if character ready ; Z = true if nothing waiting ; register DX destroyed mov al,in_flag ; if character already or al,al ; waiting, return status jnz pc_stat1 mov ah,6 ; otherwise call MS-DOS to mov dl,0ffh ; determine keyboard status int 21h jz pc_stat1 ; jump if no key ready mov in_char,al ; got key, save it for mov in_flag,0ffh ; "pc_in" routine pc_stat1: ; return to caller with ret ; Z flag set appropriately pc_stat endp pc_in proc near ; read keyboard character, ; return it in AL ; DX may be destroyed mov al,in_flag ; key already waiting? or al,al jnz pc_in1 ; yes, return it to caller call pc_stat ; try to read a character jmp pc_in pc_in1: mov in_flag,0 ; clear char-waiting flag mov al,in_char ; and return AL = character ret pc_in endp pc_out proc near ; write character in AL ; to the PC's display mov ah,0eh ; ROM BIOS function 0eh = ; "teletype output" push bx ; save register BX xor bx,bx ; assume page 0 int 10h ; transfer to ROM BIOS pop bx ; restore register BX ret ; and return to caller pc_out endp cls proc near ; clear display using ; char attribute in BH ; registers AX, CX, ; and DX destroyed mov dl,columns ; set DL,DH = X,Y of mov dh,24 ; lower right corner mov cx,0 ; set CL,CH = X,Y of ; upper left corner mov ax,600h ; ROM BIOS function 06h = ; "scroll or initialize ; window" int 10h ; transfer to ROM BIOS call home ; set cursor at (0,0) ret ; and return to caller cls endp clreol proc near ; clear from cursor to end ; of line using attribute ; in BH, registers AX, CX, ; and DX destroyed call getxy ; get current cursor position mov cx,dx ; current position = "upper ; left corner" of window; mov dl,columns ; "lower right corner" X is ; max columns, Y is same ; as upper left corner mov ax,600h ; ROM BIOS function 06h = ; "scroll or initialize ; window" int 10h ; transfer to ROM BIOS ret ; return to caller clreol endp home proc near ; put cursor at home position mov dx,0 ; set (X,Y) = (0,0) call gotoxy ; position the cursor ret ; return to caller home endp gotoxy proc near ; position the cursor ; call with DL = X, DH = Y push bx ; save registers push ax mov bh,0 ; assume page 0 mov ah,2 ; ROM BIOS function 02h = ; set cursor position int 10h ; transfer to ROM BIOS pop ax ; restore registers pop bx ret ; and return to caller gotoxy endp getxy proc near ; get cursor position, ; returns DL = X, DH = Y push ax ; save registers push bx push cx mov ah,3 ; ROM BIOS function 03h = ; get cursor position mov bh,0 ; assume page 0 int 10h ; transfer to ROM BIOS pop cx ; restore registers pop bx pop ax ret ; and return to caller getxy endp ctrl_code proc near ; process control code ; call with AL = char cmp al,cr ; if carriage return je ctrl8 ; just send it cmp al,lf ; if linefeed je ctrl8 ; just send it cmp al,bsp ; if backspace je ctrl8 ; just send it cmp al,26 ; is it cls control code? jne ctrl7 ; no, jump mov bh,dattr ; cls control code, clear call cls ; screen and home cursor jmp ctrl9 ctrl7: cmp al,escape ; is it Escape character? jne ctrl9 ; no, throw it away call esc_seq ; yes, emulate CRT terminal jmp ctrl9 ctrl8: call pc_out ; send CR, LF, or backspace ; to the display ctrl9: ret ; return to caller ctrl_code endp esc_seq proc near ; decode Televideo 950 escape ; sequence for screen control call com_in ; get next character cmp al,84 ; is it clear to end of line? jne esc_seq1 ; no, jump mov bh,dattr ; yes, clear to end of line call clreol jmp esc_seq2 ; then exit esc_seq1: cmp al,61 ; is it cursor positioning? jne esc_seq2 ; no jump call com_in ; yes, get Y parameter sub al,33 ; and remove offset mov dh,al call com_in ; get X parameter sub al,33 ; and remove offset mov dl,al call gotoxy ; position the cursor esc_seq2: ; return to caller ret esc_seq endp asc_enb proc near ; capture serial-port interrupt ; vector and enable interrupt ; save address of previous ; interrupt handler... mov ax,3500h+com_int ; function 35h = get vector int 21h ; transfer to MS-DOS mov word ptr oldvec+2,es mov word ptr oldvec,bx ; now install our handler... push ds ; save our data segment mov ax,cs ; set DS:DX = address mov ds,ax ; of our interrupt handler mov dx,offset asc_int mov ax,2500h+com_int ; function 25h = set vector int 21h ; transfer to MS-DOS pop ds ; restore data segment mov dx,com_mcr ; set modem-control register mov al,0bh ; DTR and OUT2 bits out dx,al mov dx,com_ier ; set interrupt-enable mov al,1 ; register on serial- out dx,al ; port controller in al,pic_mask ; read current 8259 mask and al,not int_mask ; set mask for COM port out pic_mask,al ; write new 8259 mask ret ; back to caller asc_enb endp asc_dsb proc near ; disable interrupt and ; release interrupt vector in al,pic_mask ; read current 8259 mask or al,int_mask ; reset mask for COM port out pic_mask,al ; write new 8259 mask push ds ; save our data segment lds dx,oldvec ; load address of ; previous interrupt handler mov ax,2500h+com_int ; function 25h = set vector int 21h ; transfer to MS-DOS pop ds ; restore data segment ret ; back to caller asc_dsb endp asc_int proc far ; interrupt service routine ; for serial port sti ; turn interrupts back on push ax ; save registers push bx push dx push ds mov ax,_DATA ; make our data segment mov ds,ax ; addressable cli ; clear interrupts for ; pointer manipulation mov dx,com_data ; DX = data port address in al,dx ; read this character mov bx,asc_in ; get buffer pointer mov [asc_buf+bx],al ; store this character inc bx ; bump pointer cmp bx,bufsiz ; time for wrap? jne asc_int1 ; no, jump xor bx,bx ; yes, reset pointer asc_int1: ; store updated pointer mov asc_in,bx sti ; turn interrupts back on mov al,20h ; send EOI to 8259 out pic_eoi,al pop ds ; restore all registers pop dx pop bx pop ax iret ; return from interrupt asc_int endp _TEXT ends _DATA segment word public 'DATA' in_char db 0 ; PC keyboard input char in_flag db 0 ; <>0 if char waiting columns db 0 ; highest numbered column in ; current display mode (39 or 79) msg1 db cr,lf db 'Display must be text mode.' db cr,lf msg1_len equ $-msg1 msg2 db 'Terminal emulator running...' db cr,lf msg2_len equ $-msg2 msg3 db 'Exit from terminal emulator.' db cr,lf msg3_len equ $-msg3 oldvec dd 0 ; original contents of serial- ; port interrupt vector asc_in dw 0 ; input pointer to ring buffer asc_out dw 0 ; output pointer to ring buffer asc_buf db bufsiz dup (?) ; communications buffer _DATA ends STACK segment para stack 'STACK' db 128 dup (?) STACK ends end talk ; defines entry point ────────────────────────────────────────────────────────────────────────── Figure 7-1. TALK.ASM: A simple terminal-emulator program for IBM PC─compatible computers. This program demonstrates use of the MS-DOS and ROM BIOS video and keyboard functions and direct control of the serial-communications adapter. The TALK program illustrates the methods that an application should use to take over and service interrupts from the serial port without running afoul of MS-DOS conventions. The program begins with some equates and conditional assembly statements that configure the program for half- or full-duplex and for the desired serial port (COM1 or COM2). At entry from MS-DOS, the main routine of the program──the procedure named talk──checks the status of the serial port, initializes the display, and calls the asc_enb routine to take over the serial-port interrupt vector and enable interrupts. The talk procedure then enters a loop that reads the keyboard and sends the characters out the serial port and then reads the serial port and puts the characters on the display──in other words, it causes the PC to emulate a simple CRT terminal. The TALK program intercepts and handles control codes (carriage return, linefeed, and so forth) appropriately. It detects escape sequences and handles them as a subset of the Televideo 950 terminal capabilities. (You can easily modify the program to emulate any other cursor-addressable terminal.) When one of the PC's special function keys is pressed, the program disables serial-port interrupts, releases the serial-port interrupt vector, and exits back to MS-DOS. There are several TALK program procedures that are worth your attention because they can easily be incorporated into other programs. These are listed in the table on the following page. ╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖ Procedure Action ────────────────────────────────────────────────────────────────────────── asc_enb Takes over the serial-port interrupt vector and enables interrupts by writing to the modem-control register of the INS8250 and the interrupt-mask register of the 8259A. asc_dsb Restores the original state of the serial-port interrupt vector and disables interrupts by writing to the interrupt-mask register of the 8259A. asc_int Services serial-port interrupts, placing received characters into a ring buffer. com_stat Tests whether characters from the serial port are waiting in the ring buffer. com_in Removes characters from the interrupt handler's ring buffer and increments the buffer pointers appropriately. Procedure Action ────────────────────────────────────────────────────────────────────────── appropriately. com_out Sends one character to the serial port. cls Calls the ROM BIOS video driver to clear the screen. clreol Calls the ROM BIOS video driver to clear from the current cursor position to the end of the line. home Places the cursor in the upper left corner of the screen. gotoxy Positions the cursor at the desired position on the display. getxy Obtains the current cursor position. pc_out Sends one character to the PC's display. Procedure Action ────────────────────────────────────────────────────────────────────────── pc_stat Gets status for the PC's keyboard. pc_in Returns a character from the PC's keyboard. ────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Chapter 8 File Management The dual heritage of MS-DOS──CP/M and UNIX/XENIX──is perhaps most clearly demonstrated in its file-management services. In general, MS-DOS provides at least two distinct operating-system calls for each major file or record operation. This chapter breaks this overlapping battery of functions into two groups and explains the usage, advantages, and disadvantages of each. I will refer to the set of file and record functions that are compatible with CP/M as FCB functions. These functions rely on a data structure called a file control block (hence, FCB) to maintain certain bookkeeping information about open files. This structure resides in the application program's memory space. The FCB functions allow the programmer to create, open, close, and delete files and to read or write records of any size at any record position within such files. These functions do not support the hierarchical (treelike) file structure that was first introduced in MS-DOS version 2.0, so they can be used only to access files in the current subdirectory for a given disk drive. I will refer to the set of file and record functions that provide compatibility with UNIX/XENIX as the handle functions. These functions allow the programmer to open or create files by passing MS-DOS a null-terminated string that describes the file's location in the hierarchical file structure (the drive and path), the file's name, and its extension. If the open or create operation is successful, MS-DOS returns a 16-bit token, or handle, that is saved by the application program and used to specify the file in subsequent operations. When you use the handle functions, the operating system maintains the data structures that contain bookkeeping information about the file inside its own memory space, and these structures are not accessible to the application program. The handle functions fully support the hierarchical file structure, allowing the programmer to create, open, close, and delete files in any subdirectory on any disk drive and to read or write records of any size at any byte offset within such files. Although we are discussing the FCB functions first in this chapter for historical reasons, new MS-DOS applications should always be written using the more powerful handle functions. Use of the FCB functions in new programs should be avoided, unless compatibility with MS-DOS version 1.0 is needed. Using the FCB Functions Understanding the structure of the file control block is the key to success with the FCB family of file and record functions. An FCB is a 37-byte data structure allocated within the application program's memory space; it is divided into many fields (Figure 8-1). Typically, the program initializes an FCB with a drive code, a filename, and an extension (conveniently accomplished with the parse-filename service, Int 21H Function 29H) and then passes the address of the FCB to MS-DOS to open or create the file. If the file is successfully opened or created, MS-DOS fills in certain fields of the FCB with information from the file's entry in the disk directory. This information includes the file's exact size in bytes and the date and time the file was created or last updated. MS-DOS also places certain other information within a reserved area of the FCB; however, this area is used by the operating system for its own purposes and varies among different versions of MS-DOS. Application programs should never modify the reserved area. For compatibility with CP/M, MS-DOS automatically sets the record-size field of the FCB to 128 bytes. If the program does not want to use this default record size, it must place the desired size (in bytes) into the record-size field after the open or create operation. Subsequently, when the program needs to read or write records from the file, it must pass the address of the FCB to MS-DOS; MS-DOS, in turn, keeps the FCB updated with information about the current position of the file pointer and the size of the file. Data is always read to or written from the current disk transfer area (DTA), whose address is set with Int 21H Function 1AH. If the application program wants to perform random record access, it must set the record number into the FCB before issuing each function call; when sequential record access is being used, MS-DOS maintains the FCB and no special intervention is needed from the application. Byte offset 00H ┌───────────────────────────────────────────────────────┐ │ Drive identification │ Note 1 01H ├───────────────────────────────────────────────────────┤ │ Filename (8 characters) │ Note 2 09H ├───────────────────────────────────────────────────────┤ │ Extension (3 characters) │ Note 2 0CH ├───────────────────────────────────────────────────────┤ │ Current block number │ Note 9 0EH ├───────────────────────────────────────────────────────┤ │ Record size │ Note 10 10H ├───────────────────────────────────────────────────────┤ │ File size (4 bytes) │ Notes 3, 6 14H ├───────────────────────────────────────────────────────┤ │ Date created/updated │ Note 7 16H ├───────────────────────────────────────────────────────┤ │ Time created/updated │ Note 8 18H ├───────────────────────────────────────────────────────┤ │ Reserved │ 20H ├───────────────────────────────────────────────────────┤ │ Current-record number │ Note 9 21H ├───────────────────────────────────────────────────────┤ │ Relative-record number (4 bytes) │ Note 5 └───────────────────────────────────────────────────────┘ Figure 8-1. Normal file control block. Total length is 37 bytes (25H bytes). See notes on pages 133─34. In general, MS-DOS functions that use FCBs accept the full address of the FCB in the DS:DX register and pass back a return code in the AL register (Figure 8-2). For file-management calls (open, close, create, and delete), this return code is zero if the function was successful and 0FFH (255) if the function failed. For the FCB-type record read and write functions, the success code returned in the AL register is again zero, but there are several failure codes. Under MS-DOS version 3.0 or later, more detailed error reporting can be obtained by calling Int 21H Function 59H (Get Extended Error Information) after a failed FCB function call. When a program is loaded under MS-DOS, the operating system sets up two FCBs in the program segment prefix, at offsets 005CH and 006CH. These are often referred to as the default FCBs, and they are included to provide upward compatibility from CP/M. MS-DOS parses the first two parameters in the command line that invokes the program (excluding any redirection directives) into the default FCBs, under the assumption that they may be file specifications. The application must determine whether they really are filenames or not. In addition, because the default FCBs overlap and are not in a particularly convenient location (especially for .EXE programs), they usually must be copied elsewhere in order to be used safely. (See Chapter 3.) ────────────────────────────────────────────────────────────────────────── ; filename was previously ; parsed into "my_fcb" mov dx,seg my_fcb ; DS:DX = address of mov ds,dx ; file control block mov dx,offset my_fcb mov ah,0fh ; function 0fh = open int 21h or al,al ; was open successful? jnz error ; no, jump to error routine . . . my_fcb db 37 dup (0) ; file control block ────────────────────────────────────────────────────────────────────────── Figure 8-2. A typical FCB file operation. This sequence of code attempts to open the file whose name was previously parsed into the FCB named my_fcb. Note that the structures of FCBs under CP/M and MS-DOS are not identical. However, the differences lie chiefly in the reserved areas of the FCBs (which should not be manipulated by application programs in any case), so well-behaved CP/M applications should be relatively easy to port into MS-DOS. It seems, however, that few such applications exist. Many of the tricks that were played by clever CP/M programmers to increase performance or circumvent the limitations of that operating system can cause severe problems under MS-DOS, particularly in networking environments. At any rate, much better performance can be achieved by thoroughly rewriting the CP/M applications to take advantage of the superior capabilities of MS-DOS. You can use a special FCB variant called an extended file control block to create or access files with special attributes (such as hidden or read-only files), volume labels, and subdirectories. An extended FCB has a 7-byte header followed by the 37-byte structure of a normal FCB (Figure 8-3). The first byte contains 0FFH, which could never be a legal drive code and thus indicates to MS-DOS that an extended FCB is being used. The next 5 bytes are reserved and are unused in current versions of MS-DOS. The seventh byte contains the attribute of the special file type that is being accessed. (Attribute bytes are discussed in more detail in Chapter 9.) Any MS-DOS function that uses a normal FCB can also use an extended FCB. The FCB file- and record-management functions may be gathered into the following broad classifications: Byte offset 00H ┌───────────────────────────────────────────────────────┐ │ 0FFH │ Note 11 01H ├───────────────────────────────────────────────────────┤ │ Reserved (5 bytes, must be zero) │ 06H ├───────────────────────────────────────────────────────┤ │ Attribute byte │ Note 12 07H ├───────────────────────────────────────────────────────┤ │ Drive identification │ Note 1 08H ├───────────────────────────────────────────────────────┤ │ Filename (8 characters) │ Note 2 10H ├───────────────────────────────────────────────────────┤ │ Extension (3 characters) │ Note 2 13H ├───────────────────────────────────────────────────────┤ │ Current-block number │ Note 9 15H ├───────────────────────────────────────────────────────┤ │ Record size │ Note 10 17H ├───────────────────────────────────────────────────────┤ │ File size (4 bytes) │ Notes 3, 6 1BH ├───────────────────────────────────────────────────────┤ │ Date created/updated │ Note 7 1DH ├───────────────────────────────────────────────────────┤ │ Time created/updated │ Note 8 1FH ├───────────────────────────────────────────────────────┤ │ Reserved │ 27H ├───────────────────────────────────────────────────────┤ │ Current-record number │ Note 9 28H ├───────────────────────────────────────────────────────┤ │ Relative-record number (4 bytes) │ Note 5 └───────────────────────────────────────────────────────┘ Figure 8-3. Extended file control block. Total length is 44 bytes (2CH bytes). See notes on pages 133─34. ╓┌─┌────────────────────────┌────────────────────────────────────────────────╖ Function Action ────────────────────────────────────────────────────────────────────────── Common FCB file operations 0FH Open file. 10H Close file. 16H Create file. Common FCB record operations 14H Perform sequential read. 15H Perform sequential write. Function Action ────────────────────────────────────────────────────────────────────────── 15H Perform sequential write. 21H Perform random read. 22H Perform random write. 27H Perform random block read. 28H Perform random block write. Other vital FCB operations 1AH Set disk transfer address. 29H Parse filename. Less commonly used FCB file operations 13H Delete file. 17H Rename file. Less commonly used FCB record operations 23H Obtain file size. 24H Set relative-record number. ────────────────────────────────────────────────────────────────────────── Function Action ────────────────────────────────────────────────────────────────────────── Several of these functions have special properties. For example, Int 21H Functions 27H (Random Block Read) and 28H (Random Block Write) allow reading and writing of multiple records of any size and also update the random-record field automatically (unlike Int 21H Functions 21H and 22H). Int 21H Function 28H can truncate a file to any desired size, and Int 21H Function 17H used with an extended FCB can alter a volume label or rename a subdirectory. Section 2 of this book, "MS-DOS Functions Reference," gives detailed specifications for each of the FCB file and record functions, along with assembly-language examples. It is also instructive to compare the preceding groups with the corresponding groups of handle-type functions listed on pages 140─41. ────────────────────────────────────────────────────────────────────────── Notes for Figures 8-1 and 8-3 1. The drive identification is a binary number: 00=default drive, 01=drive A:, 02=drive B:, and so on. If the application program supplies the drive code as zero (default drive), MS-DOS fills in the code for the actual current disk drive after a successful open or create call. 2. File and extension names must be left justified and padded with blanks. 3. The file size, date, time, and reserved fields should not be modified by applications. 4. All word fields are stored with the least significant byte at the lower address. 5. The relative-record field is treated as 4 bytes if the record size is less than 64 bytes; otherwise, only the first 3 bytes of this field are used. 6. The file-size field is in the same format as in the directory, with the less significant word at the lower address. 7. The date field is mapped as in the directory. Viewed as a 16-bit word (as it would appear in a register), the field is broken down as follows: F E D C B A 9 8 7 6 5 4 3 2 1 0 ┌─────────────────────┬─────────────────────┬─────────────────────┐ │ Year │ Month │ Day │ └─────────────────────┴─────────────────────┴─────────────────────┘ Bits Contents ──────────────────────────────────────────────────────────────────────── 00H─04H Day (1─31) 05H─08H Month (1─12) 09H─0FH Year, relative to 1980 ──────────────────────────────────────────────────────────────────────── 8. The time field is mapped as in the directory. Viewed as a 16-bit word (as it would appear in a register), the field is broken down as follows: F E D C B A 9 8 7 6 5 4 3 2 1 0 ┌───────────────────┬───────────────────────┬─────────────────────┐ │ Hours │ Minutes │ 2-second increments │ └───────────────────┴───────────────────────┴─────────────────────┘ Bits Contents ──────────────────────────────────────────────────────────────────────── 00H─04H 2-second increments (0─29) 05H─0AH Minutes (0─59) 0BH─0FH Hours (0─23) ──────────────────────────────────────────────────────────────────────── 9. The current-block and current-record numbers are used together on sequential reads and writes. This simulates the behavior of CP/M. 10. The Int 21H open (0FH) and create (16H) functions set the record-size field to 128 bytes, to provide compatibility with CP/M. If you use another record size, you must fill it in after the open or create operation. 11. An 0FFH (255) in the first byte of the structure signifies that it is an extended file control block. You can use extended FCBs with any of the functions that accept an ordinary FCB. (See also note 12.) 12. The attribute byte in an extended FCB allows access to files with the special characteristics hidden, system, or read-only. You can also use extended FCBs to read volume labels and the contents of special subdirectory files. ────────────────────────────────────────────────────────────────────────── FCB File-Access Skeleton The following is a typical program sequence to access a file using the FCB, or traditional, functions (Figure 8-4): 1. Zero out the prospective FCB. 2. Obtain the filename from the user, from the default FCBs, or from the command tail in the PSP. 3. If the filename was not obtained from one of the default FCBs, parse the filename into the new FCB using Int 21H Function 29H. 4. Open the file (Int 21H Function 0FH) or, if writing new data only, create the file or truncate any existing file of the same name to zero length (Int 21H Function 16H). 5. Set the record-size field in the FCB, unless you are using the default record size. Recall that it is important to do this after a successful open or create operation. (See Figure 8-5.) 6. Set the relative-record field in the FCB if you are performing random record I/O. 7. Set the disk transfer area address using Int 21H Function 1AH, unless the buffer address has not been changed since the last call to this function. If the application never performs a set DTA, the DTA address defaults to offset 0080H in the PSP. 8. Request the needed read- or write-record operation (Int 21H Function 14H─Sequential Read, 15H─Sequential Write, 21H─Random Read, 22H─Random Write, 27H─Random Block Read, 28H─Random Block Write). 9. If the program is not finished processing the file, go to step 6; otherwise, close the file (Int 21H Function 10H). If the file was used for reading only, you can skip the close operation under early versions of MS-DOS. However, this shortcut can cause problems under MS-DOS versions 3.0 and later, especially when the files are being accessed across a network. ────────────────────────────────────────────────────────────────────────── recsize equ 1024 ; file record size . . . mov ah,29h ; parse input filename mov al,1 ; skip leading blanks mov si,offset fname1 ; address of filename mov di,offset fcb1 ; address of FCB int 21h or al,al ; jump if name jnz name_err ; was bad . . . mov ah,29h ; parse output filename mov al,1 ; skip leading blanks mov si,offset fname2 ; address of filename mov di,offset fcb2 ; address of FCB int 21h or al,al ; jump if name jnz name_err ; was bad . . . mov ah,0fh ; open input file mov dx,offset fcb1 int 21h or al,al ; open successful? jnz no_file ; no, jump . . . mov ah,16h ; create and open mov dx,offset fcb2 ; output file int 21h or al,al ; create successful? jnz disk_full ; no, jump . . . ; set record sizes mov word ptr fcb1+0eh,recsize mov word ptr fcb2+0eh,recsize . . . mov ah,1ah ; set disk transfer mov dx,offset buffer ; address for reads int 21h ; and writes . next: . ; process next record . mov ah,14h ; sequential read from mov dx,offset fcb1 ; input file int 21h cmp al,01 ; check for end of file je file_end ; jump if end of file cmp al,03 je file_end ; jump if end of file or al,al ; other read fault? jnz bad_read ; jump if bad read . . . mov ah,15h ; sequential write to mov dx,offset fcb2 ; output file int 21h or al,al ; write successful? jnz bad_write ; jump if write failed . . . jmp next ; process next record . file_end: . ; reached end of input . mov ah,10h ; close input file mov dx,offset fcb1 int 21h . . . mov ah,10h ; close output file mov dx,offset fcb2 int 21h . . . mov ax,4c00h ; exit with return int 21h ; code of zero . . . fname1 db 'OLDFILE.DAT',0 ; name of input file fname2 db 'NEWFILE.DAT',0 ; name of output file fcb1 db 37 dup (0) ; FCB for input file fcb2 db 37 dup (0) ; FCB for output file buffer db recsize dup (?) ; buffer for file I/O ────────────────────────────────────────────────────────────────────────── Figure 8-4. Skeleton of an assembly-language program that performs file and record I/O using the FCB family of functions. Byte Offset FCB before open FCB contents FCB after open ┌────────────────────┬────────────────────┬────────────────────┐ 00H │ 00 │ Drive │ 03 │ ├────────────────────┼────────────────────┼────────────────────┤ 01H │ 4D │ │ 4D │ 02H │ 59 │ │ 59 │ 03H │ 46 │ │ 46 │ 04H │ 49 │ Filename │ 49 │ 05H │ 4C │ │ 4C │ 06H │ 45 │ │ 45 │ 07H │ 20 │ │ 20 │ 08H │ 20 │ │ 20 │ ├────────────────────┼────────────────────┼────────────────────┤ 09H │ 44 │ │ 44 │ 0AH │ 41 │ Extension │ 41 │ 0BH │ 54 │ │ 54 │ ├────────────────────┼────────────────────┼────────────────────┤ 0CH │ 00 │ │ 00 │ 0DH │ 00 │ Current block │ 00 │ ├────────────────────┼────────────────────┼────────────────────┤ 0EH │ 00 │ │ 80 │ 0FH │ 00 │ Record size │ 00 │ ├────────────────────┼────────────────────┼────────────────────┤ 10H │ 00 │ │ 80 │ 11H │ 00 │ │ 3D │ 12H │ 00 │ File size │ 00 │ 13H │ 00 │ │ 00 │ ├────────────────────┼────────────────────┼────────────────────┤ 14H │ 00 │ │ 43 │ 15H │ 00 │ File date │ 0B │ ├────────────────────┼────────────────────┼────────────────────┤ 16H │ 00 │ │ A1 │ 17H │ 00 │ File time │ 52 │ ├────────────────────┼────────────────────┼────────────────────┤ 18H │ 00 │ │ 03 │ 19H │ 00 │ │ 02 │ 1AH │ 00 │ │ 42 │ 1BH │ 00 │ │ 73 │ 1CH │ 00 │ Reserved │ 00 │ 1DH │ 00 │ │ 01 │ 1EH │ 00 │ │ 35 │ 1FH │ 00 │ │ 0F │ ├────────────────────┼────────────────────┼────────────────────┤ 20H │ 00 │ Current record │ 00 │ ├────────────────────┼────────────────────┼────────────────────┤ 21H │ 00 │ │ 00 │ 22H │ 00 │ Relative-record │ 00 │ 23H │ 00 │ number │ 00 │ 24H │ 00 │ │ 00 │ └────────────────────┴────────────────────┴────────────────────┘ Figure 8-5. A typical file control block before and after a successful open call (Int 21H Function 0FH). Points to Remember Here is a summary of the pros and cons of using the FCB-related file and record functions in your programs. Advantages: ■ Under MS-DOS versions 1 and 2, the number of files that can be open concurrently when using FCBs is unlimited. (This is not true under MS-DOS versions 3.0 and later, especially if networking software is running.) ■ File-access methods using FCBs are familiar to programmers with a CP/M background, and well-behaved CP/M applications require little change in logical flow to run under MS-DOS. ■ MS-DOS supplies the size, time, and date for a file to its FCB after the file is opened. The calling program can inspect this information. Disadvantages: ■ FCBs take up room in the application program's memory space. ■ FCBs offer no support for the hierarchical file structure (no access to files outside the current directory). ■ FCBs provide no support for file locking/sharing or record locking in networking environments. ■ In addition to the read or write call itself, file reads or writes using FCBs require manipulation of the FCB to set record size and record number, plus a previous call to a separate MS-DOS function to set the DTA address. ■ Random record I/O using FCBs for a file containing variable-length records is very clumsy and inconvenient. ■ You must use extended FCBs, which are incompatible with CP/M anyway, to access or create files with special attributes such as hidden, read-only, or system. ■ The FCB file functions have poor error reporting. This situation has been improved somewhat in MS-DOS version 3 because a program can call the added Int 21H Function 59H (Get Extended Error Information) after a failed FCB function to obtain additional information. ■ Microsoft discourages use of FCBs. FCBs will make your program more difficult to port to MS OS/2 later because MS OS/2 does not support FCBs in protected mode at all. Using the Handle Functions The handle file- and record-management functions access files in a fashion similar to that used under the UNIX/XENIX operating system. Files are designated by an ASCIIZ string (an ASCII character string terminated by a null, or zero, byte) that can contain a drive designator, path, filename, and extension. For example, the file specification C:\SYSTEM\COMMAND.COM would appear in memory as the following sequence of bytes: 43 3A 5C 53 59 53 54 45 4D 5C 43 4F 4D 4D 41 4E 44 2E 43 4F 4D 00 When a program wishes to open or create a file, it passes the address of the ASCIIZ string specifying the file to MS-DOS in the DS:DX registers (Figure 8-6). If the operation is successful, MS-DOS returns a 16-bit handle to the program in the AX register. The program must save this handle for further reference. ────────────────────────────────────────────────────────────────────────── mov ah,3dh ; function 3dh = open mov al,2 ; mode 2 = read/write mov dx,seg filename ; address of ASCIIZ mov ds,dx ; file specification mov dx,offset filename int 21h ; request open from DOS jc error ; jump if open failed mov handle,ax ; save file handle . . . filename db 'C:\MYDIR\MYFILE.DAT',0 ; filename handle dw 0 ; file handle ────────────────────────────────────────────────────────────────────────── Figure 8-6. A typical handle file operation. This sequence of code attempts to open the file designated in the ASCIIZ string whose address is passed to MS-DOS in the DS:DX registers. When the program requests subsequent operations on the file, it usually places the handle in the BX register before the call to MS-DOS. All the handle functions return with the CPU's carry flag cleared if the operation was successful, or set if the operation failed; in the latter case, the AX register contains a code describing the failure. MS-DOS restricts the number of handles that can be active at any one time──that is, the number of files and devices that can be open concurrently when using the handle family of functions──in two different ways: ■ The maximum number of concurrently open files in the system, for all active processes combined, is specified by the entry FILES=nn in the CONFIG.SYS file. This entry determines the number of entries to be allocated in the system file table; under MS-DOS version 3, the default value is 8 and the maximum is 255. After MS-DOS is booted and running, you cannot expand this table to increase the total number of files that can be open. You must use an editor to modify the CONFIG.SYS file and then restart the system. ■ The maximum number of concurrently open files for a single process is 20, assuming that sufficient entries are also available in the system file table. When a program is loaded, MS-DOS preassigns 5 of its potential 20 handles to the standard devices. Each time the process issues an open or create call, MS-DOS assigns a handle from the process's private allocation of 20, until all the handles are used up or the system file table is full. In MS-DOS versions 3.3 and later, you can expand the per-process limit of 20 handles with a call to Int 21H Function 67H (Set Handle Count). The handle file- and record-management calls may be gathered into the following broad classifications for study: ╓┌─┌────────────────────────┌────────────────────────────────────────────────╖ Function Action Function Action ────────────────────────────────────────────────────────────────────────── Common handle file operations 3CH Create file (requires ASCIIZ string). 3DH Open file (requires ASCIIZ string). 3EH Close file. Common handle record operations 42H Set file pointer (also used to find file size). 3FH Read file. 40H Write file. Less commonly used handle operations 41H Delete file. 43H Get or modify file attributes. 44H IOCTL (I/O Control). 45H Duplicate handle. 46H Redirect handle. 56H Rename file. 57H Get or set file date and time. 5AH Create temporary file (versions 3.0 and later). Function Action ────────────────────────────────────────────────────────────────────────── 5AH Create temporary file (versions 3.0 and later). 5BH Create file (fails if file already exists; versions 3.0 and later). 5CH Lock or unlock file region (versions 3.0 and later). 67H Set handle count (versions 3.3 and later). 68H Commit file (versions 3.3 and later). 6CH Extended open file (version 4). ────────────────────────────────────────────────────────────────────────── Compare the groups of handle-type functions in the preceding table with the groups of FCB functions outlined earlier, noting the degree of functional overlap. Section 2 of this book, "MS-DOS Functions Reference," gives detailed specifications for each of the handle functions, along with assembly-language examples. Handle File-Access Skeleton The following is a typical program sequence to access a file using the handle family of functions (Figure 8-7): 1. Get the filename from the user by means of the buffered input service (Int 21H Function 0AH) or from the command tail supplied by MS-DOS in the PSP. 2. Put a zero at the end of the file specification in order to create an ASCIIZ string. 3. Open the file using Int 21H Function 3DH and mode 2 (read/write access), or create the file using Int 21H Function 3CH. (Be sure to set the CX register to zero, so that you don't accidentally make a file with special attributes.) Save the handle that is returned. 4. Set the file pointer using Int 21H Function 42H. You may set the file-pointer position relative to one of three different locations: the start of the file, the current pointer position, or the end of the file. If you are performing sequential record I/O, you can usually skip this step because MS-DOS will maintain the file pointer for you automatically. 5. Read from the file (Int 21H Function 3FH) or write to the file (Int 21H Function 40H). Both of these functions require that the BX register contain the file's handle, the CX register contain the length of the record, and the DS:DX registers point to the data being transferred. Both return the actual number of bytes transferred in the AX register. In a read operation, if the number of bytes read is less than the number requested, the end of the file has been reached. In a write operation, if the number of bytes written is less than the number requested, the disk containing the file is full. Neither of these conditions is returned as an error code; that is, the carry flag is not set. 6. If the program is not finished processing the file, go to step 4; otherwise, close the file (Int 21H Function 3EH). Any normal exit from the program will also close all active handles. ────────────────────────────────────────────────────────────────────────── recsize equ 1024 ; file record size . . . mov ah,3dh ; open input file mov al,0 ; mode = read only mov dx,offset fname1 ; name of input file int 21h jc no_file ; jump if no file mov handle1,ax ; save token for file . . . mov ah,3ch ; create output file mov cx,0 ; attribute = normal mov dx,offset fname2 ; name of output file int 21h jc disk_full ; jump if create fails mov handle2,ax ; save token for file . next: . ; process next record . mov ah,3fh ; sequential read from mov bx,handle1 ; input file mov cx,recsize mov dx,offset buffer int 21h jc bad_read ; jump if read error or ax,ax ; check bytes transferred jz file_end ; jump if end of file . . . mov ah,40h ; sequential write to mov bx,handle2 ; output file mov cx,recsize mov dx,offset buffer int 21h jc bad_write ; jump if write error cmp ax,recsize ; whole record written? jne disk_full ; jump if disk is full . . . jmp next ; process next record . file_end: . ; reached end of input . mov ah,3eh ; close input file mov bx,handle1 int 21h . . . mov ah,3eh ; close output file mov bx,handle2 int 21h . . . mov ax,4c00h ; exit with return int 21h ; code of zero . . . fname1 db 'OLDFILE.DAT',0 ; name of input file fname2 db 'NEWFILE.DAT',0 ; name of output file handle1 dw 0 ; token for input file handle2 dw 0 ; token for output file buffer db recsize dup (?) ; buffer for file I/O ────────────────────────────────────────────────────────────────────────── Figure 8-7. Skeleton of an assembly-language program that performs sequential processing on an input file and writes the results to an output file using the handle file and record functions. This code assumes that the DS and ES registers have already been set to point to the segment containing the buffers and filenames. Points to Remember Here is a summary of the pros and cons of using the handle file and record operations in your program. Compare this list with the one given earlier in the chapter for the FCB family of functions. Advantages: ■ The handle calls provide direct support for I/O redirection and pipes with the standard input and output devices in a manner functionally similar to that used by UNIX/XENIX. ■ The handle functions provide direct support for directories (the hierarchical file structure) and special file attributes. ■ The handle calls support file sharing/locking and record locking in networking environments. ■ Using the handle functions, the programmer can open channels to character devices and treat them as files. ■ The handle calls make the use of random record access extremely easy. The current file pointer can be moved to any byte offset relative to the start of the file, the end of the file, or the current pointer position. Records of any length, up to an entire segment (65,535 bytes), can be read to any memory address in one operation. ■ The handle functions have relatively good error reporting in MS-DOS version 2, and error reporting has been enhanced even further in MS-DOS versions 3.0 and later. ■ Microsoft strongly encourages use of the handle family of functions in order to provide upward compatibility with MS OS/2. Disadvantages: ■ There is a limit per program of 20 concurrently open files and devices using handles in MS-DOS versions 2.0 through 3.2. ■ Minor gaps still exist in the implementation of the handle functions. For example, you must still use extended FCBs to change volume labels and to access the contents of the special files that implement directories. MS-DOS Error Codes When one of the handle file functions fails with the carry flag set, or when a program calls Int 21H Function 59H (Get Extended Error Information) following a failed FCB function or other system service, one of the following error codes may be returned: ╓┌─┌────────────────────────┌────────────────────────────────────────────────╖ Value Meaning ────────────────────────────────────────────────────────────────────────── MS-DOS version 2 error codes 01H Function number invalid 02H File not found 03H Path not found 04H Too many open files 05H Access denied 06H Handle invalid 07H Memory control blocks destroyed 08H Insufficient memory 09H Memory block address invalid 0AH (10) Environment invalid 0BH (11) Format invalid 0CH (12) Access code invalid 0DH (13) Data invalid 0EH (14) Unknown unit Value Meaning ────────────────────────────────────────────────────────────────────────── 0EH (14) Unknown unit 0FH (15) Disk drive invalid 10H (16) Attempted to remove current directory 11H (17) Not same device 12H (18) No more files Mappings to critical-error codes 13H (19) Write-protected disk 14H (20) Unknown unit 15H (21) Drive not ready 16H (22) Unknown command 17H (23) Data error (CRC) 18H (24) Bad request-structure length 19H (25) Seek error 1AH (26) Unknown media type 1BH (27) Sector not found 1CH (28) Printer out of paper 1DH (29) Write fault 1EH (30) Read fault Value Meaning ────────────────────────────────────────────────────────────────────────── 1EH (30) Read fault 1FH (31) General failure MS-DOS version 3 and later extended error codes 20H (32) Sharing violation 21H (33) File-lock violation 22H (34) Disk change invalid 23H (35) FCB unavailable 24H (36) Sharing buffer exceeded 25H─31H (37─49) Reserved 32H (50) Unsupported network request 33H (51) Remote machine not listening 34H (52) Duplicate name on network 35H (53) Network name not found 36H (54) Network busy 37H (55) Device no longer exists on network 38H (56) NetBIOS command limit exceeded 39H (57) Error in network adapter hardware 3AH (58) Incorrect response from network Value Meaning ────────────────────────────────────────────────────────────────────────── 3AH (58) Incorrect response from network 3BH (59) Unexpected network error 3CH (60) Remote adapter incompatible 3DH (61) Print queue full 3EH (62) Not enough room for print file 3FH (63) Print file was deleted 40H (64) Network name deleted 41H (65) Network access denied 42H (66) Incorrect network device type 43H (67) Network name not found 44H (68) Network name limit exceeded 45H (69) NetBIOS session limit exceeded 46H (70) Temporary pause 47H (71) Network request not accepted 48H (72) Print or disk redirection paused 49H─4FH (73─79) Reserved 50H (80) File already exists 51H (81) Reserved 52H (82) Cannot make directory Value Meaning ────────────────────────────────────────────────────────────────────────── 52H (82) Cannot make directory 53H (83) Fail on Int 24H (critical error) 54H (84) Too many redirections 55H (85) Duplicate redirection 56H (86) Invalid password 57H (87) Invalid parameter 58H (88) Net write fault ────────────────────────────────────────────────────────────────────────── Under MS-DOS versions 3.0 and later, you can also use Int 21H Function 59H to obtain other information about the error, such as the error locus and the recommended recovery action. Critical-Error Handlers In Chapter 5, we discussed how an application program can take over the Ctrl-C handler vector (Int 23H) and replace the MS-DOS default handler, to avoid losing control of the computer when the user enters a Ctrl-C or Ctrl-Break at the keyboard. Similarly, MS-DOS provides a critical-error-handler vector (Int 24H) that defines the routine to be called when unrecoverable hardware faults occur. The default MS-DOS critical-error handler is the routine that displays a message describing the error type and the cue Abort, Retry, Ignore? This message appears after such actions as the following: ■ Attempting to open a file on a disk drive that doesn't contain a floppy disk or whose door isn't closed ■ Trying to read a disk sector that contains a CRC error ■ Trying to print when the printer is off line The unpleasant thing about MS-DOS's default critical-error handler is, of course, that if the user enters an A for Abort, the application that is currently executing is terminated abruptly and never has a chance to clean up and make a graceful exit. Intermediate files may be left on the disk, files that have been extended using FCBs are not properly closed so that the directory is updated, interrupt vectors may be left pointing into the transient program area, and so forth. To write a truly bombproof MS-DOS application, you must take over the critical-error-handler vector and point it to your own routine, so that your program intercepts all catastrophic hardware errors and handles them appropriately. You can use MS-DOS Int 21H Function 25H to alter the Int 24H vector in a well-behaved manner. When your application exits, MS-DOS will automatically restore the previous contents of the Int 24H vector from information saved in the program segment prefix. MS-DOS calls the critical-error handler for two general classes of errors── disk-related and non-disk-related──and passes different information to the handler in the registers for each of these classes. For disk-related errors, MS-DOS sets the registers as shown on the following page. (Bits 3─5 of the AH register are relevant only in MS-DOS versions 3.1 and later.) ╓┌─┌──────────────────┌─────────────────┌────────────────────────────────────╖ Register Bit(s) Significance ────────────────────────────────────────────────────────────────────────── AH 7 0, to signify disk error 6 Reserved 5 0 = ignore response not allowed 1 = ignore response allowed 4 0 = retry response not allowed 1 = retry response allowed 3 0 = fail response not allowed 1 = fail response allowed 1─2 Area where disk error occurred 00 = MS-DOS area 01 = file allocation table 10 = root directory 11 = files area 0 0 = read operation 1 = write operation AL 0─7 Drive code (0 = A, 1 = B, and so forth) DI 0─7 Driver error code 8─15 Not used Register Bit(s) Significance ────────────────────────────────────────────────────────────────────────── 8─15 Not used BP:SI Segment:offset of device-driver header ────────────────────────────────────────────────────────────────────────── For non-disk-related errors, the interrupt was generated either as the result of a character-device error or because a corrupted memory image of the file allocation table was detected. In this case, MS-DOS sets the registers as follows: Register Bit(s) Significance ────────────────────────────────────────────────────────────────────────── AH 7 1, to signify a non-disk error DI 0─7 Driver error code 8─15 Not used BP:SI Segment:offset of device-driver header ────────────────────────────────────────────────────────────────────────── To determine whether the critical error was caused by a character device, use the address in the BP:SI registers to examine the device attribute word at offset 0004H in the presumed device-driver header. If bit 15 is set, then the error was indeed caused by a character device, and the program can inspect the name field of the driver's header to determine the device. At entry to a critical-error handler, MS-DOS has already disabled interrupts and set up the stack as shown in Figure 8-8. A critical-error handler cannot use any MS-DOS services except Int 21H Functions 01H through 0CH (Traditional Character I/O), Int 21H Function 30H (Get MS-DOS Version), and Int 21H Function 59H (Get Extended Error Information). These functions use a special stack so that the context of the original function (which generated the critical error) will not be lost. ┌───────┐─┐ │ Flags │ │ ├───────┤ │ Flags and CS:IP pushed │ CS │ ├─ on stack by original ├───────┤ │ Int 21H call │ IP │ │ ├───────┤═╡─SS:SP on entry to │ ES │ │ Int 21H handler ├───────┤ │ │ DS │ │ ├───────┤ │ │ BP │ │ ├───────┤ │ │ DI │ │ ├───────┤ ├─ Registers at point of │ SI │ │ original Int 21H call ├───────┤ │ │ DX │ │ ├───────┤ │ │ CX │ │ ├───────┤ │ │ BX │ │ ├───────┤ │ │ AX │ │ ├───────┤═╡ │ Flags │ │ ├───────┤ │ │ CS │ ├─ Return address for ├───────┤ │ Int 24H handler │ IP │ │ └──────┘─┘ └───── SS:SP on entry to Int 24H handler Figure 8-8. The stack at entry to a critical-error handler. The critical-error handler should return to MS-DOS by executing an IRET, passing one of the following action codes in the AL register: Code Meaning ────────────────────────────────────────────────────────────────────────── 0 Ignore the error (MS-DOS acts as though the original function call had succeeded). 1 Retry the operation. 2 Terminate the process that encountered the error. 3 Fail the function (an error code is returned to the requesting process). Versions 3.1 and later only. ────────────────────────────────────────────────────────────────────────── The critical-error handler should preserve all other registers and must not modify the device-driver header pointed to by BP:SI. A skeleton example of a critical-error handler is shown in Figure 8-9. ────────────────────────────────────────────────────────────────────────── ; prompt message used by ; critical-error handler prompt db cr,lf,'Critical Error Occurred: ' db 'Abort, Retry, Ignore, Fail? $' keys db 'aArRiIfF' ; possible user response keys keys_len equ $-keys ; (both cases of each allowed) codes db 2,2,1,1,0,0,3,3 ; codes returned to MS-DOS kernel ; for corresponding response keys ; ; This code is executed during program's initialization ; to install the new critical-error handler. ; . . . push ds ; save our data segment mov dx,seg int24 ; DS:DX = handler address mov ds,dx mov dx,offset int24 mov ax,2524h ; function 25h = set vector int 21h ; transfer to MS-DOS pop ds ; restore data segment . . . ; ; This is the replacement critical-error handler. It ; prompts the user for Abort, Retry, Ignore, or Fail, and ; returns the appropriate code to the MS-DOS kernel. ; int24 proc far ; entered from MS-DOS kernel push bx ; save registers push cx push dx push si push di push bp push ds push es int24a: mov ax,seg prompt ; display prompt for user mov ds,ax ; using function 9 (print string mov es,ax ; terminated by $ character) mov dx,offset prompt mov ah,9 int 21h mov ah,1 ; get user's response int 21h ; function 1 = read one character mov di,offset keys ; look up code for response key mov cx,keys_len cld repne scasb jnz int24a ; prompt again if bad response ; set AL = action code for MS-DOS ; according to key that was entered: ; 0 = ignore, 1 = retry, 2 = abort, ; 3 = fail mov al,[di+keys_len-1] pop es ; restore registers pop ds pop bp pop di pop si pop dx pop cx pop bx iret ; exit critical-error handler int24 endp ────────────────────────────────────────────────────────────────────────── Figure 8-9. A skeleton example of a replacement critical-error handler. Example Programs: DUMP.ASM and DUMP.C The programs DUMP.ASM (Figure 8-10) and DUMP.C (Figure 8-11) are parallel examples of the use of the handle file and record functions. The assembly-language version, in particular, illustrates features of a well-behaved MS-DOS utility: ■ The program checks the version of MS-DOS to ensure that all the functions it is going to use are really available. ■ The program parses the drive, path, and filename from the command tail in the program segment prefix. ■ The program uses buffered I/O for speed. ■ The program sends error messages to the standard error device. ■ The program sends normal program output to the standard output device, so that the dump output appears by default on the system console but can be redirected to other character devices (such as the line printer) or to a file. The same features are incorporated into the C version of the program, but some of them are taken care of behind the scenes by the C runtime library. ────────────────────────────────────────────────────────────────────────── name dump page 55,132 title DUMP--display file contents ; ; DUMP--Display contents of file in hex and ASCII ; ; Build: C>MASM DUMP; ; C>LINK DUMP; ; ; Usage: C>DUMP unit:\path\filename.exe [ >device ] ; ; Copyright (C) 1988 Ray Duncan ; cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII line feed tab equ 09h ; ASCII tab code blank equ 20h ; ASCII space code cmd equ 80h ; buffer for command tail blksize equ 16 ; input file record size stdin equ 0 ; standard input handle stdout equ 1 ; standard output handle stderr equ 2 ; standard error handle _TEXT segment word public 'CODE' assume cs:_TEXT,ds:_DATA,es:_DATA,ss:STACK dump proc far ; entry point from MS-DOS push ds ; save DS:0000 for final xor ax,ax ; return to MS-DOS, in case push ax ; function 4ch can't be used mov ax,_DATA ; make our data segment mov ds,ax ; addressable via DS register ; check MS-DOS version mov ax,3000h ; function 30h = get version int 21h ; transfer to MS-DOS cmp al,2 ; major version 2 or later? jae dump1 ; yes, proceed ; if MS-DOS 1.x, display ; error message and exit mov dx,offset msg3 ; DS:DX = message address mov ah,9 ; function 9 = print string int 21h ; transfer to MS-DOS ret ; then exit the old way dump1: ; check if filename present mov bx,offset cmd ; ES:BX = command tail call argc ; count command arguments cmp ax,2 ; are there 2 arguments? je dump2 ; yes, proceed ; missing filename, display ; error message and exit mov dx,offset msg2 ; DS:DX = message address mov cx,msg2_len ; CX = message length jmp dump9 ; go display it dump2: ; get address of filename mov ax,1 ; AX = argument number ; ES:BX still = command tail call argv ; returns ES:BX = address, ; and AX = length mov di,offset fname ; copy filename to buffer mov cx,ax ; CX = length dump3: mov al,es:[bx] ; copy one byte mov [di],al inc bx ; bump string pointers inc di loop dump3 ; loop until string done mov byte ptr [di],0 ; add terminal null byte mov ax,ds ; make our data segment mov es,ax ; addressable by ES too ; now open the file mov ax,3d00h ; function 3dh = open file ; mode 0 = read only mov dx,offset fname ; DS:DX = filename int 21h ; transfer to MS-DOS jnc dump4 ; jump, open successful ; open failed, display ; error message and exit mov dx,offset msg1 ; DS:DX = message address mov cx,msg1_len ; CX = message length jmp dump9 ; go display it dump4: mov fhandle,ax ; save file handle dump5: ; read block of file data mov bx,fhandle ; BX = file handle mov cx,blksize ; CX = record length mov dx,offset fbuff ; DS:DX = buffer mov ah,3fh ; function 3fh = read int 21h ; transfer to MS-DOS mov flen,ax ; save actual length cmp ax,0 ; end of file reached? jne dump6 ; no, proceed cmp word ptr fptr,0 ; was this the first read? jne dump8 ; no, exit normally ; display empty file ; message and exit mov dx,offset msg4 ; DS:DX = message address mov cx,msg4_len ; CX = length jmp dump9 ; go display it dump6: ; display heading at ; each 128-byte boundary test fptr,07fh ; time for a heading? jnz dump7 ; no, proceed ; display a heading mov dx,offset hdg ; DS:DX = heading address mov cx,hdg_len ; CX = heading length mov bx,stdout ; BX = standard output mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS dump7: call conv ; convert binary record ; to formatted ASCII ; display formatted output mov dx,offset fout ; DX:DX = output address mov cx,fout_len ; CX = output length mov bx,stdout ; BX = standard output mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS jmp dump5 ; go get another record dump8: ; close input file mov bx,fhandle ; BX = file handle mov ah,3eh ; function 3eh = close int 21h ; transfer to MS-DOS mov ax,4c00h ; function 4ch = terminate, ; return code = 0 int 21h ; transfer to MS-DOS dump9: ; display message on ; standard error device ; DS:DX = message address ; CX = message length mov bx,stderr ; standard error handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS mov ax,4c01h ; function 4ch = terminate, ; return code = 1 int 21h ; transfer to MS-DOS dump endp conv proc near ; convert block of data ; from input file mov di,offset fout ; clear output format mov cx,fout_len-2 ; area to blanks mov al,blank rep stosb mov di,offset fout ; convert file offset mov ax,fptr ; to ASCII for output call w2a mov bx,0 ; init buffer pointer conv1: mov al,[fbuff+bx] ; fetch byte from buffer mov di,offset foutb ; point to output area ; format ASCII part... ; store '.' as default mov byte ptr [di+bx],'.' cmp al,blank ; in range 20h-7eh? jb conv2 ; jump, not alphanumeric cmp al,7eh ; in range 20h-7eh? ja conv2 ; jump, not alphanumeric mov [di+bx],al ; store ASCII character conv2: ; format hex part... mov di,offset fouta ; point to output area add di,bx ; base addr + (offset*3) add di,bx add di,bx call b2a ; convert byte to hex inc bx ; advance through record cmp bx,flen ; entire record converted? jne conv1 ; no, get another byte ; update file pointer add word ptr fptr,blksize ret conv endp w2a proc near ; convert word to hex ASCII ; call with AX = value ; DI = addr for string ; returns AX, DI, CX destroyed push ax ; save copy of value mov al,ah call b2a ; convert upper byte pop ax ; get back copy call b2a ; convert lower byte ret w2a endp b2a proc near ; convert byte to hex ASCII ; call with AL = binary value ; DI = addr for string ; returns AX, DI, CX modified sub ah,ah ; clear upper byte mov cl,16 div cl ; divide byte by 16 call ascii ; quotient becomes the first stosb ; ASCII character mov al,ah call ascii ; remainder becomes the stosb ; second ASCII character ret b2a endp ascii proc near ; convert value 0-0fh in AL ; into "hex ASCII" character add al,'0' ; offset to range 0-9 cmp al,'9' ; is it > 9? jle ascii2 ; no, jump add al,'A'-'9'-1 ; offset to range A-F, ascii2: ret ; return AL = ASCII char ascii endp argc proc near ; count command-line arguments ; call with ES:BX = command line ; returns AX = argument count push bx ; save original BX and CX push cx ; for later mov ax,1 ; force count >= 1 argc1: mov cx,-1 ; set flag = outside argument argc2: inc bx ; point to next character cmp byte ptr es:[bx],cr je argc3 ; exit if carriage return cmp byte ptr es:[bx],blank je argc1 ; outside argument if ASCII blank cmp byte ptr es:[bx],tab je argc1 ; outside argument if ASCII tab ; otherwise not blank or tab, jcxz argc2 ; jump if already inside argument inc ax ; else found argument, count it not cx ; set flag = inside argument jmp argc2 ; and look at next character argc3: pop cx ; restore original BX and CX pop bx ret ; return AX = argument count argc endp argv proc near ; get address & length of ; command line argument ; call with ES:BX = command line ; AX = argument # ; returns ES:BX = address ; AX = length push cx ; save original CX and DI push di or ax,ax ; is it argument 0? jz argv8 ; yes, jump to get program name xor ah,ah ; initialize argument counter argv1: mov cx,-1 ; set flag = outside argument argv2: inc bx ; point to next character cmp byte ptr es:[bx],cr je argv7 ; exit if carriage return cmp byte ptr es:[bx],blank je argv1 ; outside argument if ASCII blank cmp byte ptr es:[bx],tab je argv1 ; outside argument if ASCII tab ; if not blank or tab... jcxz argv2 ; jump if already inside argument inc ah ; else count arguments found cmp ah,al ; is this the one we're looking for? je argv4 ; yes, go find its length not cx ; no, set flag = inside argument jmp argv2 ; and look at next character argv4: ; found desired argument, now ; determine its length... mov ax,bx ; save param starting address argv5: inc bx ; point to next character cmp byte ptr es:[bx],cr je argv6 ; found end if carriage return cmp byte ptr es:[bx],blank je argv6 ; found end if ASCII blank cmp byte ptr es:[bx],tab jne argv5 ; found end if ASCII tab argv6: xchg bx,ax ; set ES:BX = argument address sub ax,bx ; and AX = argument length jmp argvx ; return to caller argv7: xor ax,ax ; set AX = 0, argument not found jmp argvx ; return to caller argv8: ; special handling for argv = 0 mov ax,3000h ; check if DOS 3.0 or later int 21h ; (force AL = 0 in case DOS 1) cmp al,3 jb argv7 ; DOS 1 or 2, return null param mov es,es:[2ch] ; get environment segment from PSP xor di,di ; find the program name by xor al,al ; first skipping over all the mov cx,-1 ; environment variables... cld argv9: repne scasb ; scan for double null (can't use scasb ; SCASW since might be odd addr) jne argv9 ; loop if it was a single null add di,2 ; skip count word in environment mov bx,di ; save program name address mov cx,-1 ; now find its length... repne scasb ; scan for another null byte not cx ; convert CX to length dec cx mov ax,cx ; return length in AX argvx: ; common exit point pop di ; restore original CX and DI pop cx ret ; return to caller argv endp _TEXT ends _DATA segment word public 'DATA' fname db 64 dup (0) ; buffer for input filespec fhandle dw 0 ; token from PCDOS for input file flen dw 0 ; actual length read fptr dw 0 ; relative address in file fbuff db blksize dup (?) ; data from input file fout db 'nnnn' ; formatted output area db blank,blank fouta db 16 dup ('nn',blank) db blank foutb db 16 dup (blank),cr,lf fout_len equ $-fout hdg db cr,lf ; heading for each 128 bytes db 7 dup (blank) ; of formatted output db '0 1 2 3 4 5 6 7 ' db '8 9 A B C D E F',cr,lf hdg_len equ $-hdg msg1 db cr,lf db 'dump: file not found' db cr,lf msg1_len equ $-msg1 msg2 db cr,lf db 'dump: missing file name' db cr,lf msg2_len equ $-msg2 msg3 db cr,lf db 'dump: wrong MS-DOS version' db cr,lf,'$' msg4 db cr,lf db 'dump: empty file' db cr,lf msg4_len equ $-msg4 _DATA ends STACK segment para stack 'STACK' db 64 dup (?) STACK ends end dump ────────────────────────────────────────────────────────────────────────── Figure 8-10. The assembly-language version: DUMP.ASM. ────────────────────────────────────────────────────────────────────────── /* DUMP.C Displays the binary contents of a file in hex and ASCII on the standard output device. Compile: C>CL DUMP.C Usage: C>DUMP unit:path\filename.ext Copyright (C) 1988 Ray Duncan */ #include <stdio.h> #include <io.h> #include <fcntl.h> #define REC_SIZE 16 /* input file record size */ main(int argc, char *argv[]) { int fd; /* input file handle */ int status = 0; /* status from file read */ long fileptr = 0L; /* current file byte offset */ char filebuf[REC_SIZE]; /* data from file */ if(argc != 2) /* abort if missing filename */ { fprintf(stderr,"\ndump: wrong number of parameters\n"); exit(1); } /* open file in binary mode, abort if open fails */ if((fd = open(argv[1],O_RDONLY | O_BINARY) ) == -1) { fprintf(stderr, "\ndump: can't find file %s \n", argv[1]); exit(1); } /* read and dump records until end of file */ while((status = read(fd,filebuf,REC_SIZE) ) != 0) { dump_rec(filebuf, fileptr, status); fileptr += REC_SIZE; } close(fd); /* close input file */ exit(0); /* return success code */ } /* Display record (16 bytes) in hex and ASCII on standard output */ dump_rec(char *filebuf, long fileptr, int length) { int i; /* index to current record */ if(fileptr % 128 == 0) /* display heading if needed */ printf("\n\n 0 1 2 3 4 5 6 7 8 9 A B C D E F") printf("\n%04lX ",fileptr); /* display file offset */ /* display hex equivalent of each byte from file */ for(i = 0; i < length; i++) printf(" %02X", (unsigned char) filebuf[i]); if(length != 16) /* spaces if partial record */ for (i=0; i<(16-length); i++) printf(" "); /* display ASCII equivalent of each byte from file */ printf(" "); for(i = 0; i < length; i++) { if(filebuf[i] < 32 || filebuf[i] > 126) putchar('.'); else putchar(filebuf[i]); } } ────────────────────────────────────────────────────────────────────────── Figure 8-11. The C version: DUMP.C. The assembly-language version of the DUMP program contains a number of subroutines that you may find useful in your own programming efforts. These include the following: Subroutine Action ────────────────────────────────────────────────────────────────────────── argc Returns the number of command-line arguments. argv Returns the address and length of a particular command-line argument. w2a Converts a binary word (16 bits) into hex ASCII for output. b2a Converts a binary byte (8 bits) into hex ASCII for output. ascii Converts 4 bits into a single hex ASCII character. ────────────────────────────────────────────────────────────────────────── It is interesting to compare these two equivalent programs. The C program contains only 77 lines, whereas the assembly-language program has 436 lines. Clearly, the C source code is less complex and easier to maintain. On the other hand, if size and efficiency are important, the DUMP.EXE file generated by the C compiler is 8563 bytes, whereas the assembly-language DUMP.EXE file is only 1294 bytes and runs twice as fast as the C program. ──────────────────────────────────────────────────────────────────────────── Chapter 9 Volumes and Directories Each file in an MS-DOS system is uniquely identified by its name and its location. The location, in turn, has two components: the logical drive that contains the file and the directory on that drive where the filename can be found. Logical drives are specified by a single letter followed by a colon (for example, A:). The number of logical drives in a system is not necessarily the same as the number of physical drives; for example, it is common for large fixed-disk drives to be divided into two or more logical drives. The key aspect of a logical drive is that it contains a self-sufficient file system; that is, it contains one or more directories, zero or more complete files, and all the information needed to locate the files and directories and to determine which disk space is free and which is already in use. Directories are simply lists or catalogs. Each entry in a directory consists of the name, size, starting location, attributes, and last modification date and time of a file or another directory that the disk contains. The detailed information about the location of every block of data assigned to a file or directory is in a separate control area on the disk called the file allocation table (FAT). (See Chapter 10 for a detailed discussion of the internal format of directories and the FAT.) Every disk potentially has two distinct kinds of directories: the root directory and all other directories. The root directory is always present and has a maximum number of entries, determined when the disk is formatted; this number cannot be changed. The subdirectories of the root directory, which may or may not be present on a given disk, can be nested to any level and can grow to any size (Figure 9-1). This is the hierarchical, or tree, directory structure referred to in earlier chapters. Every directory has a name, except for the root directory, which is designated by a single backslash (\) character. MS-DOS keeps track of a "current drive" for the system and uses this drive when a file specification does not include an explicit drive code. Similarly, MS-DOS maintains a "current directory" for each logical drive. You can select any particular directory on a drive by naming in order── either from the root directory or relative to the current directory──the directories that lead to its location in the tree structure. Such a list of directories, separated by backslash delimiters, is called a path. When a complete path from the root directory is prefixed by a logical drive code and followed by a filename and extension, the resulting string is a fully qualified filename and unambiguously specifies a file. ┌────────────┐ │ Drive │ │ identifier │ └─────┬──────┘ │ ┌───────┴────────┐ │ Root directory │ │ (volume label) │ └─┬──┬──┬───┬──┬─┘ ┌───────────────────┘ │ │ │ └───────────────────┐ │ ┌───────────┘ │ └───────────┐ │ ┌────┴───┐ ┌────┴──────┐ ┌───┴────┐ ┌──────┴────┐ ┌───┴────┐ │ File A │ │ Directory │ │ File B │ │ Directory │ │ File C │ └────────┘ └─┬───────┬─┘ └────────┘ └─┬─────────┘ └─┬──────┘ │ │ │ │ │ │ │ │ ┌─────┘ │ │ │ │ │ │ │ ┌────┴──────┐ ┌──┴─────┐ ┌─────┴──┐ ┌───┴────┐ │ Directory │ │ File D │ │ File E │ │ File F │ └───────────┘ └────────┘ └────────┘ └────────┘ Figure 9-1. An MS-DOS file-system structure. Drive and Directory Control You can examine, select, create, and delete disk directories interactively with the DIR, CHDIR (CD), MKDIR (MD), and RMDIR (RD) commands. You can select a new current drive by entering the letter of the desired drive, followed by a colon. MS-DOS provides the following Int 21H functions to give application programs similar control over drives and directories: Function Action ────────────────────────────────────────────────────────────────────────── 0EH Select current drive. 19H Get current drive. 39H Create directory. 3AH Remove directory. 3BH Select current directory. 47H Get current directory. ────────────────────────────────────────────────────────────────────────── The two functions that deal with disk drives accept or return a binary drive code──0 represents drive A, 1 represents drive B, and so on. This differs from most other MS-DOS functions, which use 0 to indicate the current drive, 1 for drive A, and so on. The first three directory functions in the preceding list require an ASCIIZ string that describes the path to the desired directory. As with the handle-based file open and create functions, the address of the ASCIIZ string is passed in the DS:DX registers. On return, the carry flag is clear if the function succeeds or set if the function failed, with an error code in the AX register. The directory functions can fail for a variety of reasons, but the most common cause of an error is that some element of the indicated path does not exist. The last function in the preceding list, Int 21H Function 47H, allows you to obtain an ASCIIZ path for the current directory on the specified or default drive. MS-DOS supplies the path string without the drive identifier or a leading backslash. Int 21H Function 47H is most commonly used with Int 21H Function 19H to build fully qualified filenames. Such filenames are desirable because they remain valid if the user changes the current drive or directory. Section 2 of this book, "MS-DOS Functions Reference," gives detailed information on the drive and directory control functions. Searching Directories When you request an open operation on a file, you are implicitly performing a search of a directory. MS-DOS examines each entry of the directory to find a match for the filename you have given as an argument; if the file is found, MS-DOS copies certain information from the directory into a data structure that it can use to control subsequent read or write operations to the file. Thus, if you wish to test for the existence of a specific file, you need only perform an open operation and observe whether it is successful. (If it is, you should, of course, perform a subsequent close operation to avoid needless expenditure of handles.) Sometimes you may need to perform more elaborate searches of a disk directory. Perhaps you wish to find all the files with a certain extension, a file with a particular attribute, or the names of the subdirectories of a certain directory. Although the locations of a disk's directories and the specifics of the entries that are found in them are of necessity hardware dependent (for example, interpretation of the field describing the starting location of a file depends upon the physical disk format), MS-DOS does provide functions that will allow examination of a disk directory in a hardware-independent fashion. In order to search a disk directory successfully, you must understand two types of MS-DOS search services. The first type is the "search for first" function, which accepts a file specification──possibly including wildcard characters──and looks for the first matching file in the directory of interest. If it finds a match, the function fills a buffer owned by the requesting program with information about the file; if it does not find a match, it returns an error flag. A program can call the second type of search service, called "search for next," only after a successful "search for first." If the file specification that was originally passed to "search for first" included wildcard characters and at least one matching file was present, the program can call "search for next" as many times as necessary to find all additional matching files. Like "search for first," "search for next" returns information about the matched files in a buffer designated by the requesting program. When it can find no more matching files, "search for next" returns an error flag. As with nearly every other operation, MS-DOS provides two parallel sets of directory-searching services: Action FCB function Handle function ────────────────────────────────────────────────────────────────────────── Search for first 11H 4EH Search for next 12H 4FH ────────────────────────────────────────────────────────────────────────── The FCB directory functions allow searches to match a filename and extension, both possibly containing wildcard characters, within the current directory for the specified or current drive. The handle directory functions, on the other hand, allow a program to perform searches within any directory on any drive, regardless of the current directory. Searches that use normal FCBs find only normal files. Searches that use extended FCBs, or the handle-type functions, can be qualified with file attributes. The attribute bits relevant to searches are as follows: Bit Significance ────────────────────────────────────────────────────────────────────────── 0 Read-only file 1 Hidden file 2 System file 3 Volume label 4 Directory 5 Archive needed (set when file modified) ────────────────────────────────────────────────────────────────────────── The remaining bits of a search function's attribute parameter should be zero. When any of the preceding attribute bits are set, the search function returns all normal files plus any files with the specified attributes, except in the case of the volume-label attribute bit, which receives special treatment as described later in this chapter. Note that by setting bit 4 you can include directories in a search, exactly as though they were files. Both the FCB and handle directory-searching functions require that the disk transfer area address be set (with Int 21H Function 1AH), before the call to "search for first," to point to a working buffer for use by MS-DOS. The DTA address should not be changed between calls to "search for first" and "search for next." When it finds a matching file, MS-DOS places the information about the file in the buffer and then inspects the buffer on the next "search for next" call, to determine where to resume the search. The format of the data returned in the buffer is different for the FCB and handle functions, so read the detailed descriptions in Section 2 of this book, "MS-DOS Functions Reference," before attempting to interpret the buffer contents. Figures 9-2 and 9-3 provide equivalent examples of searches for all files in a given directory that have the .ASM extension, one example using the FCB directory functions (Int 21H Functions 11H and 12H) and the other using the handle functions (Int 21H Functions 4EH and 4FH). (Both programs use the handle write function with the standard output handle to display the matched filenames, to avoid introducing tangential differences in the listings.) ────────────────────────────────────────────────────────────────────────── start: ; set DTA address for buffer ; used by search functions mov dx,seg buff ; DS:DX = buffer address mov ds,dx mov dx,offset buff mov ah,1ah ; function 1ah = search for first int 21h ; transfer to MS-DOS ; search for first match... mov dx,offset fcb ; DS:DX = FCB address mov ah,11h ; function 11h = search for first int 21h ; transfer to MS-DOS or al,al ; any matches at all? jnz exit ; no, quit disp: ; go to a new line... mov dx,offset crlf ; DS:DX = CR-LF string mov cx,2 ; CX = string length mov bx,1 ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ; display matching file mov dx,offset buff+1 ; DS:DX = filename mov cx,11 ; CX = length mov bx,1 ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ; search for next match... mov dx,offset fcb ; DS:DX = FCB address mov ah,12h ; function 12h = search for next int 21h ; transfer to MS-DOS or al,al ; any more matches? jz disp ; yes, go show filename exit: ; final exit point mov ax,4c00h ; function 4ch = terminate, ; return code = 0 int 21h ; transfer to MS-DOS . . . crlf db 0dh,0ah ; ASCII carriage return- ; linefeed string fcb db 0 ; drive = current db 8 dup ('?') ; filename = wildcard db 'ASM' ; extension = ASM db 25 dup (0) ; remainder of FCB = zero buff db 64 dup (0) ; receives search results ────────────────────────────────────────────────────────────────────────── Figure 9-2. Example of an FCB-type directory search using Int 21H Functions 11H and 12H. This routine displays the names of all files in the current directory that have the .ASM extension. ────────────────────────────────────────────────────────────────────────── start: ; set DTA address for buffer ; used by search functions mov dx,seg buff ; DS:DX = buffer address mov ds,dx mov dx,offset buff mov ah,1ah ; function 1ah = search for first int 21h ; transfer to MS-DOS ; search for first match... mov dx,offset fname ; DS:DX = wildcard filename mov cx,0 ; CX = normal file attribute mov ah,4eh ; function 4eh = search for first int 21h ; transfer to MS-DOS jc exit ; quit if no matches at all disp: ; go to a new line... mov dx,offset crlf ; DS:DX = CR-LF string mov cx,2 ; CX = string length mov bx,1 ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ; find length of filename... mov cx,0 ; CX will be char count ; DS:SI = start of name mov si,offset buff+30 disp1: lodsb ; get next character or al,al ; is it null character? jz disp2 ; yes, found end of string inc cx ; else count characters jmp disp1 ; and get another disp2: ; display matching file... ; CX already contains length ; DS:DX = filename mov dx,offset buff+30 mov bx,1 ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ; find next matching file... mov ah,4fh ; function 4fh = search for next int 21h ; transfer to MS-DOS jnc disp ; jump if another match found exit: ; final exit point mov ax,4c00h ; function 4ch = terminate, ; return code = 0 int 21h ; transfer to MS-DOS . . . crlf db 0dh,0ah ; ASCII carriage return- ; linefeed string fname db '*.ASM',0 ; ASCIIZ filename to ; be matched buff db 64 dup (0) ; receives search results ────────────────────────────────────────────────────────────────────────── Figure 9-3. Example of a handle-type directory search using Int 21H Functions 4EH and 4FH. This routine also displays the names of all files in the current directory that have a .ASM extension. Moving Files The rename file function that was added in MS-DOS version 2.0, Int 21H Function 56H, has the little-advertised capability to move a file from one directory to another. The function has two ASCIIZ parameters: the "old" and "new" names for the file. If the old and new paths differ, MS-DOS moves the file; if the filename or extension components differ, MS-DOS renames the file. MS-DOS can carry out both of these actions in the same function call. Of course, the old and new directories must be on the same drive, because the file's actual data is not moved at all; only the information that describes the file is removed from one directory and placed in another directory. Function 56H fails if the two ASCIIZ strings include different logical-drive codes, if the file is read-only, or if a file with the same name and location as the "new" filename already exists. The FCB-based rename file service, Int 21H Function 17H, works only on the current directory and cannot be used to move files. Volume Labels Support for volume labels was first added to MS-DOS in version 2.0. A volume label is an optional name of from 1 to 11 characters that the user assigns to a disk during a FORMAT operation. You can display a volume label with the DIR, TREE, CHKDSK, or VOL command. Beginning with MS-DOS version 3.0, you can use the LABEL command to add, display, or alter the label after formatting. In MS-DOS version 4, the FORMAT program also assigns a semi-random 32-bit binary ID to each disk it formats; you can display this value, but you cannot change it. The distinction between volumes and drives is important. A volume label is associated with a specific storage medium. A drive identifier (such as A) is associated with a physical device that a storage medium can be mounted on. In the case of fixed-disk drives, the medium associated with a drive identifier does not change (hence the name). In the case of floppy disks or other removable media, the disk accessed with a given drive identifier might have any volume label or none at all. Hence, volume labels do not take the place of the logical-drive identifier and cannot be used as part of a pathname to identify a file. In fact, in MS-DOS version 2, the system does not use volume labels internally at all. In MS-DOS versions 3.0 and later, a disk driver can use volume labels to detect whether the user has replaced a disk while a file is open; this use is optional, however, and is not implemented in all systems. MS-DOS volume labels are implemented as a special type of entry in a disk's root directory. The entry contains a time-and-date stamp and has an attribute value of 8 (i.e., bit 3 set). Except for the attribute, a volume label is identical to the directory entry for a file that was created but never had any data written into it, and you can manipulate volume labels with Int 21H functions much as you manipulate files. However, a volume label receives special handling at several levels: ■ When you create a volume label after a disk is formatted, MS-DOS always places it in the root directory, regardless of the current directory. ■ A disk can contain only one volume label; attempts to create additional volume labels (even with different names) will fail. ■ MS-DOS always carries out searches for volume labels in the root directory, regardless of the current directory, and does not also return all normal files. In MS-DOS version 2, support for volume labels is not completely integrated into the handle file functions, and you must use extended FCBs instead to manipulate volume labels. For example, the code in Figure 9-4 searches for the volume label in the root directory of the current drive. You can also change volume labels with extended FCBs and the rename file function (Int 21H Function 17H), but you should not attempt to remove an existing volume label with Int 21H Function 13H under MS-DOS version 2, because this operation can damage the disk's FAT in an unpredictable manner. In MS-DOS versions 3.0 and later, you can create a volume label in the expected manner, using Int 21H Function 3CH and an attribute of 8, and you can use the handle-type "search for first" function (4EH) to obtain an existing volume label for a logical drive (Figure 9-5). However, you still must use extended FCBs to change a volume label. ────────────────────────────────────────────────────────────────────────── buff db 64 dup (?) ; receives search results xfcb db 0ffh ; flag signifying extended FCB db 5 dup (0) ; reserved db 8 ; volume attribute byte db 0 ; drive code (0 = current) db 11 dup ('?') ; wildcard filename and extension db 25 dup (0) ; remainder of FCB (not used) . . . ; set DTA address for buffer ; used by search functions mov dx,seg buff ; DS:DX = buffer address mov ds,dx mov dx,offset buff mov ah,1ah ; function 1ah = set DTA int 21h ; transfer to MS-DOS ; now search for label... ; DS:DX = extended FCB mov dx,offset xfcb mov ah,11h ; function 11h = search for first int 21h ; transfer to MS-DOS cmp al,0ffh ; search successful? je no_label ; jump if no volume label . . . ────────────────────────────────────────────────────────────────────────── Figure 9-4. A volume-label search under MS-DOS version 2, using an extended file control block. If the search is successful, the volume label is returned in buff, formatted in the filename and extension fields of an extended FCB. ────────────────────────────────────────────────────────────────────────── buff db 64 dup (?) ; receives search results wildcd db '*.*',0 ; wildcard ASCIIZ filename . . . ; set DTA address for buffer ; used by search functions mov dx,seg buff ; DS:DX = buffer address mov ds,dx mov dx,offset buff mov ah,1ah ; function 1ah = set DTA int 21h ; transfer to MS-DOS ; now search for label... ; DS:DX = ASCIIZ string mov dx,offset wildcd mov cx,8 ; CX = volume attribute mov ah,4eh ; function 4eh = search for first int 21h ; transfer to MS-DOS jc no_label ; jump if no volume label . . . ────────────────────────────────────────────────────────────────────────── Figure 9-5. A volume-label search under MS-DOS version 3, using the handle-type file functions. If the search is successful (carry flag returned clear), the volume name is placed at location buff+1EH in the form of an ASCIIZ string. ──────────────────────────────────────────────────────────────────────────── Chapter 10 Disk Internals MS-DOS disks are organized according to a rather rigid scheme that is easily understood and therefore easily manipulated. Although you will probably never need to access the special control areas of a disk directly, an understanding of their internal structure leads to a better understanding of the behavior and performance of MS-DOS as a whole. From the application programmer's viewpoint, MS-DOS presents disk devices as logical volumes that are associated with a drive code (A, B, C, and so on) and that have a volume name (optional), a root directory, and from zero to many additional directories and files. MS-DOS shields the programmer from the physical characteristics of the medium by providing a battery of disk services through Int 21H. Using these services, the programmer can create, open, read, write, close, and delete files in a uniform way, regardless of the disk drive's size, speed, number of read/write heads, number of tracks, and so forth. Requests from an application program for file operations actually go through two levels of translation before resulting in the physical transfer of data between the disk device and random-access memory: 1. Beneath the surface, MS-DOS views each logical volume, whether it is an entire physical unit such as a floppy disk or only a part of a fixed disk, as a continuous sequence of logical sectors, starting at sector 0. (A logical disk volume can also be implemented on other types of storage. For example, RAM disks map a disk structure onto an area of random-access memory.) MS-DOS translates an application program's Int 21H file-management requests into requests for transfers of logical sectors, using the information found in the volume's directories and allocation tables. (For those rare situations where it is appropriate, programs can also access logical sectors directly with Int 25H and Int 26H.) 2. MS-DOS then passes the requests for logical sectors to the disk device's driver, which maps them onto actual physical addresses (head, track, and sector). Disk drivers are extremely hardware dependent and are always written in assembly language for maximum speed. In most versions of MS-DOS, a driver for IBM-compatible floppy- and fixed-disk drives is built into the MS-DOS BIOS module (IO.SYS) and is always loaded during system initialization; you can install additional drivers for non-IBM-compatible disk devices by including the appropriate DEVICE directives in the CONFIG.SYS file. Each MS-DOS logical volume is divided into several fixed-size control areas and a files area (Figure 10-1). The size of each control area depends on several factors──the size of the volume and the version of FORMAT used to initialize the volume, for example──but all of the information needed to interpret the structure of a particular logical volume can be found on the volume itself in the boot sector. ┌───────────────────────────────────────────────────────┐ │ Boot sector │ │ Reserved area │ ├───────────────────────────────────────────────────────┤ │ File allocation table #1 │ ├───────────────────────────────────────────────────────┤ │ Possible additional copies of FAT │ ├───────────────────────────────────────────────────────┤ │ Root directory │ ├───────────────────────────────────────────────────────┤ │ │ │ Files area │ │ │ └───────────────────────────────────────────────────────┘ Figure 10-1. Map of a typical MS-DOS logical volume. The boot sector (logical sector 0) contains the OEM identification, BIOS parameter block (BPB), and disk bootstrap. The remaining sectors are divided among an optional reserved area, one or more copies of the file allocation table, the root directory, and the files area. The Boot Sector Logical sector 0, known as the boot sector, contains all of the critical information regarding the disk medium's characteristics (Figure 10-2). The first byte in the sector is always an 80x86 jump instruction──either a normal intrasegment JMP (opcode 0E9H) followed by a 16-bit displacement or a "short" JMP (opcode 0EBH) followed by an 8-bit displacement and then by an NOP (opcode 90H). If neither of these two JMP opcodes is present, the disk has not been formatted or was not formatted for use with MS-DOS. (Of course, the presence of the JMP opcode does not in itself ensure that the disk has an MS-DOS format.) Following the initial JMP instruction is an 8-byte field that is reserved by Microsoft for OEM identification. The disk-formatting program, which is specialized for each brand of computer, disk controller, and medium, fills in this area with the name of the computer manufacturer and the manufacturer's internal MS-DOS version number. 00H ┌───────────────────────────────────────────────┐ │ E9 XX XX or EB XX 90 │ 03H ├───────────────────────────────────────────────┤ │ OEM name and version │ │ (8 bytes) │ OBH ├───────────────────────────────────────────────┤─┐ │ Bytes per sector (2 bytes) │ │ ODH ├───────────────────────────────────────────────┤ │ │ Sectors per allocation unit (1 byte) │ │ 0EH ├───────────────────────────────────────────────┤ │ │ Reserved sectors, starting at 0 (2 bytes) │ │ 10H ├───────────────────────────────────────────────┤ │ │ Number of FATs (1 byte) │ B 11H ├───────────────────────────────────────────────┤ P │ Number of root-directory entries (2 bytes) │ B 13H ├───────────────────────────────────────────────┤ │ │ Total sectors in logical volume (2 bytes) │ │ 15H ├───────────────────────────────────────────────┤ │ MS-DOS │ Media descriptor byte │ │ version 2.0 16H ├───────────────────────────────────────────────┤ │ │ Number of sectors per FAT (2 bytes) │ │ 18H ├───────────────────────────────────────────────┤═╡ │ Sectors per track (2 bytes) │ │ 1AH ├───────────────────────────────────────────────┤ │ │ Number of heads (2 bytes) │ │ MS-DOS 1CH ├───────────────────────────────────────────────┤ │ version 3.0 │ Number of hidden sectors (4 bytes) │═╡ 20H ├───────────────────────────────────────────────┤ │ MS-DOS │ Total sectors in logical volume │ │ version 4.0 │ (MS-DOS 4.0 and volume size >32 MB) │ │ 24H ├───────────────────────────────────────────────┤═╡ │ Physical drive number │ │ 25H ├───────────────────────────────────────────────┤ │ │ Reserved │ │ 26H ├───────────────────────────────────────────────┤ │ │ Extended boot signature record (29H) │ │ Additional 27H ├───────────────────────────────────────────────┤ │ MS-DOS 4.0 │ 32-bit binary volume ID │ │ information 2BH ├───────────────────────────────────────────────┤ │ │ Volume label (11 bytes) │ │ 36H ├───────────────────────────────────────────────┤ │ │ Reserved (8 bytes) │ │ 3EH ├───────────────────────────────────────────────┤─┘ │ Bootstrap │ └───────────────────────────────────────────────┘ Figure 10-2. Map of the boot sector of an MS-DOS disk. Note the JMP at offset 0, the OEM identification field, the MS-DOS version 2 compatible BIOS parameter block (bytes 0BH─17H), the three additional WORD fields for MS-DOS version 3, the double-word number-of-sectors field and 32-bit binary volume ID for MS-DOS version 4.0, and the bootstrap code. The third major component of the boot sector is the BIOS parameter block (BPB) in bytes 0BH through 17H. (Additional fields are present in MS-DOS versions 3.0 and later.) This data structure describes the physical disk characteristics and allows the device driver to calculate the proper physical disk address for a given logical-sector number; it also contains information that is used by MS-DOS and various system utilities to calculate the address and size of each of the disk control areas (file allocation tables and root directory). The final element of the boot sector is the disk bootstrap routine. The disk bootstrap is usually read into memory by the ROM bootstrap, which is executed automatically when the computer is turned on. The ROM bootstrap is usually just smart enough to home the head of the disk drive (move it to track 0), read the first physical sector into RAM at a predetermined location, and jump to it. The disk bootstrap is more sophisticated. It calculates the physical disk address of the beginning of the files area, reads the files containing the operating system into memory, and transfers control to the BIOS module at location 0070:0000H. (See Chapter 2.) Figures 10-3 and 10-4 show a partial hex dump and disassembly of a PC-DOS 3.3 floppy-disk boot sector. ────────────────────────────────────────────────────────────────────────── 0 1 2 3 4 5 6 7 8 9 A B C D E F 0000 EB 34 90 49 42 4D 20 20 33 2E 33 00 02 02 01 00 .4.IBM 3.3..... 0010 02 70 00 D0 02 FD 02 00 09 00 02 00 00 00 00 00 .p.............. 0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 12 ................ 0030 00 00 00 00 01 00 FA 33 C0 8E D0 BC 00 7C 16 07 .......3.....|.. . . . 01C0 0D 0A 44 69 73 6B 20 42 6F 6F 74 20 66 61 69 6C ..Disk Boot fail 01D0 75 72 65 0D 0A 00 49 42 4D 42 49 4F 20 20 43 4F ure...IBMBIO CO 01E0 4D 49 42 4D 44 4F 53 20 20 43 4F 4D 00 00 00 00 MIBMDOS COM.... 01F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA ..............U. ────────────────────────────────────────────────────────────────────────── Figure 10-3. Partial hex dump of the boot sector (track 0, head 0, sector 1) of a PC-DOS version 3.3 floppy disk. This sector contains the OEM identification, a copy of the BIOS parameter block describing the medium, and the bootstrap routine that reads the BIOS into memory and transfers control to it. See also Figures 10-2 and 10-4. ────────────────────────────────────────────────────────────────────────── jmp $+54 ; jump to bootstrap nop db 'IBM 3.3' ; OEM identification ; BIOS parameter block dw 512 ; bytes per sector db 2 ; sectors per cluster dw 1 ; reserved sectors db 2 ; number of FATs dw 112 ; root directory entries dw 720 ; total sectors db 0fdh ; media descriptor byte dw 2 ; sectors per FAT dw 9 ; sectors per track dw 2 ; number of heads dd 0 ; hidden sectors . . . ────────────────────────────────────────────────────────────────────────── Figure 10-4. Partial disassembly of the boot sector shown in Figure 10-3. The Reserved Area The boot sector is actually part of a reserved area that can span from one to several sectors. The reserved-sectors word in the BPB, at offset 0EH in the boot sector, describes the size of this area. Remember that the number in the BPB field includes the boot sector itself, so if the value is 1 (as it is on IBM PC floppy disks), the length of the reserved area is actually 0 sectors. The File Allocation Table When a file is created or extended, MS-DOS assigns it groups of disk sectors from the files area in powers of 2. These are known as allocation units or clusters. The number of sectors per cluster for a given medium is defined in the BPB and can be found at offset 0DH in the disk's boot sector. Below are some example cluster sizes: Disk type Power of 2 Sectors/cluster ────────────────────────────────────────────────────────────────────────── 5.25" 180 KB floppy disk 0 1 5.25" 360 KB floppy disk 1 2 PC/AT fixed disk 2 4 PC/XT fixed disk 3 8 ────────────────────────────────────────────────────────────────────────── The file allocation table (FAT) is divided into fields that correspond directly to the assignable clusters on the disk. These fields are 12 bits in MS-DOS versions 1 and 2 and may be either 12 bits or 16 bits in versions 3.0 and later, depending on the size of the medium (12 bits if the disk contains fewer than 4087 clusters, 16 bits otherwise). The first two fields in the FAT are always reserved. On IBM-compatible media, the first 8 bits of the first reserved FAT entry contain a copy of the media descriptor byte, which is also found in the BPB in the boot sector. The second, third, and (if applicable) fourth bytes, which constitute the remainder of the first two reserved FAT fields, always contain 0FFH. The currently defined IBM-format media descriptor bytes are as follows: MS-DOS version where first Descriptor Medium supported ────────────────────────────────────────────────────────────────────────── 0F0H 3.5" floppy disk, 2-sided, 18-sector 3.3 0F8H Fixed disk 2.0 0F9H 5.25" floppy disk, 2-sided, 15-sector 3.0 3.5" floppy disk, 2-sided, 9-sector 3.2 0FCH 5.25" floppy disk, 1-sided, 9-sector 2.0 0FDH 5.25" floppy disk, 2-sided, 9-sector 2.0 8" floppy disk, 1-sided, single-density 0FEH 5.25" floppy disk, 1-sided, 8-sector 1.0 8" floppy disk, 1-sided, single-density 8" floppy disk, 2-sided, double-density 0FFH 5.25" floppy disk, 2-sided, 8-sector 1.1 ────────────────────────────────────────────────────────────────────────── The remainder of the FAT entries describe the use of their corresponding disk clusters. The contents of the FAT fields are interpreted as follows: Value Meaning ────────────────────────────────────────────────────────────────────────── (0)000H Cluster available (F)FF0─(F)FF6H Reserved cluster (F)FF7H Bad cluster, if not part of chain (F)FF8─(F)FFFH Last cluster of file (X)XXX Next cluster in file ────────────────────────────────────────────────────────────────────────── Each file's entry in a directory contains the number of the first cluster assigned to that file, which is used as an entry point into the FAT. From the entry point on, each FAT slot contains the cluster number of the next cluster in the file, until a last-cluster mark is encountered. At the computer manufacturer's option, MS-DOS can maintain two or more identical copies of the FAT on each volume. MS-DOS updates all copies simultaneously whenever files are extended or the directory is modified. If access to a sector in a FAT fails due to a read error, MS-DOS tries the other copies until a successful disk read is obtained or all copies are exhausted. Thus, if one copy of the FAT becomes unreadable due to wear or a software accident, the other copies may still make it possible to salvage the files on the disk. As part of its procedure for checking the integrity of a disk, the CHKDSK program compares the multiple copies (usually two) of the FAT to make sure they are all readable and consistent. The Root Directory Following the file allocation tables is an area known in MS-DOS versions 2.0 and later as the root directory. (Under MS-DOS version 1, it was the only directory on the disk.) The root directory contains 32-byte entries that describe files, other directories, and the optional volume label (Figure 10-5). An entry beginning with the byte value E5H is available for reuse; it represents a file or directory that has been erased. An entry beginning with a null (zero) byte is the logical end-of-directory; that entry and all subsequent entries have never been used. 00H ┌──────────────────────────────┐ │ Filename │ Note 1 08H ├──────────────────────────────┤ │ Extension │ 0BH ├──────────────────────────────┤ │ File attribute │ Note 2 0CH ├──────────────────────────────┤ │ Reserved │ 16H ├──────────────────────────────┤ │ Time created or last updated │ Note 3 18H ├──────────────────────────────┤ │ Date created or last updated │ Note 4 1AH ├──────────────────────────────┤ │ Starting cluster │ 1CH ├──────────────────────────────┤ │ File size, 4 bytes │ Note 5 20H └──────────────────────────────┘ Figure 10-5. Format of a single entry in a disk directory. Total length is 32 bytes (20H bytes). ────────────────────────────────────────────────────────────────────────── Notes for Figure 10-5 1. The first byte of the filename field of a directory entry may contain the following special information: Value Meaning ──────────────────────────────────────────────────────────────────────── 00H Directory entry has never been used; end of occupied portion of directory. 05H First character of filename is actually E5H. 2EH Entry is an alias for the current or parent directory. If the next byte is also 2EH, the cluster field contains the cluster number of the parent directory (zero if the parent directory is the root directory). E5H File has been erased. ──────────────────────────────────────────────────────────────────────── 2. The attribute byte of the directory entry is mapped as follows: Bit Meaning ──────────────────────────────────────────────────────────────────────── 0 Read-only; attempts to open file for write or to delete file will fail. 1 Hidden file; excluded from normal searches. 2 System file; excluded from normal searches. 3 Volume label; can exist only in root directory. 4 Directory; excluded from normal searches. 5 Archive bit; set whenever file is modified. 6 Reserved. 7 Reserved. ──────────────────────────────────────────────────────────────────────── 3. The time field is encoded as follows: Bits Contents ──────────────────────────────────────────────────────────────────────── 00H─04H Binary number of 2-second increments (0─29, corresponding to 0─58 seconds) 05H─0AH Binary number of minutes (0─59) 0BH─0FH Binary number of hours (0─23) ──────────────────────────────────────────────────────────────────────── 4. The date field is encoded as follows: Bits Contents ──────────────────────────────────────────────────────────────────────── 00H─04H Day of month (1─31) 05H─08H Month (1─12) 09H─0FH Year (relative to 1980) ──────────────────────────────────────────────────────────────────────── 5. The file-size field is interpreted as a 4-byte integer, with the low-order 2 bytes of the number stored first. ────────────────────────────────────────────────────────────────────────── The root directory has a number of special properties. Its size and position are fixed and are determined by the FORMAT program when a disk is initialized. This information can be obtained from the boot sector's BPB. If the disk is bootable, the first two entries in the root directory always describe the files containing the MS-DOS BIOS and the MS-DOS kernel. The disk bootstrap routine uses these entries to bring the operating system into memory and start it up. Figure 10-6 shows a partial hex dump of the first sector of the root directory on a bootable PC-DOS 3.3 floppy disk. ────────────────────────────────────────────────────────────────────────── 0 1 2 3 4 5 6 7 8 9 A B C D E F 0000 49 42 4D 42 49 4F 20 20 43 4F 4D 27 00 00 00 00 IBMBIO COM'.... 0010 00 00 00 00 00 00 00 60 72 0E 02 00 54 56 00 00 .......'r...TV.. 0020 49 42 4D 44 4F 53 20 20 43 4F 4D 27 00 00 00 00 IBMDOS COM'.... 0030 00 00 00 00 00 00 00 60 71 0E 18 00 CF 75 00 00 .......'q....u.. 0040 43 4F 4D 4D 41 4E 44 20 43 4F 4D 20 00 00 00 00 COMMAND COM .... 0050 00 00 00 00 00 00 00 60 71 0E 36 00 DB 62 00 00 .......'q.6..b.. 0060 42 4F 4F 54 44 49 53 4B 20 20 20 28 00 00 00 00 BOOTDISK (.... 0070 00 00 00 00 00 00 A1 00 21 00 00 00 00 00 00 00 ........!....... 0080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ . . . ────────────────────────────────────────────────────────────────────────── Figure 10-6. Partial hex dump of the first sector of the root directory for a PC-DOS 3.3 disk containing the three system files and a volume label. The Files Area The remainder of the volume after the root directory is known as the files area. MS-DOS views the sectors in this area as a pool of clusters, each containing one or more logical sectors, depending on the disk format. Each cluster has a corresponding entry in the FAT that describes its current use: available, reserved, assigned to a file, or unusable (because of defects in the medium). Because the first two fields of the FAT are reserved, the first cluster in the files area is assigned the number 2. When a file is extended under versions 1 and 2, MS-DOS searches the FAT from the beginning until it finds a free cluster (designated by a zero FAT field); it then changes that FAT field to a last-cluster mark and updates the previous last cluster of the file's chain to point to the new last cluster. Under versions 3.0 and later, however, MS-DOS searches the FAT from the most recently allocated cluster; this reduces file fragmentation and improves overall access times. Directories other than the root directory are simply a special type of file. Their storage is allocated from the files area, and their contents are 32-byte entries──in the same format as those used in the root directory──that describe files or other directories. Directory entries that describe other directories contain an attribute byte with bit 4 set, zero in the file-length field, and the date and time that the directory was created (Figure 10-7). The first cluster field points, of course, to the first cluster in the files area that belongs to the directory. (The directory's other clusters can be found only by tracing through the FAT.) All directories except the root directory contain two special directory entries with the names . and ... MS-DOS puts these entries in place when it creates a directory, and they cannot be deleted. The . entry is an alias for the current directory; its cluster field points to the cluster in which it is found. The .. entry is an alias for the directory's parent (the directory immediately above it in the tree structure); its cluster field points to the first cluster of the parent directory. If the parent is the root directory, the cluster field of the .. entry contains zero (Figure 10-8). ────────────────────────────────────────────────────────────────────────── . . . 0080 4D 59 44 49 52 20 20 20 20 20 20 10 00 00 00 00 MYDIR ..... 0090 00 00 00 00 00 00 87 9A 9B 0A 2A 00 00 00 00 00 ..........*..... . . . ────────────────────────────────────────────────────────────────────────── Figure 10-7. Extract from the root directory of an MS-DOS disk, showing the entry for a subdirectory named MYDIR. Bit 4 in the attribute byte is set, the cluster field points to the first cluster of the subdirectory file, the date and time stamps are valid, but the file length is zero. ────────────────────────────────────────────────────────────────────────── 0 1 2 3 4 5 6 7 8 9 A B C D E F 0000 2E 20 20 20 20 20 20 20 20 20 20 10 00 00 00 00 . ..... 0010 00 00 00 00 00 00 87 9A 9B 0A 2A 00 00 00 00 00 ..........*..... 0020 2E 2E 20 20 20 20 20 20 20 20 20 10 00 00 00 00 .. ..... 0030 00 00 00 00 00 00 87 9A 9B 0A 00 00 00 00 00 00 ................ 0040 4D 59 46 49 4C 45 20 20 44 41 54 20 00 00 00 00 MYFILE DAT .... 0050 00 00 00 00 00 00 98 9A 9B 0A 2B 00 15 00 00 00 ..........+..... 0060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 0070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ . . . ────────────────────────────────────────────────────────────────────────── Figure 10-8. Hex dump of the first block of the directory MYDIR. Note the . and .. entries. This directory contains exactly one file, MYFILE.DAT. Interpreting the File Allocation Table Now that we understand how the disk is structured, let's see how we can use this knowledge to find a FAT position from a cluster number. If the FAT has 12-bit entries, use the following procedure: 1. Use the directory entry to find the starting cluster of the file in question. 2. Multiply the cluster number by 1.5. 3. Use the integral part of the product as the offset into the FAT and move the word at that offset into a register. Remember that a FAT position can span a physical disk-sector boundary. 4. If the product is a whole number, AND the register with 0FFFH. 5. Otherwise, "logical shift" the register right 4 bits. 6. If the result is a value from 0FF8H through 0FFFH, the file has no more clusters. Otherwise, the result is the number of the next cluster in the file. On disks with at least 4087 clusters formatted under MS-DOS version 3.0 or later, the FAT entries use 16 bits, and the extraction of a cluster number from the table is much simpler: 1. Use the directory entry to find the starting cluster of the file in question. 2. Multiply the cluster number by 2. 3. Use the product as the offset into the FAT and move the word at that offset into a register. 4. If the result is a value from 0FFF8H through 0FFFFH, the file has no more clusters. Otherwise, the result is the number of the next cluster in the file. To convert cluster numbers to logical sectors, subtract 2, multiply the result by the number of sectors per cluster, and add the logical-sector number of the beginning of the data area (this can be calculated from the information in the BPB). As an example, let's work out the disk location of the file IBMBIO.COM, which is the first entry in the directory shown in Figure 10-6. First, we need some information from the BPB, which is in the boot sector of the medium. (See Figures 10-3 and 10-4.) The BPB tells us that there are ■ 512 bytes per sector ■ 2 sectors per cluster ■ 2 sectors per FAT ■ 2 FATs ■ 112 entries in the root directory From the BPB information, we can calculate the starting logical-sector number of each of the disk's control areas and the files area by constructing a table, as follows: Length Sector Area (sectors) numbers ────────────────────────────────────────────────────────────────────────── Boot sector 1 00H 2 FATs * 2 sectors/FAT 4 01H─04H 112 directory entries 7 05H─0BH *32 bytes/entry /512 bytes/sector Total sectors occupied by bootstrap, FATs, and 12 root directory ────────────────────────────────────────────────────────────────────────── Therefore, the first sector of the files area is 12 (0CH). The word at offset 01AH in the directory entry for IBMBIO.COM gives us the starting cluster number for that file: cluster 2. To find the logical-sector number of the first block in the file, we can follow the procedure given earlier: 1. Cluster number - 2 = 2 - 2 = 0. 2. Multiply by sectors per cluster = 0 * 2 = 0. 3. Add logical-sector number of start of the files area = 0 + 0CH = 0CH. So the calculated sector number of the beginning of the file IBMBIO.COM is 0CH, which is exactly what we expect knowing that the FORMAT program always places the system files in contiguous sectors at the beginning of the data area. Now let's trace IBMBIO.COM's chain through the file allocation table (Figures 10-9 and 10-10). This will be a little tedious, but a detailed understanding of the process is crucial. In an actual program, we would first read the boot sector using Int 25H, then calculate the address of the FAT from the contents of the BPB, and finally read the FAT into memory, again using Int 25H. From IBMBIO.COM's directory entry, we already know that the first cluster in the file is cluster 2. To examine that cluster's entry in the FAT, we multiply the cluster number by 1.5, which gives 0003H as the FAT offset, and fetch the word at that offset (which contains 4003H). Because the product of the cluster and 1.5 is a whole number, we AND the word from the FAT with 0FFFH, yielding the number 3, which is the number of the second cluster assigned to the file. ────────────────────────────────────────────────────────────────────────── 0 1 2 3 4 5 6 7 8 9 A B C D E F 0000 FD FF FF 03 40 00 05 60 00 07 80 00 09 A0 00 0B ....@..'........ 0010 C0 00 0D E0 00 0F 00 01 11 20 01 13 40 01 15 60 ......... ..@..' 0020 01 17 F0 FF 19 A0 01 1B C0 01 1D E0 01 1F 00 02 ................ 0030 21 20 02 23 40 02 25 60 02 27 80 02 29 A0 02 2B ! .#@.%'.'..)..+ . . . ────────────────────────────────────────────────────────────────────────── Figure 10-9. Hex dump of the first block of the file allocation table (track 0, head 0, sector 2) for the PC-DOS 3.3 disk whose root directory is shown in Figure 10-6. Notice that the first byte of the FAT contains the media descriptor byte for a 5.25-inch, 2-sided, 9-sector floppy disk. ────────────────────────────────────────────────────────────────────────── getfat proc near ; extracts the FAT field ; for a given cluster ; call AX = cluster # ; DS:BX = addr of FAT ; returns AX = FAT field ; other registers unchanged push bx ; save affected registers push cx mov cx,ax shl ax,1 ; cluster * 2 add ax,cx ; cluster * 3 test ax,1 pushf ; save remainder in Z flag shr ax,1 ; cluster * 1.5 add bx,ax mov ax,[bx] popf ; was cluster * 1.5 whole number? jnz getfat1 ; no, jump and ax,0fffh ; yes, isolate bottom 12 bits jmp getfat2 getfat1: mov cx,4 ; shift word right 4 bits shr ax,cx getfat2: pop cx ; restore registers and exit pop bx ret getfat endp ────────────────────────────────────────────────────────────────────────── Figure 10-10. Assembly-language procedure to access the file allocation table (assumes 12-bit FAT fields). Given a cluster number, the procedure returns the contents of that cluster's FAT entry in the AX register. This simple example ignores the fact that FAT entries can span sector boundaries. To examine cluster 3's entry in the FAT, we multiply 3 by 1.5, which gives 4.5, and fetch the word at offset 0004H (which contains 0040H). Because the product of 3 and 1.5 is not a whole number, we shift the word right 4 bits, yielding the number 4, which is the number of the third cluster assigned to IBMBIO.COM. In this manner, we can follow the chain through the FAT until we come to a cluster (number 23, in this case) whose FAT entry contains the value 0FFFH, which is an end-of-file marker in FATs with 12-bit entries. We have now established that the file IBMBIO.COM contains clusters 2 through 23 (02H─17H), from which we can calculate that logical sectors 0CH through 38H are assigned to the file. Of course, the last cluster may be only partially filled with actual data; the portion of the last cluster used is the remainder of the file's size in bytes (found in the directory entry) divided by the bytes per cluster. Fixed-Disk Partitions Fixed disks have another layer of organization beyond the logical volume structure already discussed: partitions. The FDISK utility divides a fixed disk into one or more partitions consisting of an integral number of cylinders. Each partition can contain an independent file system and, for that matter, its own copy of an operating system. The first physical sector on a fixed disk (track 0, head 0, sector 1) contains the master boot record, which is laid out as follows: Bytes Contents ────────────────────────────────────────────────────────────────────────── 000─1BDH Reserved 1BE─1CDH Partition #1 descriptor 1CE─1DDH Partition #2 descriptor 1DE─1EDH Partition #3 descriptor 1EE─1FDH Partition #4 descriptor 1FE─1FFH Signature word (AA55H) ────────────────────────────────────────────────────────────────────────── The partition descriptors in the master boot record define the size, location, and type of each partition, as follows: Byte(s) Contents ────────────────────────────────────────────────────────────────────────── 00H Active flag (0 = not bootable, 80H = bootable) 01H Starting head 02H─03H Starting cylinder/sector 04H Partition type 00H not used 01H FAT file system, 12-bit FAT entries 04H FAT file system, 16-bit FAT entries 05H extended partition 06H "huge partition" (MS-DOS versions 4.0 and later) 05H Ending head 06H─07H Ending cylinder/sector 08H─0BH Starting sector for partition, relative to beginning of disk 0CH─0FH Partition length in sectorsThe active flag, which indicates that the partition is bootable, can be set on only one partition at a time. ────────────────────────────────────────────────────────────────────────── MS-DOS treats partition types 1, 4, and 6 as normal logical volumes and assigns them their own drive identifiers during the system boot process. Partition type 5 can contain multiple logical volumes and has a special extended boot record that describes each volume. The FORMAT utility initializes MS-DOS fixed-disk partitions, creating the file system within the partition (boot record, file allocation table, root directory, and files area) and optionally placing a bootable copy of the operating system in the file system. Figure 10-11 contains a partial hex dump of a master block from a fixed disk formatted under PC-DOS version 3.3. This dump illustrates the partition descriptors for a normal partition with a 16-bit FAT and an extended partition. ────────────────────────────────────────────────────────────────────────── 0000 . . . 0180 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 80 01 01C0 01 00 04 04 D1 02 11 00 00 00 EE FF 00 00 00 00 01D0 C1 04 05 04 D1 FD 54 00 01 00 02 53 00 00 00 00 01E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA ────────────────────────────────────────────────────────────────────────── Figure 10-11. A partial hex dump of a master block from a fixed disk formatted under PC-DOS version 3.3. This disk contains two partitions. The first partition has a 16-bit FAT and is marked "active" to indicate that it contains a bootable copy of PC-DOS. The second partition is an "extended" partition. The third and fourth partition entries are not used in this example. ──────────────────────────────────────────────────────────────────────────── Chapter 11 Memory Management Current versions of MS-DOS can manage as much as 1 megabyte of contiguous random-access memory. On IBM PCs and compatibles, the memory occupied by MS-DOS and other programs starts at address 0000H and may reach as high as address 09FFFFH; this 640 KB area of RAM is sometimes referred to as conventional memory. Memory above this address is reserved for ROM hardware drivers, video refresh buffers, and the like. Computers that are not IBM compatible may use other memory layouts. The RAM area under the control of MS-DOS is divided into two major sections: ■ The operating-system area ■ The transient-program area The operating-system area starts at address 0000H──that is, it occupies the lowest portion of RAM. It holds the interrupt vector table, the operating system proper and its tables and buffers, any additional installable drivers specified in the CONFIG.SYS file, and the resident part of the COMMAND.COM command interpreter. The amount of memory occupied by the operating-system area varies with the version of MS-DOS used, the number of disk buffers, the size of installed device drivers, and so forth. The transient-program area (TPA), sometimes called the memory arena, is the remainder of memory above the operating-system area. The memory arena is dynamically allocated in blocks called arena entries. Each arena entry has a special control structure called an arena header, and all of the arena headers are chained together. Three MS-DOS Int 21H functions allow programs to allocate, resize, and release blocks of memory from the TPA: Function Action ────────────────────────────────────────────────────────────────────────── 48H Allocate memory block. 49H Release memory block. 4AH Resize memory block. ────────────────────────────────────────────────────────────────────────── MS-DOS itself uses these functions when loading a program from disk at the request of COMMAND.COM or another program. The EXEC function, which is the MS-DOS program loader, calls Int 21H Function 48H to allocate a memory block for the loaded program's environment and another for the program itself and its program segment prefix. It then reads the program from the disk into the assigned memory area. When the program terminates, MS-DOS calls Int 21H Function 49H to release all memory owned by the program. Transient programs can also employ the MS-DOS memory-management functions to dynamically manage the memory available in the TPA. Proper use of these functions is one of the most important criteria of whether a program is well behaved under MS-DOS. Well-behaved programs are most likely to be portable to future versions of the operating system and least likely to cause interference with other processes under multitasking user interfaces such as Microsoft Windows. Using the Memory-Allocation Functions The memory-allocation functions have two common uses: ■ To shrink a program's initial memory allocation so that there is enough room to load and execute another program under its control. ■ To dynamically allocate additional memory required by the program and to release the same memory when it is no longer needed. Shrinking the Initial Memory Allocation Although many MS-DOS application programs simply assume they own all memory, this assumption is a relic of MS-DOS version 1 (and CP/M), which could support only one active process at a time. Well-behaved MS-DOS programs take pains to modify only memory that they actually own and to release any memory that they don't need. Unfortunately, under current versions of MS-DOS, the amount of memory that a program will own is not easily predicted in advance. It turns out that the amount of memory allocated to a program when it is first loaded depends upon two factors: ■ The type of file the program is loaded from ■ The amount of memory available in the TPA MS-DOS always allocates all of the largest available memory block in the TPA to programs loaded from .COM (memory-image) files. Because .COM programs contain no file header that can pass segment and memory-use information to MS-DOS, MS-DOS simply assumes the worst case and gives such a program everything. MS-DOS will load the program as long as there is an available memory block as large as the size of the file plus 256 bytes for the PSP and 2 bytes for the stack. The .COM program, when it receives control, must determine whether enough memory is available to carry out its functions. MS-DOS uses more complicated rules to allocate memory to programs loaded from .EXE files. First, of course, a memory block large enough to hold the declared code, data, and stack segments must be available in the TPA. In addition, the linker sets two fields in a .EXE file's header to inform MS-DOS about the program's memory requirements. The first field, MIN_ALLOC, defines the minimum number of paragraphs required by the program, in addition to those for the code, data, and stack segments. The second, MAX_ALLOC, defines the maximum number of paragraphs of additional memory the program would use if they were available. When loading a .EXE file, MS-DOS first attempts to allocate the number of paragraphs in MAX_ALLOC plus the number of paragraphs required by the program itself. If that much memory is not available, MS-DOS assigns all of the largest available block to the program, provided that this is at least the amount specified by MIN_ALLOC plus the size of the program image. If that condition is not satisfied, the program cannot be executed. After a .COM or .EXE program is loaded and running, it can use Int 21H Function 4AH (Resize Memory Block) to release all the memory it does not immediately need. This is conveniently done right after the program receives control from MS-DOS, by calling the resize function with the segment of the program's PSP in the ES register and the number of paragraphs that the program requires to run in the BX register (Figure 11-1). ────────────────────────────────────────────────────────────────────────── . . . org 100h main proc near ; entry point from MS-DOS ; DS, ES = PSP address mov sp,offset stk ; COM program must move ; stack to safe area ; release extra memory... mov ah,4ah ; function 4Ah = ; resize memory block ; BX = paragraphs to keep mov bx,(offset stk - offset main + 10FH) / 16 int 21h ; transfer to MS-DOS jc error ; jump if resize failed . . . main endp . . . dw 64 dup (?) ; new stack area stk equ $ ; new base of stack end main ; defines entry point ────────────────────────────────────────────────────────────────────────── Figure 11-1. An example of a .COM program releasing excess memory after it receives control from MS-DOS. Int 21H Function 4AH is called with ES pointing to the program's PSP and BX containing the number of paragraphs that the program needs to execute. In this case, the new size for the program's memory block is calculated as the program image size plus the size of the PSP (256 bytes), rounded up to the next paragraph. .EXE programs use similar code. Dynamic Allocation of Additional Memory When a well-behaved program needs additional memory space──for an I/O buffer or an array of intermediate results, for example──it can call Int 21H Function 48H (Allocate Memory Block) with the desired number of paragraphs. If a sufficiently large block of unallocated memory is available, MS-DOS returns the segment address of the base of the assigned area and clears the carry flag (0), indicating that the function was successful. If no unallocated block of sufficient size is available, MS-DOS sets the carry flag (1), returns an error code in the AX register, and returns the size (in paragraphs) of the largest block available in the BX register (Figure 11-2). In this case, no memory has yet been allocated. The program can use the value returned in the BX register to determine whether it can continue in a "degraded" fashion, with less memory. If it can, it must call Int 21H Function 48H again to allocate the smaller memory block. When the MS-DOS memory manager is searching the chain of arena headers to satisfy a memory-allocation request, it can use one of the following strategies: ■ First fit: Use the arena entry at the lowest address that is large enough to satisfy the request. ■ Best fit: Use the smallest arena entry that will satisfy the request, regardless of its location. ■ Last fit: Use the arena entry at the highest address that is large enough to satisfy the request. ────────────────────────────────────────────────────────────────────────── . . . mov ah,48h ; function 48h = allocate mem bl mov bx,0800h ; 800h paragraphs = 32 KB int 21h ; transfer to MS-DOS jc error ; jump if allocation failed mov buff_seg,ax ; save segment of allocated bloc . . . mov es,buff_seg ; ES:DI = address of block xor di,di mov cx,08000h ; store 32,768 bytes mov al,0ffh ; fill buffer with -1s cld rep stosb ; now perform fast fill . . . mov cx,08000h ; length to write, bytes mov bx,handle ; handle for prev opened file push ds ; save our data segment mov ds,buff_seg ; let DS:DX = buffer address mov dx,0 mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS pop ds ; restore our data segment jc error ; jump if write failed . . . mov es,buff_seg ; ES = seg of prev allocated blo mov ah,49h ; function 49h = release mem blo int 21h ; transfer to MS-DOS jc error ; jump if release failed . error: . . handle dw 0 ; file handle buff_seg dw 0 ; segment of allocated block . . . ────────────────────────────────────────────────────────────────────────── Figure 11-2. Example of dynamic memory allocation. The program requests a 32 KB memory block from MS-DOS, fills it with -1s, writes it to disk, and then releases it. If the arena entry selected is larger than the size requested, MS-DOS divides it into two parts: one block of the size requested, which is assigned to the program that called Int 21H Function 48H, and an unowned block containing the remaining memory. The default MS-DOS allocation strategy is first fit. However, under MS-DOS versions 3.0 and later, an application program can change the strategy with Int 21H Function 58H. When a program is through with an allocated memory block, it should use Int 21H Function 49H to release the block. If it does not, MS-DOS will automatically release all memory allocations for the program when it terminates. Arena Headers Microsoft has not officially documented the internal structure of arena headers for the outside world at present. This is probably to deter programmers from trying to manipulate their memory allocations directly instead of through the MS-DOS functions provided for that purpose. Arena headers have identical structures in MS-DOS versions 2 and 3. They are 16 bytes (one paragraph) and are located immediately before the memory area that they control (Figure 11-3). An arena header contains the following information: ■ A byte signifying whether the header is a member or the last entry in the entire chain of such headers ■ A word indicating whether the area it controls is available or whether it already belongs to a program (if the latter, the word points to the program's PSP) ■ A word indicating the size (in paragraphs) of the controlled memory area (arena entry) MS-DOS inspects the chain of arena headers whenever the program requests a memory-block allocation, modification, or release function, or when a program is EXEC'd or terminated. If any of the blocks appear to be corrupted or if the chain is broken, MS-DOS displays the dreaded message Memory allocation error and halts the system. In the example illustrated in Figure 11-3, COMMAND.COM originally loaded PROGRAM1.COM into the TPA and, because it was a .COM file, COMMAND.COM allocated it all of the TPA, controlled by arena header #1. PROGRAM1.COM then used Int 21H Function 4AH (Resize Memory Block) to shrink its memory allocation to the amount it actually needed to run and loaded and executed PROGRAM2.EXE with the EXEC function (Int 21H Function 4BH). The EXEC function obtained a suitable amount of memory, controlled by arena header #2, and loaded PROGRAM2.EXE into it. PROGRAM2.EXE, in turn, needed some additional memory to store some intermediate results, so it called Int 21H Function 48H (Allocate Memory Block) to obtain the area controlled by arena header #3. The highest arena header (#4) controls all of the remaining TPA that has not been allocated to any program. ┌─────────────────────────────────────────────────┐ Top of RAM │ Unowned RAM controlled by header #4 │ controlled by MS-DOS ├─────────────────────────────────────────────────┤ │ Arena header #4 │ ├─────────────────────────────────────────────────┤ │ Memory area controlled by header #3; additional │ │ storage dynamically allocated by PROGRAM2.EXE │ ├─────────────────────────────────────────────────┤ │ Arena header #3 │ ├─────────────────────────────────────────────────┤ │ Memory area controlled by header #2, │ │ containing PROGRAM2.EXE │ ├─────────────────────────────────────────────────┤ │ Arena header #2 │ ├─────────────────────────────────────────────────┤ │ Memory area controlled by header #1, │ │ containing PROGRAM1.COM │ ├─────────────────────────────────────────────────┤ │ Arena header #1 │ └─────────────────────────────────────────────────┘ Bottom of transient- program area Figure 11-3. An example diagram of MS-DOS arena headers and the transient-program area. The environment blocks and their associated headers have been omitted from this figure to increase its clarity. Lotus/Intel/Microsoft Expanded Memory When the IBM Personal Computer and MS-DOS were first released, the 640 KB limit that IBM placed on the amount of RAM that could be directly managed by MS-DOS seemed almost unimaginably huge. But as MS-DOS has grown in both size and capabilities and the popular applications have become more powerful, that 640 KB has begun to seem a bit crowded. Although personal computers based on the 80286 and 80386 have the potential to manage up to 16 megabytes of RAM under operating systems such as MS OS/2 and XENIX, this is little comfort to the millions of users of 8086/8088-based computers and MS-DOS. At the spring COMDEX in 1985, Lotus Development Corporation and Intel Corporation jointly announced the Expanded Memory Specification 3.0 (EMS), which was designed to head off rapid obsolescence of the older PCs because of limited memory. Shortly afterward, Microsoft announced that it would support the EMS and would enhance Microsoft Windows to use the memory made available by EMS hardware and software. EMS versions 3.2 and 4.0, released in fall 1985 and summer 1987, expanded support for multitasking operating systems. The LIM EMS (as it is usually known) has been an enormous success. EMS memory boards are available from scores of manufacturers, and "EMS-aware" software──especially spreadsheets, disk caches, and terminate-and-stay- resident utilities──has become the rule rather than the exception. What Is Expanded Memory? The Lotus/Intel/Microsoft Expanded Memory Specification is a functional definition of a bank-switched memory-expansion subsystem. It consists of hardware expansion modules and a resident driver program specific to those modules. In EMS versions 3.0 and 3.2, the expanded memory is made available to application software as 16 KB pages mapped into a contiguous 64 KB area called the page frame, somewhere above the main memory area used by MS-DOS/PC-DOS (0─640 KB). The exact location of the page frame is user configurable, so it need not conflict with other hardware options. In EMS version 4.0, the pages may be mapped anywhere in memory and can have sizes other than 16 KB. The EMS provides a uniform means for applications to access as much as 8 megabytes of memory (32 megabytes in EMS 4.0). The supporting software, which is called the Expanded Memory Manager (EMM), provides a hardware-independent interface between application software and the expanded memory board(s). The EMM is supplied in the form of an installable device driver that you link into the MS-DOS/PC-DOS system by adding a line to the CONFIG.SYS file on the system boot disk. Internally, the Expanded Memory Manager consists of two major portions, which may be referred to as the driver and the manager. The driver portion mimics some of the actions of a genuine installable device driver, in that it includes initialization and output status functions and a valid device header. The second, and major, portion of the EMM is the true interface between application software and the expanded-memory hardware. Several classes of services are provided: ■ Verification of functionality of hardware and software modules ■ Allocation of expanded-memory pages ■ Mapping of logical pages into the physical page frame ■ Deallocation of expanded-memory pages ■ Support for multitasking operating systems Application programs communicate with the EMM directly, by means of software Int 67H. MS-DOS versions 3.3 and earlier take no part in (and in fact are completely oblivious to) any expanded-memory manipulations that may occur. MS-DOS version 4.0 and Microsoft Windows, on the other hand, are "EMS-aware" and can use the EMS memory when it is available. Expanded memory should not be confused with extended memory. Extended memory is the term used by IBM to refer to the memory at physical addresses above 1 megabyte that can be accessed by an 80286 or 80386 CPU in protected mode. Current versions of MS-DOS run the 80286 and 80386 in real mode (8086-emulation mode), and extended memory is therefore not directly accessible. Checking for Expanded Memory An application program can use either of two methods to test for the existence of the Expanded Memory Manager: ■ Issue an open request (Int 21H Function 3DH) using the guaranteed device name of the EMM driver: EMMXXXX0. If the open function succeeds, either the driver is present or a file with the same name coincidentally exists on the default disk drive. To rule out the latter, the application can use IOCTL (Int 21H Function 44H) subfunctions 00H and 07H to ensure that EMM is present. In either case, the application should then use Int 21H Function 3EH to close the handle that was obtained from the open function, so that the handle can be reused for another file or device. ■ Use the address that is found in the Int 67H vector to inspect the device header of the presumed EMM. Interrupt handlers and device drivers must use this method. If the EMM is present, the name field at offset 0AH of the device header contains the string EMMXXXX0. This approach is nearly foolproof and avoids the relatively high overhead of an MS-DOS open function. However, it is somewhat less well behaved because it involves inspection of memory that does not belong to the application. These two methods of testing for the existence of the Expanded Memory Manager are illustrated in Figures 11-4 and 11-5. ────────────────────────────────────────────────────────────────────────── . . . ; attempt to "open" EMM... mov dx,seg emm_name ; DS:DX = address of name mov ds,dx ; of Expanded Memory Manager mov dx,offset emm_name mov ax,3d00h ; function 3dh, mode = 00h ; = open, read only int 21h ; transfer to MS-DOS jc error ; jump if open failed ; open succeeded, be sure ; it was not a file... mov bx,ax ; BX = handle from open mov ax,4400h ; function 44h subfunction 00h ; = IOCTL get device information int 21h ; transfer to MS-DOS jc error ; jump if IOCTL call failed and dx,80h ; bit 7 = 1 if character device jz error ; jump if it was a file ; EMM is present, be sure ; it is available... ; (BX still contains handle) mov ax,4407h ; function 44h subfunction 07h ; = IOCTL get output status int 21h ; transfer to MS-DOS jc error ; jump if IOCTL call failed or al,al ; test device status jz error ; if AL = 0 EMM is not available ; now close handle ... ; (BX still contains handle) mov ah,3eh ; function 3eh = close int 21h ; transfer to MS-DOS jc error ; jump if close failed . . . emm_name db 'EMMXXXX0',0 ; guaranteed device name for ; Expanded Memory Manager ────────────────────────────────────────────────────────────────────────── Figure 11-4. Testing for the Expanded Memory Manager by means of the MS-DOS open and IOCTL functions. ────────────────────────────────────────────────────────────────────────── emm_int equ 67h ; Expanded Memory Manager ; software interrupt . . . ; first fetch contents of ; EMM interrupt vector... mov al,emm_int ; AL = EMM int number mov ah,35h ; function 35h = get vector int 21h ; transfer to MS-DOS ; now ES:BX = handler address ; assume ES:0000 points ; to base of the EMM... mov di,10 ; ES:DI = address of name ; field in device header ; DS:SI = EMM driver name mov si,seg emm_name mov ds,si mov si,offset emm_name mov cx,8 ; length of name field cld repz cmpsb ; compare names... jnz error ; jump if driver absent . . . emm_name db 'EMMXXXX0' ; guaranteed device name for ; Expanded Memory Manager ────────────────────────────────────────────────────────────────────────── Figure 11-5. Testing for the Expanded Memory Manager by inspection of the name field in the driver's device header. Using Expanded Memory After establishing that the memory-manager software is present, the application program communicates with it directly by means of the "user interrupt" 67H, bypassing MS-DOS/PC-DOS. The calling sequence for the EMM is as follows: ────────────────────────────────────────────────────────────────────────── mov ah,function ; AH determines service type . ; load other registers with . ; values specific to the . ; requested service int 67h ────────────────────────────────────────────────────────────────────────── In general, AH contains the EMM function number, AL holds the subfunction number (if any), BX holds a number of pages (if applicable), and DX contains an EMM handle. Registers DS:SI and ES:DI are used to pass the addresses of arrays or buffers. Section 4 of this book, "Lotus/Intel/Microsoft EMS Functions Reference," details each of the expanded memory functions. Upon return from an EMM function, the AH register contains zero if the function was successful; otherwise, it contains an error code with the most significant bit set (Figures 11-6 and 11-7). Other values are typically returned in the AL and BX registers or in a user-specified buffer. ╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖ Error code Meaning ────────────────────────────────────────────────────────────────────────── 00H Function successful. 80H Internal error in Expanded Memory Manager software (could be caused by corrupted memory image of driver). 81H Malfunction in expanded-memory hardware. 82H Memory manager busy. 83H Invalid handle. 84H Function requested by application not defined. 85H No more handles available. 86H Error in save or restore of mapping context. 87H Allocation request specified more logical pages than physically available in system; no pages allocated. Error code Meaning ────────────────────────────────────────────────────────────────────────── physically available in system; no pages allocated. 88H Allocation request specified more logical pages than currently available in system (request does not exceed physical pages that exist, but some are already allocated to other handles); no pages allocated. Zero pages; cannot be allocated. 8AH Logical page requested to be mapped located outside range of logical pages assigned to handle. 8BH Illegal physical page number in mapping request (not in range 0─3). 8CH Page-mapping hardware-state save area full. Error code Meaning ────────────────────────────────────────────────────────────────────────── 8DH Save of mapping context failed; save area already contains context associated with requested handle. 8EH Restore of mapping context failed; save area does not contain context for requested handle. 8FH Subfunction parameter not defined. ────────────────────────────────────────────────────────────────────────── Figure 11-6. Expanded Memory Manager error codes common to EMS versions 3.0, 3.2, and 4.0. After a call to EMM, the AH register contains zero if the function was successful or an error code in the range 80H through 8FH if the function failed. ╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖ Error code Meaning ────────────────────────────────────────────────────────────────────────── Error code Meaning ────────────────────────────────────────────────────────────────────────── 90H Attribute type not defined. 91H Feature not supported. 92H Source and destination memory regions have same handle and overlap; requested move was performed, but part of source region was overwritten. 93H Specified length for source or destination memory region is longer than actual allocated length. 94H Conventional-memory region and expanded-memory region overlap. 95H Specified offset is outside logical page. 96H Region length exceeds 1 MB. 97H Source and destination memory regions have same handle Error code Meaning ────────────────────────────────────────────────────────────────────────── 97H Source and destination memory regions have same handle and overlap; exchange cannot be performed. 98H Memory source and destination types undefined. 99H This error code currently unused. 9AH Alternate map or DMA register sets supported, but the alternate register set specified is not supported. 9BH Alternate map or DMA register sets supported, but all alternate register sets currently allocated. 9CH Alternate map or DMA register sets not supported, and specified alternate register set not zero. 9DH Alternate map or DMA register sets supported, but alternate register set specified is either not defined or not allocated. Error code Meaning ────────────────────────────────────────────────────────────────────────── or not allocated. Dedicated DMA channels not supported. 9FH Dedicated DMA channels supported, but specified DMA channel not supported. A0H No handle found for specified name. A1H Handle with this name already exists. A2H Memory address wrap; sum of the source or destination region base address and length exceeds 1 MB. A3H Invalid pointer passed to function, or contents of source array corrupted. A4H Access to function denied by operating system. ────────────────────────────────────────────────────────────────────────── Error code Meaning ────────────────────────────────────────────────────────────────────────── ────────────────────────────────────────────────────────────────────────── Figure 11-7. Expanded Memory Manager error codes unique to EMS version 4.0. Most of these errors are related to the EMS functions for use by operating systems and would not normally be encountered by application programs. An application program that uses expanded memory should regard that memory as a system resource, like a file or a device, and employ only the documented EMM services to allocate, access, and release expanded-memory pages. Such a program can use the following general strategy: 1. Establish the presence of the Expanded Memory Manager by one of the two methods demonstrated in Figures 11-4 and 11-5. 2. After the driver is known to be present, check its operational status with EMS Function 40H. 3. Check the version number of EMM with EMS Function 46H, to ensure that all services the application will request are available. 4. Obtain the segment of the page frame used by EMM with EMS Function 41H. 5. Allocate the desired number of expanded-memory pages with EMS Function 43H. If the allocation is successful, EMM returns a handle that the application can use to refer to the expanded-memory pages that it owns. This step is exactly analogous to opening a file and using the handle obtained from the open function for read/write operations on the file. 6. If the requested number of pages are not available, the application can query EMM for the actual number of pages available (EMS Function 42H) and determine whether it can continue. 7. After the application has successfully allocated the needed number of expanded-memory pages, it uses EMS Function 44H to map logical pages in and out of the physical page frame in order to store and retrieve data in expanded memory. 8. When the program finishes using its expanded-memory pages, it must release them by calling EMS Function 45H. Otherwise, the pages will be lost to use by other programs until the system is restarted. Figure 11-8 shows a skeleton program that illustrates this general approach. An interrupt handler or device driver that uses EMS follows the same general procedure outlined in steps 1 through 8, with a few minor variations. It may need to acquire an EMS handle and allocate pages before the operating system is fully functional; in particular, you cannot assume that the MS-DOS Open File or Device, IOCTL, and Get Interrupt Vector functions are available. Thus, such a handler or driver must use a modified version of the "get interrupt vector" technique (Figure 11-5) to test for the existence of EMM, fetching the contents of the Int 67H vector directly. A device driver or interrupt handler typically owns its expanded-memory pages permanently (until the system is restarted) and never deallocates them. Such a program must also take care to save and restore EMM's page-mapping context (EMS Functions 47H and 48H) whenever it accesses expanded memory, so that use of EMS by a foreground program will not be disturbed. The EMM relies on the good behavior of application software to avoid the corruption of expanded memory. If several applications that use expanded memory are running under a multitasking manager such as Microsoft Windows and one or more of them does not abide strictly by EMM conventions, the data of some or all of the applications may be destroyed. ────────────────────────────────────────────────────────────────────────── . . . mov ah,40h ; test EMM status int 67h or ah,ah jnz error ; jump if bad status from EMM mov ah,46h ; check EMM version int 67h or ah,ah jnz error ; jump if couldn't get version cmp al,030h ; make sure at least ver 3.0 jb error ; jump if wrong EMM version mov ah,41h ; get page frame segment int 67h or ah,ah jnz error ; jump if failed to get frame mov page_frame,bx ; save segment of page frame mov ah,42h ; get number of available pages int 67h or ah,ah jnz error ; jump if get pages error mov total_pages,dx ; save total EMM pages mov avail_pages,bx ; save available EMM pages or bx,bx jz error ; abort if no pages available mov ah,43h ; try to allocate EMM pages mov bx,needed_pages int 67h ; if allocation is successful or ah,ah jnz error ; jump if allocation failed mov emm_handle,dx ; save handle for allocated pages . . ; now we are ready for other . ; processing using EMM pages . ; map in EMS memory page... mov bx,log_page ; BX <- EMS logical page number mov al,phys_page ; AL <- EMS physical page (0-3) mov dx,emm_handle ; EMM handle for our pages mov ah,44h ; function 44h = map EMS page int 67h or ah,ah jnz error ; jump if mapping error . . . ; program ready to terminate, ; give up allocated EMM pages... mov dx,emm_handle ; handle for our pages mov ah,45h ; EMS function 45h = release pages int 67h or ah,ah jnz error ; jump if release failed . . . ────────────────────────────────────────────────────────────────────────── Figure 11-8. A program illustrating the general strategy for using expanded memory. Extended Memory Extended memory is RAM storage at addresses above 1 megabyte (100000H) that can be accessed by an 80286 or 80386 processor running in protected mode. IBM PC/AT─ and PS/2─compatible machines can (theoretically) have as much as 15 MB of extended memory installed, in addition to the usual 1 MB of conventional memory. Protected-mode operating systems such as Microsoft XENIX or MS OS/2 can use extended memory for execution of programs. MS-DOS, on the other hand, runs in real mode on an 80286 or 80386, and programs running under its control cannot ordinarily execute from extended memory or even address that memory for storage of data. However, the ROM BIOS contains two routines that allow real-mode programs restricted access to extended memory: ROM BIOS function Action ────────────────────────────────────────────────────────────────────────── Int 15H Function 87H Move extended-memory block. Int 15H Function 88H Get extended-memory size. ────────────────────────────────────────────────────────────────────────── These routines can be used by electronic disks (RAMdisks) and by other programs that want to use extended memory for fast storage and retrieval of information that would otherwise have to be written to a slower physical disk drive. Section 3 of this book, "IBM ROM BIOS and Mouse Functions Reference," documents both of these functions. You should use these ROM BIOS routines with caution. Data stored in extended memory is, of course, volatile; it is lost if the machine is turned off. The transfer of data to or from extended memory involves a switch from real mode to protected mode and back, which is a relatively slow process on 80286-based machines; in some cases it is only marginally faster than actually reading the data from a fixed disk. In addition, programs that use the ROM BIOS extended-memory functions are not compatible with the MS-DOS compatibility mode of MS OS/2. Finally, a major deficit in these ROM BIOS functions is that they do not make any attempt to arbitrate between two or more programs or drivers that are using extended memory for temporary storage. For example, if an application program and an installed RAMdisk driver attempt to put data in the same area of extended memory, no error will be returned to either program, but the data of one or both may be destroyed. Figure 11-9 shows an example of the code necessary to transfer data to and from extended memory. ────────────────────────────────────────────────────────────────────────── bmdt db 30h dup (0) ; block move descriptor table buff1 db 80h dup ('?') ; source buffer buff2 db 80h dup (0) ; destination buffer . . . ; copy 'buff1' to extended- ; memory address 100000h mov dx,10h ; DX:AX = destination mov ax,0 ; extended-memory address mov bx,seg buff1 ; DS:BX = source conventional- mov ds,bx ; memory address mov bx,offset buff1 mov cx,80h ; CX = bytes to move mov si,seg bmdt ; ES:SI = block move mov es,si ; descriptor table mov si,offset bmdt call putblk ; request transfer ; fill buff2 from extended- ; memory address 100000h mov dx,10h ; DX:AX = source extended- mov ax,0 ; memory address mov bx,seg buff2 ; DS:BX = destination mov ds,bx ; conventional-memory address mov bx,offset buff2 mov cx,80h ; CX = bytes to move mov si,seg bmdt ; ES:SI = block move mov es,si ; descriptor table mov si,offset bmdt call getblk ; request transfer . . . getblk proc near ; transfer block from extended ; memory to real memory ; call with ; DX:AX = source linear 32-bit ; extended-memory address ; DS:BX = segment and offset ; destination address ; CX = length in bytes ; ES:SI = block move descriptor ; table ; returns ; AH = 0 if transfer OK mov es:[si+10h],cx ; store length into descriptors mov es:[si+18h],cx ; store access rights bytes mov byte ptr es:[si+15h],93h mov byte ptr es:[si+1dh],93h mov es:[si+12h],ax ; source extended-memory address mov es:[si+14h],dl ; convert destination segment ; and offset to linear address mov ax,ds ; segment * 16 mov dx,16 mul dx add ax,bx ; + offset -> linear address adc dx,0 mov es:[si+1ah],ax ; store destination address mov es:[si+1ch],dl shr cx,1 ; convert length to words mov ah,87h ; int 15h function 87h = block move int 15h ; transfer to ROM BIOS ret ; back to caller getblk endp putblk proc near ; transfer block from real ; memory to extended memory ; call with ; DX:AX = dest linear 32-bit ; extended-memory address ; DS:BX = segment and offset ; source address ; CX = length in bytes ; ES:SI = block move descriptor ; table ; returns ; AH = 0 if transfer OK mov es:[si+10h],cx ; store length into descriptors mov es:[si+18h],cx ; store access rights bytes mov byte ptr es:[si+15h],93h mov byte ptr es:[si+1dh],93h mov es:[si+1ah],ax ; store destination extended- mov es:[si+1ch],dl ; memory address ; convert source segment and ; offset to linear address mov ax,ds ; segment * 16 mov dx,16 mul dx add ax,bx ; + offset -> linear address adc dx,0 mov es:[si+12h],ax ; store source address mov es:[si+14h],dl shr cx,1 ; convert length to words mov ah,87h ; int 15h function 87h = block move int 15h ; transfer to ROM BIOS ret ; back to caller putblk endp ────────────────────────────────────────────────────────────────────────── Figure 11-9. Moving blocks of data between conventional memory and extended memory, using the ROM BIOS extended-memory functions. For additional information on the format of the block move descriptor table, see the entry for Int 15H Function 87H in Section 3 of this book, "IBM ROM BIOS and Mouse Functions Reference." Note that you must specify the extended-memory address as a 32-bit linear address rather than as a segment and offset. ──────────────────────────────────────────────────────────────────────────── Chapter 12 The EXEC Function The MS-DOS EXEC function (Int 21H Function 4BH) allows a program (called the parent) to load any other program (called the child) from a storage device, execute it, and then regain control when the child program is finished. A parent program can pass information to the child in a command line, in default file control blocks, and by means of a set of strings called the environment block (discussed later in this chapter). All files or devices that the parent opened using the handle file-management functions are duplicated in the newly created child task; that is, the child inherits all the active handles of the parent task. Any file operations on those handles by the child, such as seeks or file I/O, also affect the file pointers associated with the parent's handles. MS-DOS suspends execution of the parent program until the child program terminates. When the child program finishes its work, it can pass an exit code back to the parent, indicating whether it encountered any errors. It can also, in turn, load other programs, and so on through many levels of control, until the system runs out of memory. The MS-DOS command interpreter, COMMAND.COM, uses the EXEC function to run its external commands and other application programs. Many popular commercial programs, such as database managers and word processors, use EXEC to run other programs (spelling checkers, for example) or to load a second copy of COMMAND.COM, thereby allowing the user to list directories or copy and rename files without closing all the application files and stopping the main work in progress. EXEC can also be used to load program overlay segments, although this use is uncommon. Making Memory Available In order for a parent program to use the EXEC function to load a child program, sufficient unallocated memory must be available in the transient program area. When the parent itself was loaded, MS-DOS allocated it a variable amount of memory, depending upon its original file type──.COM or .EXE──and any other information that was available to the loader. (See Chapter 11 for further details.) Because the operating system has no foolproof way of predicting how much memory any given program will require, it generally allocates far more memory to a program than is really necessary. Therefore, a prospective parent program's first action should be to use Int 21H Function 4AH (Resize Memory Block) to release any excess memory allocation of its own to MS-DOS. In this case, the program should call Int 21H Function 4AH with the ES register pointing to the program segment prefix of the program releasing memory and the BX register containing the number of paragraphs of memory to retain for that program. (See Figure 11-1 for an example.) ────────────────────────────────────────────────────────────────────────── WARNING A .COM program must move its stack to a safe area if it is reducing its memory allocation to less than 64 KB. ────────────────────────────────────────────────────────────────────────── Requesting the EXEC Function To load and execute a child program, the parent must execute an Int 21H with the registers set up as follows: AH = 4BH AL = 00H (subfunction to load child program) DS:DX = segment:offset of pathname for child program ES:BX = segment:offset of parameter block The parameter block, in turn, contains addresses of other information needed by the EXEC function. The Program Name The name of the program to be run, which the calling program provides to the EXEC function, must be an unambiguous file specification (no wildcard characters) and must include an explicit .COM or .EXE extension. If the path and disk drive are not supplied in the program name, MS-DOS uses the current directory and default disk drive. (The sequential search for .COM, .EXE, and .BAT files in all the locations listed in the PATH variable is not a function of EXEC, but rather of the internal logic of COMMAND.COM.) You cannot EXEC a batch file directly; instead, you must EXEC a copy of COMMAND.COM and pass the name of the batch file in the command tail, along with the /C switch. The Parameter Block The parameter block contains the addresses of four data objects: ■ The environment block ■ The command tail ■ Two default file control blocks The space reserved in the parameter block for the address of the environment block is only 2 bytes and holds a segment address. The remaining three addresses are all double-word addresses; that is, they are 4 bytes, with the offset in the first 2 bytes and the segment address in the last 2 bytes. The Environment Block Each program that the EXEC function loads inherits a data structure called an environment block from its parent. The pointer to the segment of the block is at offset 002CH in the PSP. The environment block holds certain information used by the system's command interpreter (usually COMMAND.COM) and may also hold information to be used by transient programs. It has no effect on the operation of the operating system proper. If the environment-block pointer in the EXEC parameter block contains zero, the child program acquires a copy of the parent program's environment block. Alternatively, the parent program can provide a segment pointer to a different or expanded environment. The maximum size of the environment block is 32 KB, so very large chunks of information can be passed between programs by this mechanism. The environment block for any given program is static, implying that if more than one generation of child programs is resident in RAM, each one will have a distinct and separate copy of the environment block. Furthermore, the environment block for a program that terminates and stays resident is not updated by subsequent PATH and SET commands. You will find more details about the environment block later in this chapter. The Command Tail MS-DOS copies the command tail into the child program's PSP at offset 0080H, as described in Chapter 3. The information takes the form of a count byte, followed by a string of ASCII characters, terminated by a carriage return; the carriage return is not included in the count. The command tail can include filenames, switches, or other parameters. From the child program's point of view, the command tail should provide the same information that would be present if the program had been run by a direct user command at the MS-DOS prompt. EXEC ignores any I/O-redirection parameters placed in the command tail; the parent program must provide for redirection of the standard devices before the EXEC call is made. The Default File Control Blocks MS-DOS copies the two default file control blocks pointed to by the EXEC parameter block into the child program's PSP at offsets 005CH and 006CH. To emulate the function of COMMAND.COM from the child program's point of view, the parent program should use Int 21H Function 29H (the system parse-filename service) to parse the first two parameters of the command tail into the default file control blocks before invoking the EXEC function. File control blocks are not much use under MS-DOS versions 2 and 3, because they do not support the hierarchical file structure, but some application programs do inspect them as a quick way to get at the first two switches or other parameters in the command tail. Chapter 8 discusses file control blocks in more detail. Returning from the EXEC Function In MS-DOS version 2, the EXEC function destroys the contents of all registers except the code segment (CS) and instruction pointer (IP). Therefore, before making the EXEC call, the parent program must push the contents of any other registers that are important onto the stack and then save the stack segment (SS) and stack pointer (SP) registers in variables. Upon return from a successful EXEC call (that is, the child program has finished executing), the parent program should reload SS and SP from the variables where they were saved and then pop the other saved registers off the stack. In MS-DOS versions 3.0 and later, the stack and other registers are preserved across the EXEC call in the usual fashion. Finally, the parent can use Int 21H Function 4DH to obtain the termination type and return code of the child program. The EXEC function will fail under the following conditions: ■ Not enough unallocated memory is available to load and execute the requested program file. ■ The requested program can't be found on the disk. ■ The transient portion of COMMAND.COM in highest RAM (which contains the actual loader) has been destroyed and not enough free memory is available to reload it (PC-DOS version 2 only). Figure 12-1 summarizes the calling convention for function 4BH. Figure 12-2 shows a skeleton of a typical EXEC call. This particular example uses the EXEC function to load and run the MS-DOS utility CHKDSK.COM. The SHELL.ASM program listing later in this chapter (Figure 12-5) presents a more complete example that includes the use of Int 21H Function 4AH to free unneeded memory. ────────────────────────────────────────────────────────────────────────── Called with: AH = 4BH AL = function type 00 = load and execute program 03 = load overlay ES:BX = segment:offset of parameter block DS:DX = segment:offset of program specification Returns: If call succeeded Carry flag clear. In MS-DOS version 2, all registers except for CS:IP may be destroyed. In MS-DOS versions 3.0 and later, registers are preserved in the usual fashion. If call failed Carry flag set and AX = error code. Parameter block format: If AL = 0 (load and execute program) Bytes 0─1 = segment pointer, environment block Bytes 2─3 = offset of command-line tail Bytes 4─5 = segment of command-line tail Bytes 6─7 = offset of first file control block to be copied into new PSP + 5CH Bytes 8─9 = segment of first file control block Bytes 10─11 = offset of second file control block to be copied into new PSP + 6CH Bytes 12─13 = segment of second file control block If AL = 3 (load overlay) Bytes 0─1 = segment address where file will be loaded Bytes 2─3 = relocation factor to apply to loaded image ────────────────────────────────────────────────────────────────────────── Figure 12-1. Calling convention for the EXEC function (Int 21H Function 4BH). ────────────────────────────────────────────────────────────────────────── cr egu 0dh ; ASCII carriage return . . . mov stkseg,ss ; save stack pointer mov stkptr,sp mov dx,offset pname ; DS:DX = program name mov bx,offset pars ; ES:BX = param block mov ax,4b00h ; function 4bh, subfunction 00h int 21h ; transfer to MS-DOS mov ax,_DATA ; make our data segment mov ds,ax ; addressable again mov es,ax cli ; (for bug in some 8088s) mov ss,stkseg ; restore stack pointer mov sp,stkptr sti ; (for bug in some 8088s) jc error ; jump if EXEC failed . . . stkseg dw 0 ; original SS contents stkptr dw 0 ; original SP contents pname db '\CHKDSK.COM',0 ; pathname of child program pars dw envir ; environment segment dd cmdline ; command line for child dd fcb1 ; file control block #1 dd fcb2 ; file control block #2 cmdline db 4,' *.*',cr ; command line for child fcb1 db 0 ; file control block #1 db 11 dup ('?') db 25 dup (0) fcb2 db 0 ; file control block #2 db 11 dup (' ') db 25 dup (0) envir segment para 'ENVIR' ; environment segment db 'PATH=',0 ; empty search path ; location of COMMAND.COM db 'COMSPEC=A:\COMMAND.COM',0 db 0 ; end of environment envir ends ────────────────────────────────────────────────────────────────────────── Figure 12-2. A brief example of the use of the MS-DOS EXEC call, with all necessary variables and command blocks. Note the protection of the registers for MS-DOS version 2 and the masking of interrupts during loading of SS:SP to circumvent a bug in some early 8088 CPUs. More About the Environment Block The environment block is always paragraph aligned (starts at an address that is a multiple of 16 bytes) and contains a series of ASCIIZ strings. Each of the strings takes the following form: NAME=PARAMETER An additional zero byte (Figure 12-3) indicates the end of the entire set of strings. Under MS-DOS version 3, the block of environment strings and the extra zero byte are followed by a word count and the complete drive, path, filename, and extension used by EXEC to load the program. ────────────────────────────────────────────────────────────────────────── 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF 0000 43 4F 4D 53 50 45 43 3D 43 3A 5C 43 4F 4D 4D 41 COMSPEC=C:\COMMA 0010 4E 44 2E 43 4F 4D 00 50 52 4F 4D 50 54 3D 24 70 NDcom.PROMPT=$p 0020 24 5F 24 64 20 20 20 24 74 24 68 24 68 24 68 24 $_$d $t$h$h$h$ 0030 68 24 68 24 68 20 24 71 24 71 24 67 00 50 41 54 h$h$h $q$q$g.PAT 0040 48 3D 43 3A 5C 53 59 53 54 45 4D 3B 43 3A 5C 41 H=C:\SYSTEM;C:\A 0050 53 4D 3B 43 3A 5C 57 53 3B 43 3A 5C 45 54 48 45 SM;C:\WS;C:\ETHE 0060 52 4E 45 54 3B 43 3A 5C 46 4F 52 54 48 5C 50 43 RNET;C:\FORTH\PC 0070 33 31 3B 00 00 01 00 43 3A 5C 46 4F 52 54 48 5C 31;....C:\FORTH\ 0080 50 43 33 31 5C 46 4F 52 54 48 2E 43 4F 4D 00 20 PC31\FORTH.COM. ────────────────────────────────────────────────────────────────────────── Figure 12-3. Dump of a typical environment block under MS-DOS version 3. This particular example contains the default COMSPEC parameter and two relatively complex PATH and PROMPT control strings that were set up by entries in the user's AUTOEXEC file. Note the path and file specification of the executing program following the double zeros at offset 0073H that denote the end of the environment block. Under normal conditions, the environment block inherited by a program will contain at least three strings: COMSPEC=variable PATH=variable PROMPT=variable MS-DOS places these three strings into the environment block at system initialization, during the interpretation of SHELL, PATH, and PROMPT directives in the CONFIG.SYS and AUTOEXEC.BAT files. The strings tell the MS-DOS command interpreter, COMMAND.COM, the location of its executable file (to enable it to reload the transient portion), where to search for executable external commands or program files, and the format of the user prompt. You can add other strings to the environment block, either interactively or in batch files, with the SET command. Transient programs can use these strings for informational purposes. For example, the Microsoft C Compiler looks in the environment block for INCLUDE, LIB, and TMP strings to tell it where to find its #include files and library files and where to build its temporary working files. Example Programs: SHELL.C and SHELL.ASM As a practical example of use of the MS-DOS EXEC function, I have included a small command interpreter called SHELL, with equivalent Microsoft C (Figure 12-4) and Microsoft Macro Assembler (Figure 12-5) source code. The source code for the assembly-language version is considerably more complex than the code for the C version, but the names and functionality of the various procedures are quite parallel. ────────────────────────────────────────────────────────────────────────── /* SHELL.C Simple extendable command interpreter for MS-DOS versions 2.0 and later Copyright 1988 Ray Duncan Compile: C>CL SHELL.C Usage: C>SHELL */ #include <stdio.h> #include <process.h> #include <stdlib.h> #include <signal.h> /* macro to return number of elements in a structure */ #define dim(x) (sizeof(x) / sizeof(x[0])) unsigned intrinsic(char *); /* function prototypes */ void extrinsic(char *); void get_cmd(char *); void get_comspec(char *); void break_handler(void); void cls_cmd(void); void dos_cmd(void); void exit_cmd(void); struct cmd_table { /* intrinsic commands table */ char *cmd_name; int (*cmd_fxn)(); } commands[] = { "CLS", cls_cmd, "DOS", dos_cmd, "EXIT", exit_cmd, }; static char com_spec[64]; /* COMMAND.COM filespec */ main(int argc, char *argv[]) { char inp_buf[80]; /* keyboard input buffer */ get_comspec(com_spec); /* get COMMAND.COM filespec */ /* register new handler for Ctrl-C interrupts */ if(signal(SIGINT, break_handler) == (int(*)()) -1) { fputs("Can't capture Control-C Interrupt", stderr); exit(1); } while(1) /* main interpreter loop */ { get_cmd(inp_buf); /* get a command */ if (! intrinsic(inp_buf) ) /* if it's intrinsic, run its subroutine */ extrinsic(inp_buf); /* else pass to COMMAND.COM */ } } /* Try to match user's command with intrinsic command table. If a match is found, run the associated routine and return true; else return false. */ unsigned intrinsic(char *input_string) { int i, j; /* some scratch variables */ /* scan off leading blanks */ while(*input_string == '\x20') input_string++ ; /* search command table */ for(i=0; i < dim(commands); i++) { j = strcmp(commands[i].cmd_name, input_string); if(j == 0) /* if match, run routine */ { (*commands[i].cmd_fxn)(); return(1); /* and return true */ } } return(0); /* no match, return false */ } /* Process an extrinsic command by passing it to an EXEC'd copy of COMMAND.COM. */ void extrinsic(char *input_string) { int status; status = system(input_string); /* call EXEC function */ if(status) /* if failed, display error message */ fputs("\nEXEC of COMMAND.COM failed\n", stderr); } /* Issue prompt, get user's command from standard input, fold it to uppercase. */ void get_cmd(char *buffer) { printf("\nsh: "); /* display prompt */ gets(buffer); /* get keyboard entry */ strupr(buffer); /* fold to uppercase */ } /* Get the full path and file specification for COMMAND.COM from the COMSPEC variable in the environment. */ void get_comspec(char *buffer) { strcpy(buffer, getenv("COMSPEC")); if(buffer[0] == NULL) { fputs("\nNo COMSPEC in environment\n", stderr); exit(1); } } /* This Ctrl-C handler keeps SHELL from losing control. It just reissues the prompt and returns. */ void break_handler(void) { signal(SIGINT, break_handler); /* reset handler */ printf("\nsh: "); /* display prompt */ } /* These are the subroutines for the intrinsic commands. */ void cls_cmd(void) /* CLS command */ { printf("\033[2J"); /* ANSI escape sequence */ } /* to clear screen */ void dos_cmd(void) /* DOS command */ { int status; /* run COMMAND.COM */ status = spawnlp(P_WAIT, com_spec, com_spec, NULL); if (status) fputs("\nEXEC of COMMAND.COM failed\n",stderr); } void exit_cmd(void) /* EXIT command */ { exit(0); /* terminate SHELL */ } ────────────────────────────────────────────────────────────────────────── Figure 12-4. SHELL.C: A table-driven command interpreter written in Microsoft C. ────────────────────────────────────────────────────────────────────────── name shell page 55,132 title SHELL.ASM--simple MS-DOS shell ; ; SHELL.ASM Simple extendable command interpreter ; for MS-DOS versions 2.0 and later ; ; Copyright 1988 by Ray Duncan ; ; Build: C>MASM SHELL; ; C>LINK SHELL; ; ; Usage: C>SHELL; ; stdin equ 0 ; standard input handle stdout equ 1 ; standard output handle stderr equ 2 ; standard error handle cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII linefeed blank equ 20h ; ASCII blank code escape equ 01bh ; ASCII escape code _TEXT segment word public 'CODE' assume cs:_TEXT,ds:_DATA,ss:STACK shell proc far ; at entry DS = ES = PSP mov ax,_DATA ; make our data segment mov ds,ax ; addressable mov ax,es:[002ch] ; get environment segment mov env_seg,ax ; from PSP and save it ; release unneeded memory... ; ES already = PSP segment mov bx,100h ; BX = paragraphs needed mov ah,4ah ; function 4ah = resize block int 21h ; transfer to MS-DOS jnc shell1 ; jump if resize OK mov dx,offset msg1 ; resize failed, display mov cx,msg1_length ; error message and exit jmp shell4 shell1: call get_comspec ; get COMMAND.COM filespec jnc shell2 ; jump if it was found mov dx,offset msg3 ; COMSPEC not found in mov cx,msg3_length ; environment, display error jmp shell4 ; message and exit shell2: mov dx,offset shell3 ; set Ctrl-C vector (int 23h) mov ax,cs ; for this program's handler mov ds,ax ; DS:DX = handler address mov ax,2523h ; function 25h = set vector int 21h ; transfer to MS-DOS mov ax,_DATA ; make our data segment mov ds,ax ; addressable again mov es,ax shell3: ; main interpreter loop call get_cmd ; get a command from user call intrinsic ; check if intrinsic function jnc shell3 ; yes, it was processed call extrinsic ; no, pass it to COMMAND.COM jmp shell3 ; then get another command shell4: ; come here if error detected ; DS:DX = message address ; CX = message length mov bx,stderr ; BX = standard error handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS mov ax,4c01h ; function 4ch = terminate with ; return code = 1 int 21h ; transfer to MS-DOS shell endp intrinsic proc near ; decode user entry against ; the table "COMMANDS" ; if match, run the routine, ; and return carry = false ; if no match, carry = true ; return carry = true mov si,offset commands ; DS:SI = command table intr1: cmp byte ptr [si],0 ; end of table? je intr7 ; jump, end of table found mov di,offset inp_buf ; no, let DI = addr of user input intr2: cmp byte ptr [di],blank ; scan off any leading blanks jne intr3 inc di ; found blank, go past it jmp intr2 intr3: mov al,[si] ; next character from table or al,al ; end of string? jz intr4 ; jump, entire string matched cmp al,[di] ; compare to input character jnz intr6 ; jump, found mismatch inc si ; advance string pointers inc di jmp intr3 intr4: cmp byte ptr [di],cr ; be sure user's entry je intr5 ; is the same length... cmp byte ptr [di],blank ; next character in entry jne intr6 ; must be blank or return intr5: call word ptr [si+1] ; run the command routine clc ; return carry flag = false ret ; as success flag intr6: lodsb ; look for end of this or al,al ; command string (null byte) jnz intr6 ; not end yet, loop add si,2 ; skip over routine address jmp intr1 ; try to match next command intr7: stc ; command not matched, exit ret ; with carry = true intrinsic endp extrinsic proc near ; process extrinsic command ; by passing it to ; COMMAND.COM with a ; " /C " command tail mov al,cr ; find length of command mov cx,cmd_tail_length ; by scanning for carriage mov di,offset cmd_tail+1 ; return cld repnz scasb mov ax,di ; calculate command-tail sub ax,offset cmd_tail+2 ; length without carriage mov cmd_tail,al ; return, and store it ; set command-tail address mov word ptr par_cmd,offset cmd_tail call exec ; and run COMMAND.COM ret extrinsic endp get_cmd proc near ; prompt user, get command ; display the shell prompt mov dx,offset prompt ; DS:DX = message address mov cx,prompt_length ; CX = message length mov bx,stdout ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ; get entry from user mov dx,offset inp_buf ; DS:DX = input buffer mov cx,inp_buf_length ; CX = max length to read mov bx,stdin ; BX = standard input handle mov ah,3fh ; function 3fh = read int 21h ; transfer to MS-DOS mov si,offset inp_buf ; fold lowercase characters mov cx,inp_buf_length ; in entry to uppercase gcmd1: cmp byte ptr [si],'a' ; check if 'a-z' jb gcmd2 ; jump, not in range cmp byte ptr [si],'z' ; check if 'a-z' ja gcmd2 ; jump, not in range sub byte ptr [si],'a'-'A' ; convert to uppercase gcmd2: inc si ; advance through entry loop gcmd1 ret ; back to caller get_cmd endp get_comspec proc near ; get location of COMMAND.COM ; from environment "COMSPEC=" ; returns carry = false ; if COMSPEC found ; returns carry = true ; if no COMSPEC mov si,offset com_var ; DS:SI = string to match... call get_env ; search environment block jc gcsp2 ; jump if COMSPEC not found ; ES:DI points past "=" mov si,offset com_spec ; DS:SI = local buffer gcsp1: mov al,es:[di] ; copy COMSPEC variable mov [si],al ; to local buffer inc si inc di or al,al ; null char? (turns off carry) jnz gcsp1 ; no, get next character gcsp2: ret ; back to caller get_comspec endp get_env proc near ; search environment ; call DS:SI = "NAME=" ; uses contents of "ENV_SEG" ; returns carry = false and ES:DI ; pointing to parameter if found, ; returns carry = true if no match mov es,env_seg ; get environment segment xor di,di ; initialize env offset genv1: mov bx,si ; initialize pointer to name cmp byte ptr es:[di],0 ; end of environment? jne genv2 ; jump, end not found stc ; no match, return carry set ret genv2: mov al,[bx] ; get character from name or al,al ; end of name? (turns off carry) jz genv3 ; yes, name matched cmp al,es:[di] ; compare to environment jne genv4 ; jump if match failed inc bx ; advance environment inc di ; and name pointers jmp genv2 genv3: ; match found, carry = clear, ret ; ES:DI = variable genv4: xor al,al ; scan forward in environment mov cx,-1 ; for zero byte cld repnz scasb jmp genv1 ; go compare next string get_env endp exec proc near ; call MS-DOS EXEC function ; to run COMMAND.COM mov stkseg,ss ; save stack pointer mov stkptr,sp ; now run COMMAND.COM mov dx,offset com_spec ; DS:DX = filename mov bx,offset par_blk ; ES:BX = parameter block mov ax,4b00h ; function 4bh = EXEC ; subfunction 0 = ; load and execute int 21h ; transfer to MS-DOS mov ax,_DATA ; make data segment mov ds,ax ; addressable again mov es,ax cli ; (for bug in some 8088s) mov ss,stkseg ; restore stack pointer mov sp,stkptr sti ; (for bug in some 8088s) jnc exec1 ; jump if no errors ; display error message mov dx,offset msg2 ; DS:DX = message address mov cx,msg2_length ; CX = message length mov bx,stderr ; BX = standard error handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS exec1: ret ; back to caller exec endp cls_cmd proc near ; intrinsic CLS command mov dx,offset cls_str ; send the ANSI escape mov cx,cls_str_length ; sequence to clear mov bx,stdout ; the screen mov ah,40h int 21h ret cls_cmd endp dos_cmd proc near ; intrinsic DOS command ; set null command tail mov word ptr par_cmd,offset nultail call exec ; and run COMMAND.COM ret dos_cmd endp exit_cmd proc near ; intrinsic EXIT command mov ax,4c00h ; call MS-DOS terminate int 21h ; function with ; return code of zero exit_cmd endp _TEXT ends STACK segment para stack 'STACK' ; declare stack segment dw 64 dup (?) STACK ends _DATA segment word public 'DATA' commands equ $ ; "intrinsic" commands table ; each entry is ASCIIZ string ; followed by the offset ; of the procedure to be ; executed for that command db 'CLS',0 dw cls_cmd db 'DOS',0 dw dos_cmd db 'EXIT',0 dw exit_cmd db 0 ; end of table com_var db 'COMSPEC=',0 ; environment variable ; COMMAND.COM filespec com_spec db 80 dup (0) ; from environment COMSPEC= nultail db 0,cr ; null command tail for ; invoking COMMAND.COM ; as another shell cmd_tail db 0,' /C ' ; command tail for invoking ; COMMAND.COM as a transient inp_buf db 80 dup (0) ; command line from standard input inp_buf_length equ $-inp_buf cmd_tail_length equ $-cmd_tail-1 prompt db cr,lf,'sh: ' ; SHELL's user prompt prompt_length equ $-prompt env_seg dw 0 ; segment of environment block msg1 db cr,lf db 'Unable to release memory.' db cr,lf msg1_length equ $-msg1 msg2 db cr,lf db 'EXEC of COMMAND.COM failed.' db cr,lf msg2_length equ $-msg2 msg3 db cr,lf db 'No COMSPEC variable in environment.' db cr,lf msg3_length equ $-msg3 cls_str db escape,'[2J' ; ANSI escape sequence cls_str_length equ $-cls_str ; to clear the screen ; EXEC parameter block par_blk dw 0 ; environment segment par_cmd dd cmd_tail ; command line dd fcb1 ; file control block #1 dd fcb2 ; file control block #2 fcb1 db 0 ; file control block #1 db 11 dup (' ') db 25 dup (0) fcb2 db 0 ; file control block #2 db 11 dup (' ') db 25 dup (0) stkseg dw 0 ; original SS contents stkptr dw 0 ; original SP contents _DATA ends end shell ────────────────────────────────────────────────────────────────────────── Figure 12-5. SHELL.ASM: A simple table-driven command interpreter written in Microsoft Macro Assembler. The SHELL program is table driven and can easily be extended to provide a powerful customized user interface for almost any application. When SHELL takes control of the system, it displays the prompt sh: and waits for input from the user. After the user types a line terminated by a carriage return, SHELL tries to match the first token in the line against its table of internal (intrinsic) commands. If it finds a match, it calls the appropriate subroutine. If it does not find a match, it calls the MS-DOS EXEC function and passes the user's input to COMMAND.COM with the /C switch, essentially using COMMAND.COM as a transient command processor under its own control. As supplied in these listings, SHELL "knows" exactly three internal commands: Command Action ────────────────────────────────────────────────────────────────────────── CLS Uses the ANSI standard control sequence to clear the display screen and home the cursor. DOS Runs a copy of COMMAND.COM. EXIT Exits SHELL, returning control of the system to the next lower command interpreter. ────────────────────────────────────────────────────────────────────────── You can quickly add new intrinsic commands to either the C version or the assembly-language version of SHELL. Simply code a procedure with the appropriate action and insert the name of that procedure, along with the text string that defines the command, into the table COMMANDS. In addition, you can easily prevent SHELL from passing certain "dangerous" commands (such as MKDIR or ERASE) to COMMAND.COM simply by putting the names of the commands to be screened out into the intrinsic command table with the address of a subroutine that prints an error message. To summarize, the basic flow of both versions of the SHELL program is as follows: 1. The program calls MS-DOS Int 21H Function 4AH (Resize Memory Block) to shrink its memory allocation, so that the maximum possible space will be available for COMMAND.COM if it is run as an overlay. (This is explicit in the assembly-language version only. To keep the example code simple, the number of paragraphs to be reserved is coded as a generous literal value, rather than being figured out at runtime from the size and location of the various program segments.) 2. The program searches the environment for the COMSPEC variable, which defines the location of an executable copy of COMMAND.COM. If it can't find the COMSPEC variable, it prints an error message and exits. 3. The program puts the address of its own handler in the Ctrl-C vector (Int 23H) so that it won't lose control if the user enters a Ctrl-C or a Ctrl-Break. 4. The program issues a prompt to the standard output device. 5. The program reads a buffered line from the standard input device to get the user's command. 6. The program matches the first blank-delimited token in the line against its table of intrinsic commands. If it finds a match, it executes the associated procedure. 7. If the program does not find a match in the table of intrinsic commands, it synthesizes a command-line tail by appending the user's input to the /C switch and then EXECs a copy of COMMAND.COM, passing the address of the synthesized command tail in the EXEC parameter block. 8. The program repeats steps 4 through 7 until the user enters the command EXIT, which is one of the intrinsic commands, and which causes SHELL to terminate execution. In its present form, SHELL allows COMMAND.COM to inherit a full copy of the current environment. However, in some applications it may be helpful, or safer, to pass a modified copy of the environment block so that the secondary copy of COMMAND.COM will not have access to certain information. Using EXEC to Load Overlays Loading overlays with the EXEC function is much less complex than using EXEC to run another program. The overlay can be constructed as either a memory image (.COM) or relocatable (.EXE) file and need not be the same type as the program that loads it. The main program, called the root segment, must carry out the following steps to load and execute an overlay: 1. Make a memory block available to receive the overlay. The program that calls EXEC must own the memory block for the overlay. 2. Set up the overlay parameter block to be passed to the EXEC function. This block contains the segment address of the block that will receive the overlay, plus a segment relocation value to be applied to the contents of the overlay file (if it is a .EXE file). These are normally the same value. 3. Call the MS-DOS EXEC function to load the overlay by issuing an Int 21H with the registers set up as follows: AH = 4BH AL = 03H (EXEC subfunction to load overlay) DS:DX = segment:offset of overlay file pathname ES:BX = segment:offset of overlay parameter block Upon return from the EXEC function, the carry flag is clear if the overlay was found and loaded. The carry flag is set if the file could not be found or if some other error occurred. 4. Execute the code within the overlay by transferring to it with a far call. The overlay should be designed so that either the entry point or a pointer to the entry point is at the beginning of the module after it is loaded. This technique allows you to maintain the root and overlay modules separately, because the root module does not contain any "magical" knowledge of addresses within the overlay segment. To prevent users from inadvertently running an overlay directly from the command line, you should assign overlay files an extension other than .COM or .EXE. It is most convenient to relate overlays to their root segment by assigning them the same filename but a different extension, such as .OVL or .OV1, .OV2, and so on. Figure 12-6 shows the use of EXEC to load and execute an overlay. ────────────────────────────────────────────────────────────────────────── . . . ; allocate memory for overlay mov bx,1000h ; get 64 KB (4096 paragraphs) mov ah,48h ; function 48h = allocate block int 21h ; transfer to MS-DOS jc error ; jump if allocation failed mov pars,ax ; set load address for overlay mov pars+2,ax ; set relocation segment for overlay ; set segment of entry point mov word ptr entry+2,ax mov stkseg,ss ; save root's stack pointer mov stkptr,sp mov ax,ds ; set ES = DS mov es,ax mov dx,offset oname ; DS:DX = overlay pathname mov bx,offset pars ; ES:BX = parameter block mov ax,4b03h ; function 4bh, subfunction 03h int 21h ; transfer to MS-DOS mov ax,_DATA ; make our data segment mov ds,ax ; addressable again mov es,ax cli ; (for bug in some early 8088s) mov ss,stkseg ; restore stack pointer mov sp,stkptr sti ; (for bug in some early 8088s) jc error ; jump if EXEC failed ; otherwise EXEC succeeded... push ds ; save our data segment call dword ptr entry ; now call the overlay pop ds ; restore our data segment . . . oname db 'OVERLAY.OVL',0 ; pathname of overlay file pars dw 0 ; load address (segment) for file dw 0 ; relocation (segment) for file entry dd 0 ; entry point for overlay stkseg dw 0 ; save SS register stkptr dw 0 ; save SP register ────────────────────────────────────────────────────────────────────────── Figure 12-6. A code skeleton for loading and executing an overlay with the EXEC function. The overlay file may be in either .COM or .EXE format. ──────────────────────────────────────────────────────────────────────────── Chapter 13 Interrupt Handlers Interrupts are signals that cause the computer's central processing unit to suspend what it is doing and transfer to a program called an interrupt handler. Special hardware mechanisms that are designed for maximum speed force the transfer. The interrupt handler determines the cause of the interrupt, takes the appropriate action, and then returns control to the original process that was suspended. Interrupts are typically caused by events external to the central processor that require immediate attention, such as the following: ■ Completion of an I/O operation ■ Detection of a hardware failure ■ "Catastrophes" (power failures, for example) In order to service interrupts more efficiently, most modern processors support multiple interrupt types, or levels. Each type usually has a reserved location in memory, called an interrupt vector, that specifies where the interrupt-handler program for that interrupt type is located. This design speeds processing of an interrupt because the computer can transfer control directly to the appropriate routine; it does not need a central routine that wastes precious machine cycles determining the cause of the interrupt. The concept of interrupt types also allows interrupts to be prioritized, so that if several interrupts occur simultaneously, the most important one can be processed first. CPUs that support interrupts must also have the capability to block interrupts while they are executing critical sections of code. Sometimes the CPU can block interrupt levels selectively, but more frequently the effect is global. While an interrupt is being serviced, the CPU masks all other interrupts of the same or lower priority until the active handler has completed its execution; similarly, it can preempt the execution of a handler if a different interrupt with higher priority requires service. Some CPUs can even draw a distinction between selectively masking interrupts (they are recognized, but their processing is deferred) and simply disabling them (the interrupt is thrown away). The creation of interrupt handlers has traditionally been considered one of the most arcane of programming tasks, suitable only for the elite cadre of system hackers. In reality, writing an interrupt handler is, in itself, straightforward. Although the exact procedure must, of course, be customized for the characteristics of the particular CPU and operating system, the guidelines on the following page are applicable to almost any computer system. A program preparing to handle interrupts must do the following: 1. Disable interrupts, if they were previously enabled, to prevent them from occurring while interrupt vectors are being modified. 2. Initialize the vector for the interrupt of interest to point to the program's interrupt handler. 3. Ensure that, if interrupts were previously disabled, all other vectors point to some valid handler routine. 4. Enable interrupts again. The interrupt handler itself must follow a simple but rigid sequence of steps: 1. Save the system context (registers, flags, and anything else that the handler will modify and that wasn't saved automatically by the CPU). 2. Block any interrupts that might cause interference if they were allowed to occur during this handler's processing. (This is often done automatically by the computer hardware.) 3. Enable any interrupts that should still be allowed to occur during this handler's processing. 4. Determine the cause of the interrupt. 5. Take the appropriate action for the interrupt: receive and store data from the serial port, set a flag to indicate the completion of a disk-sector transfer, and so forth. 6. Restore the system context. 7. Reenable any interrupt levels that were blocked during this handler's execution. 8. Resume execution of the interrupted process. As in writing any other program, the key to success in writing an interrupt handler is to program defensively and cover all the bases. The main reason interrupt handlers have acquired such a mystical reputation is that they are so difficult to debug when they contain obscure errors. Because interrupts can occur asynchronously──that is, because they can be caused by external events without regard to the state of the currently executing process──bugs in interrupt handlers can cause the system as a whole to behave quite unpredictably. Interrupts and the Intel 80x86 Family The Intel 80x86 family of microprocessors supports 256 levels of prioritized interrupts, which can be triggered by three types of events: ■ Internal hardware interrupts ■ External hardware interrupts ■ Software interrupts Internal Hardware Interrupts Internal hardware interrupts, sometimes called faults, are generated by certain events encountered during program execution, such as an attempt to divide by zero. The assignment of such events to certain interrupt numbers is wired into the processor and is not modifiable (Figure 13-1). ╓┌─┌──────────┌─────────┌────────────────────┌──────────┌─────────┌──────────╖ Interrupt Vector Interrupt 8086/88 80286 80386 level address trigger ────────────────────────────────────────────────────────────────────────── 00H 00H─03H Divide-by-zero x x x 01H 04H─07H Single step x x x 02H 08H─0BH Nonmaskable x x x Interrupt Vector Interrupt 8086/88 80286 80386 level address trigger ────────────────────────────────────────────────────────────────────────── 02H 08H─0BH Nonmaskable x x x interrupt (NMI) 03H 0CH─0FH Breakpoint x x x 04H 10H─13H Overflow x x x 05H 14H─17H BOUND exceeded x x 06H 18H─1BH Invalid opcode x x 07H 1CH─1FH Processor extension x x not available 08H 20H─23H Double fault x x 09H 24H─27H Segment overrun x x 0AH 28H─2BH Invalid task-state x x segment 0BH 2CH─2FH Segment not present x x 0CH 30H─33H Stack segment x x overrun 0DH 34H─37H General protection x x fault 0EH 38H─3BH Page fault x Interrupt Vector Interrupt 8086/88 80286 80386 level address trigger ────────────────────────────────────────────────────────────────────────── 0EH 38H─3BH Page fault x 0FH 3CH─3FH Reserved 10H 40H─43H Numeric coprocessor x x error 11H─1FH 44H─7FH Reserved ────────────────────────────────────────────────────────────────────────── Figure 13-1. Internal interrupts (faults) on the Intel 8086/88, 80286, and 80386 microprocessors. External Hardware Interrupts External hardware interrupts are triggered by peripheral device controllers or by coprocessors such as the 8087/80287. These can be tied to either the CPU's nonmaskable-interrupt (NMI) pin or its maskable-interrupt (INTR) pin. The NMI line is usually reserved for interrupts caused by such catastrophic events as a memory parity error or a power failure. Instead of being wired directly to the CPU, the interrupts from external devices can be channeled through a device called the Intel 8259A Programmable Interrupt Controller (PIC). The CPU controls the PIC through a set of I/O ports, and the PIC, in turn, signals the CPU through the INTR pin. The PIC allows the interrupts from specific devices to be enabled and disabled, and their priorities to be adjusted, under program control. A single PIC can handle only eight levels of interrupts. However, PICs can be cascaded together in a treelike structure to handle as many levels as desired. For example, 80286- and 80386-based machines with a PC/AT-compatible architecture use two PICs wired together to obtain 16 individually configurable levels of interrupts. INTR interrupts can be globally enabled and disabled with the CPU's STI and CLI instructions. As you would expect, these instructions have no effect on interrupts received on the CPU's NMI pin. The manufacturer of the computer system and/or the manufacturer of the peripheral device assigns external devices to specific 8259A PIC interrupt levels. These assignments are realized as physical electrical connections and cannot be modified by software. Software Interrupts Any program can trigger software interrupts synchronously simply by executing an INT instruction. MS-DOS uses Interrupts 20H through 3FH to communicate with its modules and with application programs. (For instance, the MS-DOS function dispatcher is reached by executing an Int 21H.) The IBM PC ROM BIOS and application software use other interrupts, with either higher or lower numbers, for various purposes (Figure 13-2). These assignments are simply conventions and are not wired into the hardware in any way. ╓┌─┌──────────────────┌────────────────────────────────────┌─────────────────╖ Interrupt Usage Machine ────────────────────────────────────────────────────────────────────────── 00H Divide-by-zero PC, AT, PS/2 01H Single step PC, AT, PS/2 02H NMI PC, AT, PS/2 03H Breakpoint PC, AT, PS/2 Interrupt Usage Machine ────────────────────────────────────────────────────────────────────────── 03H Breakpoint PC, AT, PS/2 04H Overflow PC, AT, PS/2 05H ROM BIOS PrintScreen PC, AT, PS/2 BOUND exceeded AT, PS/2 06H Reserved PC Invalid opcode AT, PS/2 07H Reserved PC 80287/80387 not present AT, PS/2 08H IRQ0 timer tick PC, AT, PS/2 Double fault AT, PS/2 09H IRQ1 keyboard PC, AT, PS/2 80287/80387 segment overrun AT, PS/2 0AH IRQ2 reserved PC IRQ2 cascade from slave 8259A PIC AT, PS/2 Invalid task-state segment (TSS) AT, PS/2 0BH IRQ3 serial communications (COM2) PC, AT, PS/2 Segment not present AT, PS/2 0CH IRQ4 serial communications (COM1) PC, AT, PS/2 Stack segment overflow AT, PS/2 Interrupt Usage Machine ────────────────────────────────────────────────────────────────────────── Stack segment overflow AT, PS/2 0DH IRQ5 fixed disk PC IRQ5 parallel printer (LPT2) AT Reserved PS/2 General protection fault AT, PS/2 0EH IRQ6 floppy disk PC, AT, PS/2 Page fault AT, PS/2 0FH IRQ7 parallel printer (LPT1) PC, AT, PS/2 10H ROM BIOS video driver PC, AT, PS/2 Numeric coprocessor fault AT, PS/2 11H ROM BIOS equipment check PC, AT, PS/2 12H ROM BIOS conventional-memory size PC, AT, PS/2 13H ROM BIOS disk driver PC, AT, PS/2 14H ROM BIOS communications driver PC, AT, PS/2 15H ROM BIOS cassette driver PC ROM BIOS I/O system extensions AT, PS/2 16H ROM BIOS keyboard driver PC, AT, PS/2 17H ROM BIOS printer driver PC, AT, PS/2 18H ROM BASIC PC, AT, PS/2 Interrupt Usage Machine ────────────────────────────────────────────────────────────────────────── 18H ROM BASIC PC, AT, PS/2 19H ROM BIOS bootstrap PC, AT, PS/2 1AH ROM BIOS time of day AT, PS/2 1BH ROM BIOS Ctrl-Break PC, AT, PS/2 1CH ROM BIOS timer tick PC, AT, PS/2 1DH ROM BIOS video parameter table PC, AT, PS/2 1EH ROM BIOS floppy-disk parameters PC, AT, PS/2 1FH ROM BIOS font (characters 80H─FFH) PC, AT, PS/2 20H MS-DOS terminate process 21H MS-DOS function dispatcher 22H MS-DOS terminate address 23H MS-DOS Ctrl-C handler address 24H MS-DOS critical-error handler address 25H MS-DOS absolute disk read 26H MS-DOS absolute disk write 27H MS-DOS terminate and stay resident 28H MS-DOS idle interrupt 29H MS-DOS reserved Interrupt Usage Machine ────────────────────────────────────────────────────────────────────────── 29H MS-DOS reserved 2AH MS-DOS network redirector 2BH─2EH MS-DOS reserved 2FH MS-DOS multiplex interrupt 30H─3FH MS-DOS reserved 40H ROM BIOS floppy-disk driver (if PC, AT, PS/2 fixed disk installed) 41H ROM BIOS fixed-disk parameters PC ROM BIOS fixed-disk parameters AT, PS/2 (drive 0) 42H ROM BIOS default video driver (if PC, AT, PS/2 EGA installed) 43H EGA, MCGA, VGA character table PC, AT, PS/2 44H ROM BIOS font (characters 00H─7FH) PCjr 46H ROM BIOS fixed-disk parameters AT, PS/2 (drive 1) 4AH ROM BIOS alarm handler AT, PS/2 5AH Cluster adapter PC, AT 5BH Used by cluster program PC, AT Interrupt Usage Machine ────────────────────────────────────────────────────────────────────────── 5BH Used by cluster program PC, AT 60H─66H User interrupts PC, AT, PS/2 67H LIM EMS driver PC, AT, PS/2 68H─6FH Unassigned 70H IRQ8 CMOS real-time clock AT, PS/2 71H IRQ9 software diverted to IRQ2 AT, PS/2 72H IRQ10 reserved AT, PS/2 73H IRQ11 reserved AT, PS/2 74H IRQ12 reserved AT IRQ12 mouse PS/2 75H IRQ13 numeric coprocessor AT, PS/2 76H IRQ14 fixed-disk controller AT, PS/2 77H IRQ15 reserved AT, PS/2 78H─7FH Unassigned 80H─F0H BASIC PC, AT, PS/2 F1H─FFH Not used PC, AT, PS/2 ────────────────────────────────────────────────────────────────────────── Figure 13-2. Interrupts with special significance on the IBM PC, PC/AT, and PS/2 and compatible computers. Note that the IBM ROM BIOS uses several interrupts in the range 00H─1FH, even though they were reserved by Intel for CPU faults. IRQ numbers refer to Intel 8259A PIC priority levels. The Interrupt-Vector Table The bottom 1024 bytes of system memory are called the interrupt-vector table. Each 4-byte position in the table corresponds to an interrupt type (0 through 0FFH) and contains the segment and offset of the interrupt handler for that level. Interrupts 0 through 1FH (the lowest levels) are used for internal hardware interrupts; MS-DOS uses Interrupts 20H through 3FH; all the other interrupts are available for use by either external hardware devices or system drivers and application software. When an 8259A PIC or other device interrupts the CPU by means of the INTR pin, it must also place the interrupt type as an 8-bit number (0 through 0FFH) on the system bus, where the CPU can find it. The CPU then multiplies this number by 4 to find the memory address of the interrupt vector to be used. Servicing an Interrupt When the CPU senses an interrupt, it pushes the program status word (which defines the various CPU flags), the code segment (CS) register, and the instruction pointer (IP) onto the machine stack and disables the interrupt system. It then uses the 8-bit number that was jammed onto the system bus by the interrupting device to fetch the address of the handler from the vector table and resumes execution at that address. Usually the handler immediately reenables the interrupt system (to allow higher-priority interrupts to occur), saves any registers it is going to use, and then processes the interrupt as quickly as possible. Some external devices also require a special acknowledgment signal so that they will know the interrupt has been recognized. If the interrupt was funneled through an 8259A PIC, the handler must send a special code called end of interrupt (EOI) to the PIC through its control port to tell it when interrupt processing is completed. (The EOI has no effect on the CPU itself.) Finally, the handler executes the special IRET (INTERRUPT RETURN) instruction that restores the original state of the CPU flags, the CS register, and the instruction pointer (Figure 13-3). Whether an interrupt was triggered by an external device or forced by software execution of an INT instruction, there is no discernible difference in the system state at the time the interrupt handler receives control. This fact is convenient when you are writing and testing external interrupt handlers because you can debug them to a large extent simply by invoking them with software drivers. ────────────────────────────────────────────────────────────────────────── pic_ctl equ 20h ; control port for 8259A ; interrupt controller . . . sti ; turn interrupts back on, push ax ; save registers push bx push cx push dx push si push di push bp push ds push es mov ax,cs ; make local data addressable mov ds,ax . ; do some stuff appropriate . ; for this interrupt here . mov al,20h ; send EOI to 8259A PIC mov dx,pic_ctl out dx,al pop es ; restore registers pop ds pop bp pop di pop si pop dx pop cx pop bx pop ax iret ; resume previous processing ────────────────────────────────────────────────────────────────────────── Figure 13-3. Typical handler for hardware interrupts on the 80x86 family of microprocessors. In real life, the interrupt handler would need to save and restore only the registers that it actually modified. Also, if the handler made extensive use of the machine stack, it would need to save and restore the SS and SP registers of the interrupted process and use its own local stack. Interrupt Handlers and MS-DOS The introduction of an interrupt handler into your program brings with it considerable hardware dependence. It goes without saying (but I am saying it again here anyway) that you should avoid such hardware dependence in MS-DOS applications whenever possible, to ensure that your programs will be portable to any machine running current versions of MS-DOS and that they will run properly under future versions of the operating system. Valid reasons do exist, however, for writing your own interrupt handler for use under MS-DOS: ■ To supersede the MS-DOS default handler for an internal hardware interrupt (such as divide-by-zero, BOUND exceeded, and so forth). ■ To supersede the MS-DOS default handler for a defined system exception, such as the critical-error handler or Ctrl-C handler. ■ To chain your own interrupt handler onto the default system handler for a hardware device, so that both the system's actions and your own will occur on an interrupt. (A typical example of this is the "clock-tick" interrupt.) ■ To service interrupts not supported by the default MS-DOS device drivers (such as the serial communications port, which can be used at much higher speeds with interrupts than with polling). ■ To provide a path of communication between a program that terminates and stays resident and other application software. MS-DOS provides the following facilities to enable you to install well-behaved interrupt handlers in a manner that does not interfere with operating-system functions or other interrupt handlers: Function Action ────────────────────────────────────────────────────────────────────────── Int 21H Function 25H Set interrupt vector. Int 21H Function 35H Get interrupt vector. Int 21H Function 31H Terminate and stay resident. ────────────────────────────────────────────────────────────────────────── These functions allow you to examine or modify the contents of the system interrupt-vector table and to reserve memory for the use of a handler without running afoul of other processes in the system or causing memory use conflicts. Section 2 of this book, "MS-DOS Functions Reference," describes each of these functions in detail, with programming examples. Handlers for external hardware interrupts under MS-DOS must operate under some fairly severe restrictions: ■ Because the current versions of MS-DOS are not reentrant, a hardware interrupt handler should never call the MS-DOS functions during the actual interrupt processing. ■ The handler must reenable interrupts as soon as it gets control, to avoid crippling other devices or destroying the accuracy of the system clock. ■ A program should access the 8259A PIC with great care. The program should not access the PIC unless that program is known to be the only process in the system concerned with that particular interrupt level. And it is vital that the handler issue an end-of-interrupt code to the 8259A PIC before performing the IRET; otherwise, the processing of further interrupts for that priority level or lower priority levels will be blocked. Restrictions on handlers that replace the MS-DOS default handlers for internal hardware interrupts or system exceptions (such as Ctrl-C or critical errors) are not quite so stringent, but you must still program the handlers with extreme care to avoid destroying system tables or leaving the operating system in an unstable state. The following are a few rules to keep in mind when you are writing an interrupt driver: ■ Use Int 21H Function 25H (Set Interrupt Vector) to modify the interrupt vector; do not write directly to the interrupt-vector table. ■ If your program is not the only process in the system that uses this interrupt level, chain back to the previous handler after performing your own processing on an interrupt. ■ If your program is not going to stay resident, fetch and save the current contents of the interrupt vector before modifying it and then restore the original contents when your program exits. ■ If your program is going to stay resident, use one of the terminate- and-stay-resident functions (preferably Int 21H Function 31H) to reserve the proper amount of memory for your handler. ■ If you are going to process hardware interrupts, keep the time that interrupts are disabled and the total length of the service routine to an absolute minimum. Remember that even after interrupts are reenabled with an STI instruction, interrupts of the same or lower priority remain blocked if the interrupt was received through the 8259A PIC. ZERODIV, an Example Interrupt Handler The listing ZERODIV.ASM (Figure 13-4) illustrates some of the principles and guidelines on the previous pages. It is an interrupt handler for the divide-by-zero internal interrupt (type 0). ZERODIV is loaded as a .COM file (usually by a command in the system's AUTOEXEC file) but makes itself permanently resident in memory as long as the system is running. The ZERODIV program has two major portions: the initialization portion and the interrupt handler. The initialization procedure (called init in the program listing) is executed only once, when the ZERODIV program is executed from the MS-DOS level. The init procedure takes over the type 0 interrupt vector, prints a sign-on message, then performs a terminate-and-stay-resident exit to MS-DOS. This special exit reserves the memory occupied by the ZERODIV program, so that it is not overwritten by subsequent application programs. The interrupt handler (called zdiv in the program listing) receives control when a divide-by-zero interrupt occurs. The handler preserves all registers and then prints a message to the user asking whether to continue or to abort the program. We can use the MS-DOS console I/O functions within this particular interrupt handler because we can safely presume that the application was in control when the interrupt occurred; thus, there should be no chance of accidentally making overlapping calls upon the operating system. If the user enters a C to continue, the handler simply restores all the registers and performs an IRET (INTERRUPT RETURN) to return control to the application. (Of course, the results of the divide operation will be useless.) If the user enters Q to quit, the handler exits to MS-DOS. Int 21H Function 4CH is particularly convenient in this case because it allows the program to pass a return code and at the same time is the only termination function that does not rely on the contents of any of the segment registers. For an example of an interrupt handler for external (communications port) interrupts, see the TALK terminal-emulator program in Chapter 7. You may also want to look again at the discussions of Ctrl-C and critical-error exception handlers in Chapters 5 and 8. ────────────────────────────────────────────────────────────────────────── name zdivide page 55,132 title ZERODIV--Divide-by-zero handler ; ; ZERODIV.ASM--Terminate-and-stay-resident handler ; for divide-by-zero interrupts ; ; Copyright 1988 Ray Duncan ; ; Build: C>MASM ZERODIV; ; C>LINK ZERODIV; ; C>EXE2BIN ZERODIV.EXE ZERODIV.COM ; C>DEL ZERODIV.EXE ; ; Usage: C>ZERODIV ; cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII linefeed beep equ 07h ; ASCII bell code backsp equ 08h ; ASCII backspace code _TEXT segment word public 'CODE' org 100H assume cs:_TEXT,ds:_TEXT,es:_TEXT,ss:_TEXT init proc near ; entry point at load time ; capture vector for ; interrupt zero... mov dx,offset zdiv ; DS:DX = handler address mov ax,2500h ; function 25h = set vector ; interrupt type = 0 int 21h ; transfer to MS-DOS ; print sign-on message mov dx,offset msg1 ; DS:DX = message address mov ah,9 ; function 09h = display string int 21h ; transfer to MS-DOS ; DX = paragraphs to reserve mov dx,((offset pgm_len+15)/16)+10h mov ax,3100h ; function 31h = terminate and ; stay resident int 21h ; transfer to MS-DOS init endp zdiv proc far ; this is the divide-by- ; zero interrupt handler sti ; enable interrupts push ax ; save registers push bx push cx push dx push si push di push bp push ds push es mov ax,cs ; make data addressable mov ds,ax ; display message ; "Continue or Quit?" mov dx,offset msg2 ; DS:DX = message address mov ah,9 ; function 09h = display string int 21h ; transfer to MS-DOS zdiv1: mov ah,1 ; function 01h = read keyboard int 21h ; transfer to MS-DOS or al,20h ; fold char to lowercase cmp al,'c' ; is it C or Q? je zdiv3 ; jump, it's a C cmp al,'q' je zdiv2 ; jump, it's a Q ; illegal entry, send beep ; and erase the character mov dx,offset msg3 ; DS:DX = message address mov ah,9 ; function 09h = display string int 21h ; transfer to MS-DOS jmp zdiv1 ; try again zdiv2: ; user chose "Quit" mov ax,4cffh ; terminate current program int 21h ; with return code = 255 zdiv3: ; user chose "Continue" ; send CR-LF pair mov dx,offset msg4 ; DS:DX = message address mov ah,9 ; function 09h = print string int 21h ; transfer to MS-DOS ; what CPU type is this? xor ax,ax ; to find out, we'll put push ax ; zero in the CPU flags popf ; and see what happens pushf pop ax and ax,0f000h ; 8086/8088 forces cmp ax,0f000h ; bits 12-15 true je zdiv5 ; jump if 8086/8088 ; otherwise we must adjust ; return address to bypass ; the divide instruction... mov bp,sp ; make stack addressable lds bx,[bp+18] ; get address of the ; faulting instruction mov bl,[bx+1] ; get addressing byte and bx,0c7h ; isolate mod & r/m fields cmp bl,6 ; mod 0, r/m 6 = direct jne zdiv4 ; not direct, jump add word ptr [bp+18],4 jmp zdiv5 zdiv4: mov cl,6 ; otherwise isolate mod shr bx,cl ; field and get instruction mov bl,cs:[bx+itab] ; size from table add [bp+18],bx zdiv5: pop es ; restore registers pop ds pop bp pop di pop si pop dx pop cx pop bx pop ax iret ; return from interrupt zdiv endp msg1 db cr,lf ; load-time sign-on message db 'Divide by Zero Interrupt ' db 'Handler installed.' db cr,lf,'$' msg2 db cr,lf,lf ; interrupt-time message db 'Divide by Zero detected: ' db cr,lf,'Continue or Quit (C/Q) ? ' db '$' msg3 db beep ; used if bad entry db backsp,' ',backsp,'$' msg4 db cr,lf,'$' ; carriage return-linefeed ; instruction size table itab db 2 ; mod = 0 db 3 ; mod = 1 db 4 ; mod = 2 db 2 ; mod = 3 pgm_len equ $-init ; program length _TEXT ends end init ────────────────────────────────────────────────────────────────────────── Figure 13-4. A simple example of an interrrupt handler for use within the MS-DOS environment. ZERODIV makes itself permanently resident in memory and handles the CPU's internal divide-by-zero interrupt. ──────────────────────────────────────────────────────────────────────────── Chapter 14 Installable Device Drivers Device drivers are the modules of an operating system that control the hardware. They isolate the operating-system kernel from the specific characteristics and idiosyncrasies of the peripheral devices interfaced to the central processor. Thus, the driver's relationship to the kernel is analogous to the operating system's relationship to application programs. The installable device drivers that were introduced in MS-DOS version 2 give the user great flexibility. They allow the user to customize and configure the computer for a wide range of peripheral devices, with a minimum of troublesome interactions and without having to "patch" the operating system. Even the most inexperienced user can install a new device into a system by plugging in a card, copying a driver file to the boot disk, and editing the system configuration file. For those inclined to do their own programming, the MS-DOS installable device drivers are interfaced to the hardware-independent kernel through a simple and clearly defined scheme of function codes and data structures. Given adequate information about the hardware, any competent assembly- language programmer can expect to successfully interface even the most bizarre device to MS-DOS without altering the operating system in the slightest and without acquiring any special or proprietary knowledge about its innards. In retrospect, installable device drivers have proven to be one of the key usability features of MS-DOS. I feel that they have been largely responsible for the rapid proliferation and competitive pricing of high-speed mass-storage devices for MS-DOS machines, and for the growing confidence of the average user toward "tampering with" (upgrading) his or her machine. MS-DOS Device-Driver Types Drivers written for MS-DOS fall into two distinct classes: ■ Block-device drivers ■ Character-device drivers A driver's class determines what functions it must support, how it is viewed by MS-DOS, and how it makes the associated physical device appear to behave when an application program makes a request for I/O. Character-Device Drivers Character-device drivers control peripheral devices that perform input and output one character (or byte) at a time, such as a terminal or printer. A single character-device driver ordinarily supports a single hardware unit. Each character device has a one-to-eight-character logical name, and an application program can use this name to open the device for input or output, as though it were a file. The logical name is strictly a means of identification for MS-DOS and has no physical equivalent on the device. MS-DOS's built-in character-device drivers for the console, serial port, and printer are unique in that an application program can access them in three different ways: ■ It can open them by name (CON, AUX, PRN, etc.) for input and output, like any other character device. ■ It can use the special-purpose MS-DOS function calls (Int 21H Functions 01-0CH). ■ It can use the default handles (standard input, standard output, standard error, standard auxiliary, and standard printer), which do not need to be opened to be used. The number of additional character-device drivers that can be installed is limited only by available memory and by the requirement that each driver have a unique logical name. If more than one driver uses the same logical name, the last driver to be loaded will supersede any others and will receive all I/O requests addressed to that logical name. This fact can occasionally be turned to advantage; for example, it allows the user to replace the system's default CON driver, which does not support cursor positioning or character attributes, with the more powerful ANSI.SYS driver. ASCII vs Binary Mode MS-DOS regards a handle associated with a character device to be in either ASCII (cooked) mode or binary (raw) mode. The mode affects MS-DOS's buffering of data for read and write requests. The driver itself is not aware of the mode, and the mode does not affect its operation. An application can select the mode of a handle with the IOCTL function (Int 21H Function 44H). During ASCII-mode input, MS-DOS requests characters one at a time from the driver and places them into its own internal buffer, echoing each to the screen (if the input device is the keyboard) and checking each character for a Ctrl-C (03H). When the number of characters requested by the application program has been received, when a Ctrl-Z is detected, or when the Enter key is pressed (in the case of the keyboard), MS-DOS terminates the input and copies the data from its internal buffer into the requesting program's buffer. Similarly, during ASCII-mode output, MS-DOS passes the characters to the device driver one at a time and checks for a Ctrl-C pending at the keyboard between each character. When a Ctrl-C is detected, MS-DOS aborts the input or output operation and transfers to the routine whose address is stored in the Int 23H vector. In binary mode, MS-DOS reads or writes the exact number of bytes requested by the application program, without regard to any control characters such as Enter or Ctrl-C. MS-DOS passes the entire request through to the driver in a single operation, instead of breaking it into single-character reads or writes, and transfers the characters directly to or from the requesting program's buffer. Block-Device drivers Block-device drivers usually control random-access mass-storage devices such as floppy-disk drives and fixed disks, although they can also be used to control non-random-access devices such as magnetic-tape drives. Block devices transfer data in chunks, rather than one byte at a time. The size of the blocks may be either fixed (disk drives) or variable (tape drives). A block driver can support more than one hardware unit, map a single physical unit onto two or more logical units, or both. Block devices do not have file-like logical names, as character devices do. Instead, MS-DOS assigns drive designators to the block-device units or logical drives in an alphabetic sequence: A, B, and so forth. Each logical drive contains a file system: boot block, file allocation table, root directory, and so forth. (See Chapter 10.) A block-device driver's position in the chain of all drivers determines the first letter assigned to that driver. The number of logical drive units that the driver supports determines the total number of letters assigned to it. Block-device drivers always read or write exactly the number of sectors requested (barring hardware or addressing errors) and never filter or otherwise manipulate the contents of the blocks being transferred. Structure of an MS-DOS Device Driver A device driver consists of three major parts (Figure 14-1): ■ A device header ■ A strategy (strat) routine ■ An interrupt (intr) routine We'll discuss each of these in more detail as we work through this chapter. ┌────────────────────────┬─────────────────────────┐ │ │ Initialization │ │ ├─────────────────────────┤ │ │ Media check │ │ ├─────────────────────────┤ │ │ Build BPB │ │ ├─────────────────────────┤ │ │ IOCTL read and write │ │ ├─────────────────────────┤ │ │ Status │ │ ├─────────────────────────┤ │ │ Read │ │ ├─────────────────────────┤ │ │ Write, write/verify │ │ ├─────────────────────────┤ │ Interrupt routine │ Output until busy │ │ ├─────────────────────────┤ │ │ Flush buffers │ │ ├─────────────────────────┤ │ │ Device open │ │ ├─────────────────────────┤ │ │ Device close │ │ ├─────────────────────────┤ │ │ Check whether removable │ │ ├─────────────────────────┤ │ │ Generic IOCTL │ │ ├─────────────────────────┤ │ │ Get/Set logical device │ │ └─────────────────────────┤ ├──────────────────────────────────────────────────┤ │ Strategy routine │ ├──────────────────────────────────────────────────┤ │ Device-driver header │ └──────────────────────────────────────────────────┘ Figure 14-1. General structure of an MS-DOS installable device driver. The Device Header The device header (Figure 14-2) lies at the beginning of the driver. It contains a link to the next driver in the chain, a set of attribute flags for the device (Figure 14-3), offsets to the executable strategy and interrupt routines for the device, and the logical-device name (if it is a character device such as PRN or COM1) or the number of logical units (if it is a block device). Byte offset 00H ┌──────────────────────────────────────────────┐ │ Link to next driver, offset │ 02H ├──────────────────────────────────────────────┤ │ Link to next driver, segment │ 04H ├──────────────────────────────────────────────┤ │ Device attribute word │ 06H ├──────────────────────────────────────────────┤ │ Strategy entry point, offset │ 08H ├──────────────────────────────────────────────┤ │ Interrupt entry point, offset │ 0AH ├──────────────────────────────────────────────┤ │ Logical name (8 bytes) if character device │ │ Number of units (1 byte) if block device, │ │ followed by 7 bytes of reserved space │ └──────────────────────────────────────────────┘ Figure 14-2. Device-driver header. The offsets to the strat and intr routines are offsets from the same segment used to point to the device header. ╓┌─┌──────────────┌──────────────────────────────────────────────────────────╖ Bit Significance ────────────────────────────────────────────────────────────────────────── 15 1 if character device, 0 if block device 14 1 if IOCTL read and write supported 13 for block devices: 1 if BIOS parameter block in boot sector should be used to determine media characteristics, 0 if media ID byte should be used for character devices: 1 if output until busy supported 12 Reserved (should be 0) 11 1 if open/close/removable media supported (MS-DOS 3.0 and later) Bit Significance ────────────────────────────────────────────────────────────────────────── later) 7─10 Reserved (should be 0) 6 1 if generic IOCTL and get/set logical drive supported (MS-DOS 3.2 and later) 5 Reserved (should be 0) 4 1 if CON driver and Int 29H fast-output function supported 3 1 if current CLOCK$ device 2 1 if current NUL device 1 for block devices: 1 if driver supports 32-bit sector addressing (MS-DOS 4.0) for character devices: 1 if standard output device (stdout) 0 1 if current standard input device (stdin) ────────────────────────────────────────────────────────────────────────── Figure 14-3. Device attribute word in device header. In block-device drivers, only bits 6, 11, and 13─15 (and bit 1 in MS-DOS version 4.0) have significance; the remainder should always be zero. The Strategy Routine MS-DOS calls the strategy routine (strat) for the device when the driver is first loaded and installed, and again whenever an application program issues an I/O request for the device. MS-DOS passes the strategy routine a double-word pointer to a data structure called a request header. This structure contains information about the type of operation to be performed. In current versions of MS-DOS, the strategy routine never actually performs any I/O operation but simply saves the pointer to the request header. The strat routine must not make any Int 21H function calls. The first 13 bytes of the request header are the same for all device-driver functions and are therefore referred to as the static portion of the header. The number and contents of the subsequent bytes vary according to the type of function being requested (Figure 14-4). Both MS-DOS and the driver read and write information in the request header. The request header's most important component is a command code, or function number, passed in its third byte to select a driver subfunction such as read, write, or status. Other information passed to the driver in the header includes unit numbers, transfer addresses, and sector or byte counts. ────────────────────────────────────────────────────────────────────────── ; ; MS-DOS request header structure definition ; Request struc ; request header template structure Rlength db ? ; 0 length of request header Unit db ? ; 1 unit number for this request Command db ? ; 2 request header's command code Status dw ? ; 3 driver's return status word Reserve db 8 dup (?) ; 5 reserved area Media db ? ; 13 media descriptor byte Address dd ? ; 14 memory address for transfer Count dw ? ; 18 byte/sector count value Sector dw ? ; 20 starting sector value Request ends ; end of request header template ────────────────────────────────────────────────────────────────────────── Figure 14-4. Format of request header. Only the first 13 bytes are common to all driver functions; the number and definition of the subsequent bytes vary, depending upon the function type. The structure shown here is the one used by the read and write subfunctions of the driver. The Interrupt Routine The last and most complex part of a device driver is the interrupt routine (intr), which MS-DOS calls immediately after it calls the strategy routine. The interrupt routine implements the device driver proper; it performs (or calls other resident routines to perform) the actual input or output operations, based on the information passed in the request header. The strat routine may not make any Int 21H function calls, except for a restricted set during driver initialization. When an I/O function is completed, the interrupt routine uses the status field in the request header to inform the DOS kernel about the outcome of the requested I/O operation. It can use other fields in the request header to pass back such useful information as counts of the actual sectors or bytes transferred. The interrupt routine usually consists of the following elements: ■ A collection of subroutines to implement the various function types that may be requested by MS-DOS (sometimes called the command-code routines) ■ A centralized entry point that saves all affected registers, extracts the desired function code from the request header, and branches to the appropriate command-code routine (typically accomplished with a jump table) ■ A centralized exit point that stores status and error codes into the request header (Figures 14-5 and 14-6) and restores the previous contents of the affected registers The command-code routines that implement the various functions supported by an installable device driver are discussed in detail in the following pages. Bit(s) Significance ────────────────────────────────────────────────────────────────────────── 15 Error 12─14 Reserved 9 Busy 8 Done 0─7 Error code if bit 15 = 1 ────────────────────────────────────────────────────────────────────────── Figure 14-5. Values for the return status word of the request header. Code Meaning ────────────────────────────────────────────────────────────────────────── 0 Write-protect violation 1 Unknown unit 2 Drive not ready 3 Unknown command 4 Data error (CRC) 5 Bad request-structure length 6 Seek error 7 Unknown medium 8 Sector not found 9 Printer out of paper 0AH Write fault 0BH Read fault 0CH General failure 0D─0EH Reserved 0FH Invalid disk change (MS-DOS versions 3.0 and later) ────────────────────────────────────────────────────────────────────────── Figure 14-6. Driver error codes returned in bits 0 through 7 of the return status word of the request header. Although its name suggests otherwise, the interrupt routine is never entered asynchronously (on an I/O completion interrupt, for example). Thus, the division of function between strategy and interrupt routines is completely artificial in the current versions of MS-DOS. The Command-Code Routines A total of 20 command codes are defined for MS-DOS device drivers. The command codes (which are not consecutive), the names of the associated driver-interrupt routines, and the MS-DOS versions in which they are first supported are as follows: ╓┌─┌───────────┌────────────────────────┌───────────┌────────────┌───────────╖ Command Function Character Block MS-DOS code driver driver version ────────────────────────────────────────────────────────────────────────── 0 Init (Initialization) X X 2.0 1 Media Check X 2.0 2 Build BPB X 2.0 3 IOCTL Read X X 2.0 4 Read X X 2.0 5 Nondestructive Read X 2.0 6 Input Status X 2.0 7 Flush Input Buffers X 2.0 8 Write X X 2.0 9 Write with Verify X 2.0 10 Output Status X 2.0 11 Flush Output Buffers X 2.0 Command Function Character Block MS-DOS code driver driver version ────────────────────────────────────────────────────────────────────────── 11 Flush Output Buffers X 2.0 12 IOCTL Write X X 2.0 13 Device Open X X 3.0 14 Device Close X X 3.0 15 Removable Media X 3.0 16 Output Until Busy X 3.0 19 Generic IOCTL X X 3.2 23 Get Logical Device X 3.2 24 Set Logical Device X 3.2 ────────────────────────────────────────────────────────────────────────── As you can see from the preceding table, a driver's interrupt section must support functions 0 through 12 under all versions of MS-DOS. Drivers tailored for MS-DOS 3.0 and 3.1 can optionally support an additional four functions, and MS-DOS drivers for versions 3.2 and later can support three more (for a total of 20). MS-DOS inspects the bits in the attribute word of the device-driver header to determine which of the optional functions a driver supports, if any. Some of the functions are relevant only for character-device drivers and some only for block-device drivers; a few have meaning to both types. In any case, both driver types should have an executable routine present for each function, even if it does nothing except set the done flag in the status word of the request header. In the command-code descriptions that follow, RH refers to the request header whose address was passed to the strategy routine in ES:BX, BYTE is an 8-bit parameter, WORD is a 16-bit parameter, and DWORD is a far pointer (a 16-bit offset followed by a 16-bit segment). Function 00H (0): Driver Initialization MS-DOS requests the driver's initialization function (init) only once, when the driver is first loaded. This function performs any necessary device hardware initialization, setup of interrupt vectors, and so forth. The initialization routine must return the address of the position where free memory begins after the driver code (the break address), so that MS-DOS knows where it can build certain control structures and then load the next installable driver. If this is a block-device driver, init must also return the number of units and the address of a BPB pointer array. MS-DOS uses the number of units returned by a block driver in the request header to assign drive identifiers. For example, if the current maximum drive is D and the driver being initialized supports four units, MS-DOS will assign it the drive letters E, F, G, and H. Although the device-driver header also has a field for number of units, MS-DOS does not inspect it. The BPB pointer array is an array of word offsets to BIOS parameter blocks (Figure 14-7). Each unit defined by the driver must have one entry in the array, although the entries can all point to the same BPB to conserve memory. During the operating-system boot sequence, MS-DOS scans all the BPBs defined by all the units in all the block-device drivers to determine the largest sector size that exists on any device in the system and uses this information to set its cache buffer size. The operating-system services that the initialization code can invoke at load time are very limited only Int 21H Functions 01H through 0CH and 30H. These are just adequate to check the MS-DOS version number and display a driver-identification or error message. Many programmers position the initialization code at the end of the driver and return that address as the location of the first free memory, so that MS-DOS will reclaim the memory occupied by the initialization routine after the routine is finished with its work. If the initialization routine finds that the device is missing or defective and wants to abort the installation of the driver completely so that it does not occupy any memory, it should return number of units as zero and set the free memory address to CS:0000H. (A character-device driver that wants to abort its installation should clear bit 15 of the attribute word in the driver header and then set the units field and free memory address as though it were a block-device driver.) Byte(s) Contents ────────────────────────────────────────────────────────────────────────── 00─01H Bytes per sector 02H Sectors per allocation unit (power of 2) 03H─04H Number of reserved sectors (starting at sector 0) 05H Number of file allocation tables 06H─07H Maximum number of root-directory entries 08H─09H Total number of sectors in medium 0AH Media descriptor byte 0BH─0CH Number of sectors occupied by a single FAT 0DH─0EH Sectors per track (versions 3.0 and later) 0FH─10H Number of heads (versions 3.0 and later) 11H─12H Number of hidden sectors (versions 3.0 and later) 13H─14H High-order word of number of hidden sectors (version 4.0) 15H─18H If bytes 8─9 are zero, total number of sectors in medium (version 4.0) 19H─1EH Reserved, should be zero (version 4.0) ────────────────────────────────────────────────────────────────────────── Figure 14-7. Structure of a BIOS parameter block (BPB). Every formatted disk contains a copy of its BPB in the boot sector. (See Chapter 10.) The initialization function is called with ────────────────────────────────────────────────────────────────────────── RH + 2 BYTE Command code = 0 RH + 18 DWORD Pointer to character after equal sign on CONFIG.SYS line that loaded driver (this information is read-only) RH + 22 BYTE Drive number for first unit of this block driver (0 = A, 1 = B, and so forth) (MS-DOS version 3 only) ────────────────────────────────────────────────────────────────────────── It returns: ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status RH + 13 BYTE Number of units (block devices only) RH + 14 DWORD Address of first free memory above driver (break address) RH + 18 DWORD BPB pointer array (block devices only) ────────────────────────────────────────────────────────────────────────── Function 01H (1): Media Check The media-check function applies only to block devices, and in character-device drivers it should do nothing except set the done flag. This function is called when a drive-access call other than a simple file read or write is pending. MS-DOS passes to the function the media descriptor byte for the disk that it assumes is in the drive (Figure 14-8). If feasible, the media-check routine returns a code indicating whether the disk has been changed since the last transfer. If the media-check routine can assert that the disk has not been changed, MS-DOS can bypass rereading the FAT before a directory access, which improves overall performance. Code Meaning ────────────────────────────────────────────────────────────────────────── 0F0H 3.5", 2-sided, 18-sector 0F8H fixed disk 0F9H 3.5", 2-sided, 9-sector 0F9H 5.25", 2-sided, 15-sector 0FCH 5.25", 1-sided, 9-sector 0FDH 5.25", 2-sided, 9-sector 0FEH 5.25", 1-sided, 8-sector 0FFH 5.25", 2-sided, 8-sector ────────────────────────────────────────────────────────────────────────── Figure 14-8. Current valid MS-DOS codes for the media descriptor byte of the request header, assuming bit 13 in the attribute word of the driver header is zero. MS-DOS responds to the results of the media-check function in the following ways: ■ If the disk has not been changed, MS-DOS proceeds with the disk access. ■ If the disk has been changed, MS-DOS invalidates all buffers associated with this unit, including buffers containing data waiting to be written (this data is simply lost), performs a BUILD BPB call, and then reads the disk's FAT and directory. ■ If the disk-change status is unknown, the action taken by MS-DOS depends upon the state of its internal buffers. If data that needs to be written out is present in the buffers, MS-DOS assumes no disk change has occurred and writes the data (taking the risk that, if the disk really was changed, the file structure on the new disk may be damaged). If the buffers are empty or have all been previously flushed to the disk, MS-DOS assumes that the disk was changed, and then proceeds as described above for the disk-changed return code. If bit 11 of the device-header attribute word is set (that is, the driver supports the optional open/close/removable-media functions), the host system is MS-DOS version 3.0 or later, and the function returns the disk-changed code (-1), the function must also return the segment and offset of the ASCIIZ volume label for the previous disk in the drive. (If the driver does not have the volume label, it can return a pointer to the ASCIIZ string NO NAME.) If MS-DOS determines that the disk was changed with unwritten data still present in its buffers, it issues a critical-error 0FH (invalid disk change). Application programs can trap this critical error and prompt the user to replace the original disk. The media-check function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code RH + 2 BYTE Command code = 1 RH + 13 BYTE Media descriptor byte ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status RH + 14 BYTE Media-change code: -1 if disk changed 0 if don't know whether disk changed 1 if disk not changed RH + 15 DWORD Pointer to previous volume label, if device attribute bit 11 = 1 and disk has been changed (MS-DOS versions 3.0 and later) ────────────────────────────────────────────────────────────────────────── Function 02H (2): Build BIOS Parameter Block (BPB) The build BPB function applies only to block devices, and in character-device drivers should do nothing except set the done flag. The kernel uses this function to get a pointer to the valid BPB (see Figure 14-7) for the current disk and calls it when the disk-changed code is returned by the media-check routine or the don't-know code is returned and there are no dirty buffers (buffers with changed data that have not yet been written to disk). Thus, a call to this function indicates that the disk has been legally changed. The build BPB function receives a pointer to a one-sector buffer in the request header. If bit 13 in the driver header's attribute word is zero, the buffer contains the first sector of the FAT (which includes the media identification byte) and should not be altered by the driver. If bit 13 is set, the driver can use the buffer as scratch space. The build BPB function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code RH + 2 BYTE Command code = 2 RH + 13 BYTE Media descriptor byte RH + 14 DWORD Buffer address ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status RH + 18 DWORD Pointer to new BPB ────────────────────────────────────────────────────────────────────────── Under MS-DOS versions 3.0 and later, if bit 11 of the header's device attribute word is set, this routine should also read the volume label off the disk and save it. Function 03H (3): I/O-Control Read The IOCTL read function allows the device driver to pass information directly to the application program. This function is called only if bit 14 is set in the device attribute word. MS-DOS performs no error check on IOCTL I/O calls. The IOCTL read function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code (block devices) RH + 2 BYTE Command code = 3 RH + 13 BYTE Media descriptor byte RH + 14 DWORD Transfer address RH + 18 WORD Byte/sector count RH + 20 WORD Starting sector number (block devices) ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status RH + 18 WORD Actual bytes or sectors transferred ────────────────────────────────────────────────────────────────────────── Function 04H (4): Read The read function transfers data from the device into the specified memory buffer. If an error is encountered during the read, the function must set the error status and, in addition, report the number of bytes or sectors successfully transferred; it is not sufficient to simply report an error. The read function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code (block devices) RH + 2 BYTE Command code = 4 RH + 13 BYTE Media descriptor byte RH + 14 DWORD Transfer address RH + 18 WORD Byte/sector count RH + 20 WORD Starting sector number (block devices) ────────────────────────────────────────────────────────────────────────── For block-device read operations in MS-DOS version 4, if the logical unit is larger than 32 MB and bit 1 of the driver's attribute word is set, the following request structure is used instead: ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code RH + 2 BYTE Command code = 4 RH + 13 BYTE Media descriptor byte RH + 14 DWORD Transfer address RH + 18 WORD Sector count RH + 20 WORD Contains -1 to signal use of 32-bit sector number RH + 26 DWORD 32-bit starting sector number ────────────────────────────────────────────────────────────────────────── The read function returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status RH + 18 WORD Actual bytes or sectors transferred RH + 22 DWORD Pointer to volume label if error 0FH is returned (MS-DOS versions 3.0 and later) ────────────────────────────────────────────────────────────────────────── Under MS-DOS versions 3.0 and later, this routine can use the count of open files maintained by the open and close functions (0DH and 0EH) and the media descriptor byte to determine whether the disk has been illegally changed. Function 05H (5): Nondestructive Read The nondestructive read function applies only to character devices, and in block devices it should do nothing except set the done flag. It returns the next character that would be obtained with a read function (command code 4), without removing that character from the driver's internal buffer. MS-DOS uses this function to check the console driver for pending Control-C characters during other operations. The nondestructive read function is called with ────────────────────────────────────────────────────────────────────────── RH + 2 BYTE Command code = 5 ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status If busy bit = 0, at least one character is waiting If busy bit = 1, no characters are waiting RH + 13 BYTE Character (if busy bit = 0) ────────────────────────────────────────────────────────────────────────── Function 06H (6): Input Status The input-status function applies only to character devices, and in block-device drivers it should do nothing except set the done flag. This function returns the current input status for the device, allowing MS-DOS to test whether characters are waiting in a type-ahead buffer. If the character device does not have a type-ahead buffer, the input-status routine should always return the busy bit equal to zero, so that MS-DOS will not wait forever to call the read (04H) or nondestructive read (05H) function. The input-status function is called with ────────────────────────────────────────────────────────────────────────── RH + 2 BYTE Command code = 6 ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status: If busy bit = 1, read request goes to physical device. If busy bit = 0, characters already in device buffer and read request returns quickly. ────────────────────────────────────────────────────────────────────────── Function 07H (7): Flush Input Buffers The flush-input-buffers function applies only to character devices, and in block-device drivers it should do nothing except set the done flag. This function causes any data waiting in the input buffer to be discarded. The flush-input-buffers function is called with ────────────────────────────────────────────────────────────────────────── RH + 2 BYTE Command code = 7 ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status ────────────────────────────────────────────────────────────────────────── Function 08H (8): Write The write function transfers data from the specified memory buffer to the device. If an error is encountered during the write, the write function must set the error status and, in addition, report the number of bytes or sectors successfully transferred; it is not sufficient to simply report an error. The write function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code (block devices) RH + 2 BYTE Command code = 8 RH + 13 BYTE Media descriptor byte RH + 14 DWORD Transfer address RH + 18 WORD Byte/sector count RH + 20 WORD Starting sector number (block devices) ────────────────────────────────────────────────────────────────────────── For block-device write operations in MS-DOS version 4, if the logical unit is larger than 32 MB and bit 1 of the driver's attribute word is set, the following request structure is used instead: ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code RH + 2 BYTE Command code = 8 RH + 13 BYTE Media descriptor byte RH + 14 DWORD Transfer address RH + 18 WORD Sector count RH + 20 WORD Contains -1 to signal use of 32-bit sector number RH + 26 DWORD 32-bit starting sector number ────────────────────────────────────────────────────────────────────────── The write function returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status RH + 18 WORD Actual bytes or sectors transferred RH + 22 DWORD Pointer to volume label if error 0FH returned (MS-DOS versions 3.0 and later) ────────────────────────────────────────────────────────────────────────── Under MS-DOS versions 3.0 and later, this routine can use the reference count of open files maintained by the open and close functions (0DH and 0EH) and the media descriptor byte to determine whether the disk has been illegally changed. Function 09H (9): Write with Verify The write-with-verify function transfers data from the specified memory buffer to the device. If feasible, it should perform a read-after-write verification of the data to confirm that the data was written correctly. Otherwise, Function 09H is exactly like Function 08H. Function 0AH (10): Output Status The output-status function applies only to character devices, and in block-device drivers it should do nothing except set the done flag. This function returns the current output status for the device. The output-status function is called with ────────────────────────────────────────────────────────────────────────── RH + 2 BYTE Command code = 10 (0AH) ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status: If busy bit = 1, write request waits for completion of current request. If busy bit = 0, device idle and write request starts immediately. ────────────────────────────────────────────────────────────────────────── Function 0BH (11): Flush Output Buffers The flush-output-buffers function applies only to character devices, and in block-device drivers it should do nothing except set the done flag. This function empties the output buffer, if any, and discards any pending output requests. The flush-output-buffers function is called with ────────────────────────────────────────────────────────────────────────── RH + 2 BYTE Command code = 11 (0BH) ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status ────────────────────────────────────────────────────────────────────────── Function 0CH (12): I/O-Control Write The IOCTL write function allows an application program to pass control information directly to the driver. This function is called only if bit 14 is set in the device attribute word. MS-DOS performs no error check on IOCTL I/O calls. The IOCTL write function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code (block devices) RH + 2 BYTE Command code = 12 (0CH) RH + 13 BYTE Media descriptor byte RH + 14 DWORD Transfer address RH + 18 WORD Byte/sector count RH + 20 WORD Starting sector number (block devices) ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status RH + 18 WORD Actual bytes or sectors transferred ────────────────────────────────────────────────────────────────────────── Function 0DH (13): Device Open The device-open function is supported only under MS-DOS versions 3.0 and later and is called only if bit 11 is set in the device attribute word of the device header. On block devices, the device-open function can be used to manage local buffering and to increment a reference count of the number of open files on the device. This capability must be used with care, however, because programs that access files through FCBs frequently fail to close them, thus invalidating the open-files count. One way to protect against this possibility is to reset the open-files count to zero, without flushing the buffers, whenever the answer to a media-change call is yes and a subsequent build BPB call is made to the driver. On character devices, the device-open function can be used to send a device-initialization string (which can be set into the driver by an application program by means of an IOCTL write function) or to deny simultaneous access to a character device by more than one process. Note that the predefined handles for the CON, AUX, and PRN devices are always open. The device-open function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code (block devices) RH + 2 BYTE Command code = 13 (0DH) ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status ────────────────────────────────────────────────────────────────────────── Function 0EH (14): Device Close The device-close function is supported only under MS-DOS versions 3.0 and later and is called only if bit 11 is set in the device attribute word of the device header. On block devices, this function can be used to manage local buffering and to decrement a reference count of the number of open files on the device; when the count reaches zero, all files have been closed and the driver should flush buffers because the user may change disks. On character devices, the device-close function can be used to send a device-dependent post-I/O string such as a formfeed. (This string can be set into the driver by an application program by means of an IOCTL write function.) Note that the predefined handles for the CON, PRN, and AUX devices are never closed. The device-close function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code (block devices) RH + 2 BYTE Command code = 14 (0EH) ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status ────────────────────────────────────────────────────────────────────────── Function 0FH (15): Removable Media The removable-media function is supported only under MS-DOS versions 3.0 and later and only on block devices; in character-device drivers it should do nothing except set the done flag. This function is called only if bit 11 is set in the device attribute word in the device header. The removable-media function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code RH + 2 BYTE Command code = 15 (0FH) ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status: If busy bit = 1, medium nonremovable If busy bit = 0, medium removable ────────────────────────────────────────────────────────────────────────── Function 10H (16): Output Until Busy The output-until-busy function is supported only under MS-DOS versions 3.0 and later, and only on character devices; in block-device drivers it should do nothing except set the done flag. This function transfers data from the specified memory buffer to a device, continuing to transfer bytes until the device is busy. It is called only if bit 13 of the device attribute word is set in the device header. This function is an optimization included specifically for the use of print spoolers. It is not an error for this function to return a number of bytes transferred that is less than the number of bytes requested. The output-until-busy function is called with ────────────────────────────────────────────────────────────────────────── RH + 2 BYTE Command code = 16 (10H) RH + 14 DWORD Transfer address RH + 18 WORD Byte count ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status RH + 18 WORD Actual bytes transferred ────────────────────────────────────────────────────────────────────────── Function 13H (19) Generic IOCTL The generic IOCTL function is supported only under MS-DOS versions 3.2 and later and is called only if bit 6 is set in the device attribute word of the device header. This function corresponds to the MS-DOS generic IOCTL service supplied to application programs by Int 21H Function 44H Subfunctions 0CH and 0DH. The generic IOCTL function is passed a category (major) code, a function (minor) code, the contents of the SI and DI registers at the point of the IOCTL call, and the segment and offset of a data buffer. This buffer in turn contains other information whose format depends on the major and minor IOCTL codes passed in the request header. The driver must interpret the major and minor codes in the request header and the contents of the additional buffer to determine which operation it will carry out, then set the done flag in the request-header status word, and return any other applicable information in the request header or the data buffer. Services that the generic IOCTL function may invoke, if the driver supports them, include configuration of the driver for nonstandard disk formats, reading and writing entire disk tracks of data, and formatting and verifying tracks. The generic IOCTL function has been designed to be open-ended, so that it can be used to easily extend the device-driver definition under future versions of MS-DOS. The generic IOCTL function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit number (block devices) RH + 2 BYTE Command code = 19 (13H) RH + 13 BYTE Category (major) code RH + 14 BYTE Function (minor) code RH + 15 WORD SI register contents RH + 17 WORD DI register contents RH + 19 DWORD Address of generic IOCTL data packet ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status ────────────────────────────────────────────────────────────────────────── Function 17H (23): Get Logical Device The get-logical-device function is supported only under MS-DOS versions 3.2 and later and only on block devices; in character-device drivers it should do nothing except set the done bit in the status word. This function is called only if bit 6 is set in the device attribute word of the device header. It corresponds to the get-logical-device-map service supplied to application programs through Int 21H Function 44H Subfunction 0EH. The get-logical-device function returns a code for the last drive letter used to reference the device; if only one drive letter is assigned to the device, the returned unit code should be zero. Thus, this function can be used to determine whether more than one drive letter is assigned to the same physical device. The get-logical-device function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code RH + 2 BYTE Command code = 23 (17H) ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Last unit referenced, or zero RH + 3 WORD Status ────────────────────────────────────────────────────────────────────────── Function 18H (24): Set Logical Device The set-logical-device function is supported only under MS-DOS versions 3.2 and later and only on block devices; in character-device drivers it should do nothing except set the done bit in the status word. This function is called only if bit 6 is set in the device attribute word of the device header. It corresponds to the set-logical-device-map service supplied to application programs by MS-DOS through Int 21H Function 44H Subfunction 0FH. The set-logical-device function informs the driver of the next logical-drive identifier that will be used to reference the physical device. The unit code passed by the MS-DOS kernel in this case is zero-based relative to the number of logical drives supported by this particular driver. For example, if the driver supports two floppy-disk units (A and B), only one physical floppy-disk drive exists in the system, and the set-logical-device function is called with a unit number of 1, the driver is being informed that the next read or write request from the kernel will be directed to drive B. The set-logical-device function is called with ────────────────────────────────────────────────────────────────────────── RH + 1 BYTE Unit code RH + 2 BYTE Command code = 24 (18H) ────────────────────────────────────────────────────────────────────────── It returns ────────────────────────────────────────────────────────────────────────── RH + 3 WORD Status ────────────────────────────────────────────────────────────────────────── The Processing of a Typical I/O Request An application program requests an I/O operation from MS-DOS by loading registers with the appropriate values and executing an Int 21H. This results in the following sequence of actions: 1. MS-DOS inspects its internal tables and determines which device driver should receive the I/O request. 2. MS-DOS creates a request-header data packet in a reserved area of memory. (Disk I/O requests are transformed from file and record information into logical-sector requests by MS-DOS's interpretation of the disk directory and FAT.) 3. MS-DOS calls the device driver's strat entry point, passing the address of the request header in the ES:BX registers. 4. The device driver saves the address of the request header in a local variable and performs a FAR RETURN. 5. MS-DOS calls the device driver's intr entry point. 6. The interrupt routine saves all registers, retrieves the address of the request header that was saved by the strategy routine, extracts the function code, and branches to the appropriate command-code subroutine to perform the function. 7. If a data transfer on a block device was requested, the driver's read or write subroutine translates the logical-sector number into a head, track, and physical-sector address for the requested unit and then performs the I/O operation. Because a multiple-sector transfer can be requested in a single request header, a single request by MS-DOS to the driver can result in multiple read or write commands to the disk controller. 8. When the requested function is complete, the interrupt routine sets the status word and any other required information into the request header, restores all registers to their state at entry, and performs a FAR RETURN. 9. MS-DOS translates the driver's return status into the appropriate return code and carry-flag status for the MS-DOS Int 21H function that was requested and returns control to the application program. Note that a single request by an application program can result in MS-DOS passing many request headers to the driver. For example, attempting to open a file in a subdirectory on a previously unaccessed disk drive might require the following actions: ■ Reading the disk's boot sector to get the BPB ■ Reading from one to many sectors of the root directory to find the entry for the subdirectory and obtain its starting-cluster number ■ Reading from one to many sectors of both the FAT and the subdirectory itself to find the entry for the desired file The CLOCK Driver: A Special Case MS-DOS uses the CLOCK device for marking file control blocks and directory entries with the date and time, as well as for providing the date and time services to application programs. This device has a unique type of interaction with MS-DOS──a 6-byte sequence is read from or written to the driver that obtains or sets the current date and time. The sequence has the following format: ┌─────────┬─────────┬─────────┬─────────┬─────────┬─────────┐ │ 0 │ 1 │ 2 │ 3 │ 4 │ 5 │ │ Days │ Days │ Minutes │ Hours │Seconds/ │ Seconds │ │low byte │high byte│ │ │ 100 │ │ └─────────┴─────────┴─────────┴─────────┴─────────┴─────────┘ The value passed for days is a 16-bit integer representing the number of days elapsed since January 1, 1980. The clock driver can have any logical-device name because MS-DOS uses the CLOCK bit in the device attribute word of the driver's device header to identify the device, rather than its name. On IBM PC systems, the clock device has the logical-device name CLOCK$. Writing and Installing a Device Driver Now that we have discussed the structure and capabilities of installable device drivers for the MS-DOS environment, we can discuss the mechanical steps of assembling and linking them. Assembly Device drivers for MS-DOS always have an origin of zero but are otherwise assembled, linked, and converted into an executable module as though they were .COM files. (Although MS-DOS is also capable of loading installable drivers in the .EXE file format, this introduces unnecessary complexity into writing and debugging drivers and offers no significant advantages. In addition, it is not possible to use .EXE-format drivers with some IBM versions of MS-DOS because the .EXE loader is located in COMMAND.COM, which is not present when the installable device drivers are being loaded.) The driver should not have a declared stack segment and must, in general, follow the other restrictions outlined in Chapter 3 for memory-image (.COM) programs. A driver can be loaded anywhere, so beware that you do not make any assumptions in your code about the driver's location in physical memory. Figure 14-9 presents a skeleton example that you can follow as you read the next few pages. ────────────────────────────────────────────────────────────────────────── name driver page 55,132 title DRIVER.ASM Device-Driver Skeleton ; ; DRIVER.ASM MS-DOS device-driver skeleton ; ; The driver command-code routines are stubs only and have ; no effect but to return a nonerror "done" status. ; ; Copyright 1988 Ray Duncan ; _TEXT segment word public 'CODE' assume cs:_TEXT,ds:_TEXT,es:NOTHING org 0 MaxCmd equ 24 ; maximum allowed command code: ; 12 for MS-DOS 2 ; 16 for MS-DOS 3.0-3.1 ; 24 for MS-DOS 3.2-3.3 cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII linefeed eom equ '$' ; end-of-message signal Header: ; device-driver header dd -1 ; link to next device driver dw 0c840h ; device attribute word dw Strat ; "strategy" routine entry point dw Intr ; "interrupt" routine entry point db 'SKELETON' ; logical-device name RHPtr dd ? ; pointer to request header, passed ; by MS-DOS kernel to strategy routine Dispatch: ; interrupt-routine command-code ; dispatch table: dw Init ; 0 = initialize driver dw MediaChk ; 1 = media check dw BuildBPB ; 2 = build BPB dw IoctlRd ; 3 = IOCTL read dw Read ; 4 = read dw NdRead ; 5 = nondestructive read dw InpStat ; 6 = input status dw InpFlush ; 7 = flush input buffers dw Write ; 8 = write dw WriteVfy ; 9 = write with verify dw OutStat ; 10 = output status dw OutFlush ; 11 = flush output buffers dw IoctlWt ; 12 = IOCTL write dw DevOpen ; 13 = device open (MS-DOS 3.0+) dw DevClose ; 14 = device close (MS-DOS 3.0+) dw RemMedia ; 15 = removable media (MS-DOS 3.0+) dw OutBusy ; 16 = output until busy (MS-DOS 3.0+) dw Error ; 17 = not used dw Error ; 18 = not used dw GenIOCTL ; 19 = generic IOCTL (MS-DOS 3.2+) dw Error ; 20 = not used dw Error ; 21 = not used dw Error ; 22 = not used dw GetLogDev ; 23 = get logical device (MS-DOS 3.2+) dw SetLogDev ; 24 = set logical device (MS-DOS 3.2+) Strat proc far ; device-driver strategy routine, ; called by MS-DOS kernel with ; ES:BX = address of request header ; save pointer to request header mov word ptr cs:[RHPtr],bx mov word ptr cs:[RHPtr+2],es ret ; back to MS-DOS kernel Strat endp Intr proc far ; device-driver interrupt routine, ; called by MS-DOS kernel immediately ; after call to strategy routine push ax ; save general registers push bx push cx push dx push ds push es push di push si push bp push cs ; make local data addressable pop ds ; by setting DS = CS les di,[RHPtr] ; let ES:DI = request header ; get BX = command code mov bl,es:[di+2] xor bh,bh cmp bx,MaxCmd ; make sure it's legal jle Intr1 ; jump, function code is ok call Error ; set error bit, "unknown command" code jmp Intr2 Intr1: shl bx,1 ; form index to dispatch table ; and branch to command-code routine call word ptr [bx+Dispatch] les di,[RHPtr] ; ES:DI = addr of request header Intr2: or ax,0100h ; merge 'done' bit into status and mov es:[di+3],ax ; store status into request header pop bp ; restore general registers pop si pop di pop es pop ds pop dx pop cx pop bx pop ax ret ; back to MS-DOS kernel ; Command-code routines are called by the interrupt routine ; via the dispatch table with ES:DI pointing to the request ; header. Each routine should return AX = 0 if function was ; completed successfully or AX = (8000h + error code) if ; function failed. MediaChk proc near ; function 1 = media check xor ax,ax ret MediaChk endp BuildBPB proc near ; function 2 = build BPB xor ax,ax ret BuildBPB endp IoctlRd proc near ; function 3 = IOCTL read xor ax,ax ret IoctlRd endp Read proc near ; function 4 = read (input) xor ax,ax ret Read endp NdRead proc near ; function 5 = nondestructive read xor ax,ax ret NdRead endp InpStat proc near ; function 6 = input status xor ax,ax ret InpStat endp InpFlush proc near ; function 7 = flush input buffers xor ax,ax ret InpFlush endp Write proc near ; function 8 = write (output) xor ax,ax ret Write endp WriteVfy proc near ; function 9 = write with verify xor ax,ax ret endp OutStat proc near ; function 10 = output status xor ax,ax ret OutStat endp OutFlush proc near ; function 11 = flush output buffers xor ax,ax ret OutFlush endp IoctlWt proc near ; function 12 = IOCTL write xor ax,ax ret IoctlWt endp DevOpen proc near ; function 13 = device open xor ax,ax ret DevOpen endp DevClose proc near ; function 14 = device close xor ax,ax ret DevClose endp RemMedia proc near ; function 15 = removable media xor ax,ax ret RemMedia endp OutBusy proc near ; function 16 = output until busy xor ax,ax ret OutBusy endp GenIOCTL proc near ; function 19 = generic IOCTL xor ax,ax ret GenIOCTL endp GetLogDev proc near ; function 23 = get logical device xor ax,ax ret GetLogDev endp SetLogDev proc near ; function 24 = set logical device xor ax,ax ret SetLogDev endp Error proc near ; bad command code in request header mov ax,8003h ; error bit + "unknown command" code ret endp Init proc near ; function 0 = initialize driver push es ; save address of request header push di mov ax,cs ; convert load address to ASCII mov bx,offset Ident1 call hexasc mov ah,9 ; display driver sign-on message mov dx,offset Ident int 21h pop di ; restore request-header address pop es ; set address of free memory ; above driver (break address) mov word ptr es:[di+14],offset Init mov word ptr es:[di+16],cs xor ax,ax ; return status ret Init endp hexasc proc near ; converts word to hex ASCII ; call with AX = value, ; DS:BX = address for string ; returns AX, BX destroyed push cx ; save registers push dx mov dx,4 ; initialize character counter mov cx,4 ; isolate next four bits rol ax,cl mov cx,ax and cx,0fh add cx,'0' ; convert to ASCII cmp cx,'9' ; is it 0-9? jbe hexasc2 ; yes, jump add cx,'A'-'9'-1 ; add fudge factor for A-F hexasc2: ; store this character mov [bx],cl inc bx ; bump string pointer dec dx ; count characters converted jnz hexasc1 ; loop, not four yet pop dx ; restore registers pop cx ret ; back to caller hexasc endp Ident db cr,lf,lf db 'Advanced MS-DOS Example Device Driver' db cr,lf db 'Device driver header at: ' Ident1 db 'XXXX:0000' db cr,lf,lf,eom Intr endp _TEXT ends end ────────────────────────────────────────────────────────────────────────── Figure 14-9. DRIVER.ASM: A functional skeleton from which you can implement your own working device driver. The driver's device header must be located at the beginning of the file (offset 0000H). Both words in the link field in the header should be set to -1. The attribute word must be set up correctly for the device type and other options. The offsets to the strategy and interrupt routines must be relative to the same segment base as the device header itself. If the driver is for a character device, the name field should be filled in properly with the device's logical name. The logical name can be any legal 8-character filename, padded with spaces and without a colon. Beware of accidentally duplicating the names of existing character devices, unless you are intentionally superseding a resident driver. MS-DOS calls the strategy and interrupt routines for the device by means of an intersegment call (CALL FAR) when the driver is first loaded and installed and again whenever an application program issues an I/O request for the device. MS-DOS uses the ES:BX registers to pass the strat routine a double-word pointer to the request header; this address should be saved internally in the driver so that it is available for use during the subsequent call to the intr routine. The command-code routines for function codes 0 through 12 (0CH) must be present in every installable device driver, regardless of device type. Functions 13 (0DH) and above are optional for drivers used with MS-DOS versions 3.0 and later and can be handled in one of the following ways: ■ Don't implement them, and leave the associated bits in the device header cleared. The resulting driver will work in either version 2 or version 3 but does not take full advantage of the augmented functionality of version 3. ■ Implement them, and test the MS-DOS version during the initialization sequence, setting bits 6 and 11 of the device header appropriately. Write all command-code routines so that they test this bit and adjust to accommodate the host version of MS-DOS. Such a driver requires more work and testing but will take full advantage of both the version 2 and the version 3 environments. ■ Implement them, and assume that all the version 3 facilities are available. With this approach, the resulting driver may not work properly under version 2. Remember that device drivers must preserve the integrity of MS-DOS. The driver must preserve all registers, including flags (especially the direction flag and interrupt enable bits), and if the driver makes heavy use of the stack, it should switch to an internal stack of adequate depth (the MS-DOS stack has room for only 40 to 50 bytes when a driver is called). If you install a new CON driver, be sure to set the bits for standard input and standard output in the device attribute word in the device header. You'll recall that one file can contain multiple drivers. In this case, the device-header link field of each driver should point to the segment offset of the next, all using the same segment base, and the link field for the last driver in the file should be set to -1,-1. The initialization routines for all the drivers in the file should return the same break address. Linking Use the standard MS-DOS linker to transform the .OBJ file that is output from the assembler into a relocatable .EXE module. Then, use the EXE2BIN utility (see Chapter 4) to convert the .EXE file into a memory-image program. The extension on the final driver file can be anything, but .BIN and .SYS are most commonly used in MS-DOS systems, and it is therefore wise to follow one of these conventions. Installation After the driver is assembled, linked, and converted to a .BIN or .SYS file, copy it to the root directory of a bootable disk. If it is a character-device driver, do not use the same name for the file as you used for the logical device listed in the driver's header, or you will not be able to delete, copy, or rename the file after the driver is loaded. Use your favorite text editor to add the line DEVICE=[D:][PATH]FILENAME.EXT to the CONFIG.SYS file on the bootable disk. (In this line, D: is an optional drive designator and FILENAME.EXT is the name of the file containing your new device driver. You can include a path specification in the entry if you prefer not to put the driver file in your root directory.) Now restart your computer system to load the modified CONFIG.SYS file. During the MS-DOS boot sequence, the SYSINIT module (which is part of IO.SYS) reads and processes the CONFIG.SYS file. It loads the driver into memory and inspects the device header. If the driver is a character-device driver, SYSINIT links it into the device chain ahead of the other character devices; if it is a block-device driver, SYSINIT places it behind all previously linked block devices and the resident block devices (Figures 14-10, 14-11, and 14-12). It accomplishes the linkage by updating the link field in the device header to point to the segment and offset of the next driver in the chain. The link field of the last driver in the chain contains -1,-1. Next, SYSINIT calls the strat routine with a request header that contains a command code of zero, and then it calls the intr routine. The driver executes its initialization routine and returns the break address, telling MS-DOS how much memory to reserve for this driver. Now MS-DOS can proceed to the next entry in the CONFIG.SYS file. You cannot supersede a built-in block-device driver──you can only add supplemental block devices. However, you can override the default system driver for a character device (such as CON) with an installed driver by giving it the same logical-device name in the device header. When processing a character I/O request, MS-DOS always scans the list of installed drivers before it scans the list of default devices and takes the first match. NUL │ CON │ AUX │ PRN │ CLOCK │ Any other resident block or character devices Figure 14-10. MS-DOS device-driver chain before any installable device drivers have been loaded. NUL │ Installable character- device drivers │ CON │ AUX │ PRN │ CLOCK │ Any other resident block or character devices │ Installable block- device drivers Figure 14-11. MS-DOS device-driver chain after installable device drivers have been loaded. Address Attribute Strategy Interrupt Type Units Name routine routine ────────────────────────────────────────────────────────────────────────── 00E3:0111 8004 0FD5 0FE0 C NUL 0070:0148 8013 008E 0099 C CON 0070:01DD 8000 008E 009F C AUX 0070:028E 8000 008E 00AE C PRN 0070:0300 8008 008E 00C3 C CLOCK 0070:03CC 0000 008E 00C9 B 02 0070:01EF 8000 008E 009F C COM1 0070:02A0 8000 008E 00AE C LPT1 0070:06F0 8000 008E 00B4 C LPT2 0070:0702 8000 008E 00BA C LPT3 0070:0714 8000 008E 00A5 C COM2 End of device chain ────────────────────────────────────────────────────────────────────────── Figure 14-12. Example listing of device chain under MS-DOS version 2.1, "plain vanilla" IBM PC with no fixed disks or user device drivers. (C=character device, B=block device) Debugging a Device Driver The most important thing to remember when testing new device drivers is to maintain adequate backups and a viable fallback position. Don't modify the CONFIG.SYS file and install the new driver on your fixed disk before it is proven! Be prudent──create a bootable floppy disk and put the modified CONFIG.SYS file and the new driver on that for debugging. When everything is working properly, copy the finished product to its permanent storage medium. The easiest way to test a new device driver is to write a simple assembly-language front-end routine that sets up a simulated request packet and then performs FAR CALLs to the strat and intr entry points, exactly as MS-DOS would. You can then link the driver and the front end together into a .COM or .EXE file that can be run under the control of CodeView or another debugger. This arrangement makes it easy to trace each of the command-code routines individually, to observe the results of the I/O, and to examine the status codes returned in the request header. Tracing the installed driver when it is linked into the MS-DOS system in the normal manner is more difficult. Breakpoints must be chosen carefully, to yield the maximum possible information per debugging run. Because current versions of MS-DOS maintain only one request header internally, the request header that was being used by the driver you are tracing will be overwritten as soon as your debugger makes an output request to display information. You will find it helpful to add a routine to your initialization subroutine that displays the driver's load address on the console when you boot MS-DOS; you can then use this address to inspect the device-driver header and set breakpoints within the body of the driver. Debugging a device driver can also be somewhat sticky when interrupt handling is involved, especially if the device uses the same interrupt-request priority level (IRQ level) as other peripherals in the system. Cautious, conservative programming is needed to avoid unexpected and unreproducible interactions with other device drivers and interrupt handlers. If possible, prove out the basic logic of the driver using polled I/O, rather than interrupt-driven I/O, and introduce interrupt handling only when you know the rest of the driver's logic to be solid. Typical device-driver errors or problems that can cause system crashes or strange system behavior include the following: ■ Failure to set the linkage address of the last driver in a file to -1 ■ Overflow of the MS-DOS stack by driver-initialization code, corrupting the memory image of MS-DOS (can lead to unpredictable behavior during boot; remedy is to use a local stack) ■ Incorrect break-address reporting by the initialization routine (can lead to a system crash if the next driver loaded overwrites vital parts of the driver) ■ Improper BPBs supplied by the build BPB routine, or incorrect BPB pointer array supplied by the initialization routine (can lead to many confusing problems, ranging from out-of-memory errors to system boot failure) ■ Incorrect reporting of the number of bytes or sectors successfully transferred at the time an I/O error occurs (can manifest itself as a system crash after you enter R to the Abort, Retry, Ignore? prompt) Although the interface between the DOS kernel and the device driver is fairly simple, it is also quite strict. The command-code routines must perform exactly as they are defined, or the system will behave erratically. Even a very subtle discrepancy in the action of a command-code routine can have unexpectedly large global effects. ──────────────────────────────────────────────────────────────────────────── Chapter 15 Filters A filter is, essentially, a program that operates on a stream of characters. The source and destination of the character stream can be files, another program, or almost any character device. The transformation applied by the filter to the character stream can range from an operation as simple as character substitution to one as elaborate as generating splines from sets of coordinates. The standard MS-DOS package includes three simple filters: SORT, which alphabetically sorts text on a line-by-line basis; FIND, which searches a text stream to match a specified string; and MORE, which displays text one screenful at a time. System Support for Filters The operation of a filter program relies on two MS-DOS features that first appeared in version 2.0: standard devices and redirectable I/O. The standard devices are represented by five handles that are originally established by COMMAND.COM. Each process inherits these handles from its immediate parent. Thus, the standard device handles are already open when a process acquires control of the system, and it can use them with Interrupt 21H Functions 3FH and 40H for read and write operations without further preliminaries. The default assignments of the standard device handles are as follows: Handle Name Default device ────────────────────────────────────────────────────────────────────────── 0 stdin (standard input) CON 1 stdout (standard output) CON 2 stderr (standard error) CON 3 stdaux (standard auxiliary) AUX 4 stdprn (standard printer) PRN ────────────────────────────────────────────────────────────────────────── The CON device is assigned by default to the system's keyboard and video display. AUX and PRN are respectively associated by default with COM1 (the first physical serial port) and LPT1 (the first parallel printer port). You can use the MODE command to redirect LPT1 to one of the serial ports; the MODE command will also redirect PRN. When executing a program by entering its name at the COMMAND.COM prompt, you can redirect the standard input, the standard output, or both from their default device (CON) to another file, a character device, or a process. You do this by including one of the special characters <, >, >>, and | in the command line, in the form shown on the following page. Symbol Effect ────────────────────────────────────────────────────────────────────────── < file Takes standard input from the specified file instead of the keyboard. < device Takes standard input from the named device instead of the keyboard. > file Sends standard output to the specified file instead of the display. >> file Appends standard output to the current contents of the specified file instead of sending it to the display. > device Sends standard output to the named device instead of the display. p1 | p2 Routes standard output of program p1 to become the standard input of program p2. (Output of p1 is said to be piped to p2.) ────────────────────────────────────────────────────────────────────────── For example, the command C>SORT <MYFILE.TXT >PRN <Enter> causes the SORT filter to read its input from the file MYFILE.TXT, sort the lines alphabetically, and write the resulting text to the character device PRN (the logical name for the system's list device). The redirection requested by the <, >, >>, and | characters takes place at the level of COMMAND.COM and is invisible to the program it affects. Any other process can achieve a similar effect by redirecting the standard input and standard output with Int 21H Function 46H before calling the EXEC function (Int 21H Function 4BH) to run a child process. Note that if a program circumvents MS-DOS to perform its input and output, either by calling ROM BIOS functions or by manipulating the keyboard or video controller directly, redirection commands placed in the program's command line do not have the expected effect. How Filters Work By convention, a filter program reads its text from the standard input device and writes the results of its operations to the standard output device. When it reaches the end of the input stream, the filter simply terminates. As a result, filters are both flexible and simple. Filter programs are flexible because they do not know, and do not care about, the source of the data they process or the destination of their output. Thus, any character device that has a logical name within the system (CON, AUX, COM1, COM2, PRN, LPT1, LPT2, LPT3, and so on), any file on any block device (local or network) known to the system, or any other program can supply a filter's input or accept its output. If necessary, you can concatenate several functionally simple filters with pipes to perform very complex operations. Although flexible, filters are also simple because they rely on their parent processes to supply standard input and standard output handles that have already been appropriately redirected. The parent must open or create any necessary files, check the validity of logical character-device names, and load and execute the preceding or following process in a pipe. The filter concerns itself only with the transformation it applies to the data. Building a Filter Creating a new filter for MS-DOS is a straightforward process. In its simplest form, a filter need only use the handle-oriented read (Interrupt 21H Function 3FH) and write (Interrupt 21H Function 40H) functions to get characters or lines from standard input and send them to standard output, performing any desired alterations on the text stream on a character-by-character or line-by-line basis. Figures 15-1 and 15-2 contain prototype character-oriented filters in both assembly language and C. In these examples, the translate routine, which is called for each character transferred from the standard input to the standard output, does nothing at all. As a result, both filters function rather like a very slow COPY command. You can quickly turn these primitive filters into useful programs by substituting your own translate routine. If you try out these programs, you'll notice that the C prototype filter runs much faster than its MASM equivalent. This is because the C runtime library is performing hidden blocking and deblocking of the input and output stream, whereas the MASM filter is doing exactly what it appears to be doing: making two calls to MS-DOS for each character processed. You can easily restore the MASM filter's expected speed advantage by adapting it to read and write lines instead of single characters. ────────────────────────────────────────────────────────────────────────── name proto page 55,132 title PROTO.ASM--prototype filter ; ; PROTO.ASM: prototype character-oriented filter ; ; Copyright 1988 Ray Duncan ; stdin equ 0 ; standard input handle stdout equ 1 ; standard output handle stderr equ 2 ; standard error handle cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII linefeed _TEXT segment word public 'CODE' assume cs:_TEXT,ds:_DATA,ss:STACK main proc far ; entry point from MS-DOS mov ax,_DATA ; set DS = our data segment mov ds,ax main1: ; read char from stdin... mov dx,offset char ; DS:DX = buffer address mov cx,1 ; CX = length to read mov bx,stdin ; BX = standard input handle mov ah,3fh ; function 3fh = read int 21h ; transfer to MS-DOS jc main3 ; if error, terminate cmp ax,1 ; any character read? jne main2 ; if end of file, terminate call translate ; translate character ; write char to stdout... mov dx,offset char ; DS:DX = buffer address mov cx,1 ; CX = length to write mov bx,stdout ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS jc main3 ; if error, terminate cmp ax,1 ; was character written? jne main3 ; if disk full, terminate jmp main1 ; get another character main2: ; end of file reached mov ax,4c00h ; function 4ch = terminate ; return code = 0 int 21h ; transfer to MS-DOS main3: ; error or disk full mov ax,4c01h ; function 4ch = terminate ; return code = 1 int 21h ; transfer to MS-DOS main endp ; ; Perform any necessary translation on character ; from standard input stored in variable 'char'. ; This example simply leaves character unchanged. ; translate proc near ret ; does nothing translate endp _TEXT ends _DATA segment word public 'DATA' char db 0 ; storage for input character _DATA ends STACK segment para stack 'STACK' dw 64 dup (?) STACK ends end main ; defines program entry point ────────────────────────────────────────────────────────────────────────── Figure 15-1. PROTO.ASM, the source code for a prototype character-oriented MASM filter. ────────────────────────────────────────────────────────────────────────── /* PROTO.C: prototype character-oriented filter Copyright 1988 Ray Duncan */ #include <stdio.h> main(int argc, char *argv[]) { char ch; while((ch=getchar()) != EOF) /* read a character */ { ch = translate(ch); /* translate it if necessary */ putchar(ch); /* write the character */ } exit(0); /* terminate at end of file */ } /* Perform any necessary translation on character from input file. This example simply returns the same character. */ int translate(char ch) { return (ch); } ────────────────────────────────────────────────────────────────────────── Figure 15-2. PROTO.C, the source code for a prototype character-oriented C filter. The CLEAN Filter As a more practical example of MS-DOS filters, let's look at a simple but very useful filter called CLEAN. Figures 15-3 and 15-4 show the assembly-language and C source code for this filter. CLEAN processes a text stream by stripping the high bit from all characters, expanding tabs to spaces, and throwing away all control codes except carriage returns, linefeeds, and formfeeds. Consequently, CLEAN can transform almost any kind of word-processed document file into a plain ASCII text file. ────────────────────────────────────────────────────────────────────────── name clean page 55,132 title CLEAN--Text-file filter ; ; CLEAN.ASM Filter to turn document files into ; normal text files. ; ; Copyright 1988 Ray Duncan ; ; Build: C>MASM CLEAN; ; C>LINK CLEAN; ; ; Usage: C>CLEAN <infile >outfile ; ; All text characters are passed through with high ; bit stripped off. Formfeeds, carriage returns, ; and linefeeds are passed through. Tabs are expanded ; to spaces. All other control codes are discarded. ; tab equ 09h ; ASCII tab code lf equ 0ah ; ASCII linefeed ff equ 0ch ; ASCII formfeed cr equ 0dh ; ASCII carriage return blank equ 020h ; ASCII space code eof equ 01ah ; Ctrl-Z end-of-file tabsiz equ 8 ; width of tab stop bufsiz equ 128 ; size of input and ; output buffers stdin equ 0000 ; standard input handle stdout equ 0001 ; standard output handle stderr equ 0002 ; standard error handle _TEXT segment word public 'CODE' assume cs:_TEXT,ds:_DATA,es:_DATA,ss:STACK clean proc far ; entry point from MS-DOS push ds ; save DS:0000 for final xor ax,ax ; return to MS-DOS, in case push ax ; function 4ch can't be used mov ax,_DATA ; make data segment addressable mov ds,ax mov es,ax mov ah,30h ; check version of MS-DOS int 21h cmp al,2 ; MS-DOS 2.0 or later? jae clean1 ; jump if version OK ; MS-DOS 1, display error ; message and exit... mov dx,offset msg1 ; DS:DX = message address mov ah,9 ; function 9 = display string int 21h ; transfer to MS-DOS ret ; then exit the old way clean1: call init ; initialize input buffer clean2: call getc ; get character from input jc clean9 ; exit if end of stream and al,07fh ; strip off high bit cmp al,blank ; is it a control char? jae clean4 ; no, write it cmp al,eof ; is it end of file? je clean8 ; yes, write EOF and exit cmp al,tab ; is it a tab? je clean6 ; yes, expand it to spaces cmp al,cr ; is it a carriage return? je clean3 ; yes, go process it cmp al,lf ; is it a linefeed? je clean3 ; yes, go process it cmp al,ff ; is it a formfeed? jne clean2 ; no, discard it clean3: mov column,0 ; if CR, LF, or FF, jmp clean5 ; reset column to zero clean4: inc column ; if non-control character, ; increment column counter clean5: call putc ; write char to stdout jnc clean2 ; if disk not full, ; get another character ; write failed... mov dx,offset msg2 ; DS:DX = error message mov cx,msg2_len ; CX = message length mov bx,stderr ; BX = standard error handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS mov ax,4c01h ; function 4ch = terminate ; return code = 1 int 21h ; transfer to MS-DOS clean6: mov ax,column ; tab code detected cwd ; tabsiz - (column MOD tabsiz) mov cx,tabsiz ; is number of spaces needed idiv cx ; to move to next tab stop sub cx,dx add column,cx ; also update column counter clean7: push cx ; save spaces counter mov al,blank ; write an ASCII space call putc pop cx ; restore spaces counter loop clean7 ; loop until tab stop jmp clean2 ; get another character clean8: call putc ; write EOF mark clean9: call flush ; write last output buffer mov ax,4c00h ; function 4ch = terminate ; return code = 0 int 21h ; transfer to MS-DOS clean endp getc proc near ; get character from stdin ; returns carry = 1 if ; end of input, else ; AL = char, carry = 0 mov bx,iptr ; get input buffer pointer cmp bx,ilen ; end of buffer reached? jne getc1 ; not yet, jump ; more data is needed... mov bx,stdin ; BX = standard input handle mov cx,bufsiz ; CX = length to read mov dx,offset ibuff ; DS:DX = buffer address mov ah,3fh ; function 3fh = read int 21h ; transfer to MS-DOS jc getc2 ; jump if read failed or ax,ax ; was anything read? jz getc2 ; jump if end of input mov ilen,ax ; save length of data xor bx,bx ; reset buffer pointer getc1: mov al,[ibuff+bx] ; get character from buffer inc bx ; bump buffer pointer mov iptr,bx ; save updated pointer clc ; return character in AL ret ; and carry = 0 (clear) getc2: stc ; end of input stream ret ; return carry = 1 (set) getc endp putc proc near ; send character to stdout, ; returns carry = 1 if ; error, else carry = 0 mov bx,optr ; store character into mov [obuff+bx],al ; output buffer inc bx ; bump buffer pointer cmp bx,bufsiz ; buffer full? jne putc1 ; no, jump mov bx,stdout ; BX = standard output handle mov cx,bufsiz ; CX = length to write mov dx,offset obuff ; DS:DX = buffer address mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS jc putc2 ; jump if write failed cmp ax,cx ; was write complete? jne putc2 ; jump if disk full xor bx,bx ; reset buffer pointer putc1: mov optr,bx ; save buffer pointer clc ; write successful, ret ; return carry = 0 (clear) putc2: stc ; write failed or disk full, ret ; return carry = 1 (set) putc endp init proc near ; initialize input buffer mov bx,stdin ; BX = standard input handle mov cx,bufsiz ; CX = length to read mov dx,offset ibuff ; DS:DX = buffer address mov ah,3fh ; function 3fh = read int 21h ; transfer to MS-DOS jc init1 ; jump if read failed mov ilen,ax ; save actual bytes read init1: ret init endp flush proc near ; flush output buffer mov cx,optr ; CX = bytes to write jcxz flush1 ; exit if buffer empty mov dx,offset obuff ; DS:DX = buffer address mov bx,stdout ; BX = standard output handle mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS flush1: ret flush endp _TEXT ends _DATA segment word public 'DATA' ibuff db bufsiz dup (0) ; input buffer obuff db bufsiz dup (0) ; output buffer iptr dw 0 ; ibuff pointer ilen dw 0 ; bytes in ibuff optr dw 0 ; obuff pointer column dw 0 ; current column counter msg1 db cr,lf db 'clean: need MS-DOS version 2 or greater.' db cr,lf,'$' msg2 db cr,lf db 'clean: disk is full.' db cr,lf msg2_len equ $-msg2 _DATA ends STACK segment para stack 'STACK' dw 64 dup (?) STACK ends end clean ────────────────────────────────────────────────────────────────────────── Figure 15-3. CLEAN.ASM, the source code for the MASM version of the CLEAN filter. ────────────────────────────────────────────────────────────────────────── /* CLEAN.C Filter to turn document files into normal text files. Copyright 1988 Ray Duncan Compile: C>CL CLEAN.C Usage: C>CLEAN <infile >outfile All text characters are passed through with high bit stripped off. Formfeeds, carriage returns, and linefeeds are passed through. Tabs are expanded to spaces. All other control codes are discarded. */ #include <stdio.h> #define TAB_WIDTH 8 /* width of a tab stop */ #define TAB '\x09' /* ASCII tab character */ #define LF '\x0A' /* ASCII linefeed */ #define FF '\x0C' /* ASCII formfeed */ #define CR '\x0D' /* ASCII carriage return */ #define BLANK '\x20' /* ASCII space code */ #define EOFMK '\x1A' /* Ctrl-Z end of file */ main(int argc, char *argv[]) { char c; /* character from stdin */ int col = 0; /* column counter */ while((c = getchar()) != EOF) /* read input character */ { c &= 0x07F; /* strip high bit */ switch(c) /* decode character */ { case LF: /* if linefeed or */ case CR: /* carriage return, */ col=0; /* reset column count */ case FF: /* if formfeed, carriage */ wchar(c); /* return, or linefeed, */ break; /* pass character through */ case TAB: /* if tab, expand to spaces*/ do wchar(BLANK); while((++col % TAB_WIDTH) != 0); break; default: /* discard other control */ if(c >= BLANK) /* characters, pass text */ { /* characters through */ wchar(c); col++; /* bump column counter */ } break; } } wchar(EOFMK); /* write end-of-file mark */ exit(0); } /* Write a character to the standard output. If write fails, display error message and terminate. */ wchar(char c) { if((putchar(c) == EOF) && (c != EOFMK)) { fputs("clean: disk full",stderr); exit(1); } } ────────────────────────────────────────────────────────────────────────── Figure 15-4. CLEAN.C, the source code for the C version of the CLEAN filter. When using the CLEAN filter, you must specify the source and destination files with redirection parameters in the command line; otherwise, CLEAN will simply read the keyboard and write to the display. For example, to filter the document file MYFILE.DOC and leave the result in the file MYFILE.TXT, you would enter the following command: C>CLEAN <MYFILE.DOC >MYFILE.TXT <Enter> (Note that the original file, MYFILE.DOC, is unchanged.) One valuable application of this filter is to rescue assembly-language source files. If you accidentally edit such a source file in document mode, the resulting file may cause the assembler to generate spurious or confusing error messages. CLEAN lets you turn the source file back into something the assembler can cope with, without losing the time you spent to edit it. Another handy application for CLEAN is to list a word-processed document in raw form on the printer, using a command such as C>CLEAN <MYFILE.DOC >PRN <Enter> Contrasting the C and assembly-language versions of this filter provides some interesting statistics. The C version contains 79 lines and compiles to a 5889-byte .EXE file, whereas the assembly-language version contains 265 lines and builds an 1107-byte .EXE file. The size and execution-speed advantages of implementing such tools in assembly language is obvious, even compared with such an excellent compiler as the Microsoft C Optimizing Compiler. However, you must balance performance considerations against the time and expense required for programming, particularly when a program will not be used very often. ──────────────────────────────────────────────────────────────────────────── Chapter 16 Compatibility and Portability At the beginning of this book, we surveyed the history of MS-DOS and saw that new versions come along nearly every year, loosely coupled to the introduction of new models of personal computers. We then focused on each of the mainstream issues of MS-DOS applications programming: the user interface; mass storage; memory management; control of "child" processes; and special classes of programs, such as filters, interrupt handlers, and device drivers. It's now time to close the circle and consider two global concerns of MS-DOS programming: compatibility and portability. For your programs to remain useful in a constantly evolving software and hardware environment, you must design them so that they perform reliably on any reasonable machine configuration and exploit available system resources; in addition, you should be able to upgrade them easily for new versions of MS-DOS, for new machines, and, for that matter, for completely new environments such as MS OS/2. Degrees of Compatibility If we look at how existing MS-DOS applications use the operating system and hardware, we find that we can assign them to one of four categories: ■ MS-DOS─compatible applications ■ ROM BIOS─compatible applications ■ Hardware-compatible applications ■ "Ill-behaved" applications MS-DOS─compatible applications use only the documented MS-DOS function calls and do not call the ROM BIOS or access the hardware directly. They use ANSI escape sequences for screen control, and their input and output is redirectable. An MS-DOS─compatible application will run on any machine that supports MS-DOS, regardless of the machine configuration. Because of the relatively poor performance of MS-DOS's built-in display and serial port drivers, few popular programs other than compilers, assemblers, and linkers fall into this category. ROM BIOS─compatible applications use the documented MS-DOS and ROM BIOS function calls but do not access the hardware directly. As recently as three years ago, this strategy might have significantly limited a program's potential market. Today, the availability of high-quality IBM-compatible ROM BIOSes from companies such as Phoenix has ensured the dominance of the IBM ROM BIOS standard; virtually no machines are being sold in which a program cannot rely as much on the ROM BIOS interface as it might on the MS-DOS interface. However, as we noted in Chapters 6 and 7, the ROM BIOS display and serial drivers are still not adequate to the needs of high-performance interactive applications, so the popular programs that fall into this category are few. Hardware-compatible applications generally use MS-DOS functions for mass storage, memory management, and the like, and use a mix of MS-DOS and ROM BIOS function calls and direct hardware access for their user interfaces. The amount of hardware dependence in such programs varies widely. For example, some programs only write characters and attributes into the video controller's regen buffer and use the ROM BIOS to switch modes and position the cursor; others bypass the ROM BIOS video driver altogether and take complete control of the video adapter. As this book is written, the vast majority of the popular MS-DOS "productivity" applications (word processors, databases, telecommunications programs, and so on) can be placed somewhere in this category. "Ill-behaved" applications are those that rely on undocumented MS-DOS function calls or data structures, interception of MS-DOS or ROM BIOS interrupts, or direct access to mass storage devices (bypassing the MS-DOS file system). These programs tend to be extremely sensitive to their environment and typically must be "adjusted" in order to work with each new MS-DOS version or PC model. Virtually all popular terminate- and-stay-resident (TSR) utilities, network programs, and disk repair/optimization packages are in this category. Writing Well-Behaved MS-DOS Applications Your choice of MS-DOS functions, ROM BIOS functions, or direct hardware access to solve a particular problem must always be balanced against performance needs; and, of course, the user is the final judge of a program's usefulness and reliability. Nevertheless, you can follow some basic guidelines, outlined below, to create well-behaved applications that are likely to run properly under future versions of MS-DOS and under multitasking program managers that run on top of MS-DOS, such as Microsoft Windows. Program structure Design your programs as .EXE files with separate code, data, and stack segments; shun the use of .COM files. Use the Microsoft conventions for segment names and attributes discussed in Chapter 3. Inspect the environment block at runtime to locate your program's overlays or data files; don't "hard-wire" a directory location into the program. Check host capabilities Obtain the MS-DOS version number with Int 21H Function 30H during your program's initialization and be sure that all of the functions your program requires are actually available. If you find that the host MS-DOS version is inadequate, be careful about which functions you call to display an error message and to terminate. Use the enhanced capabilities of MS-DOS versions 3 and 4 when your program is running under those versions. For example, you can specify a sharing mode when opening a file with Int 21H Function 3DH, you can create temporary or unique files with Int 21H Functions 5AH and 5BH, and you can obtain extended error information (including a recommended recovery strategy) with Int 21H Function 59H. Section 2 of this book contains version-dependency information for each MS-DOS function. Input and output Use the handle file functions exclusively and extend full path support throughout your application (being sure to allow for the maximum possible path length during user input of filenames). Use buffered I/O whenever possible. The device drivers in MS-DOS versions 2.0 and later can handle strings as long as 64 KB, and performance will be improved if you write fewer, larger records as opposed to many short ones. Avoid the use of FCBs, the Int 25H or Int 26H functions, or the ROM BIOS disk driver. If you must use FCBs, close them when you are done with them and don't move them around while they are open. Avoid reopening FCBs that are already open or reclosing FCBs that have already been closed──these seemingly harmless practices can cause problems when network software is running. Memory management During your program's initialization, release any memory that is not needed by the program. (This is especially important for .COM programs.) If your program requires extra memory for buffers or tables, allocate that memory dynamically when it is needed and release it as soon as it is no longer required. Use expanded memory, when it is available, to minimize your program's demands on conventional memory. As a general rule, don't touch any memory that is not owned by your program. To set or inspect interrupt vectors, use Int 21H Functions 25H and 35H rather than editing the interrupt vector table directly. If you alter the contents of interrupt vectors, save their original values and restore them before the program exits. Process management To isolate your program from dependencies on PSP structure and relocation information, use the EXEC function (Int 21H Function 4BH) when loading overlays or other programs. Terminate your program with Int 21H Function 4CH, passing a zero return code if the program executes successfully and a nonzero code if an error is encountered. Your program's parent can then test this return code with Int 21H Function 4DH or, in a batch file, with the IF ERRORLEVEL statement. Exception handling Install Ctrl-C (Int 23H) and critical-error (Int 24H) handlers so that your program cannot be terminated unexpectedly by the user's entry of Ctrl-C or Ctrl-Break or by a hardware I/O failure. This is particularly important if your program uses expanded memory or installs its own interrupt handlers. ROM BIOS and Hardware-Compatible Applications When you feel the need to introduce ROM BIOS or hardware dependence for performance reasons, keep it isolated to small, well-documented procedures that can be easily modified when the hardware changes. Use macros and equates to hide hardware characteristics and to avoid spreading "magic numbers" throughout your program. Check host capabilities If you use ROM BIOS functions in your program, you must check the machine model at runtime to be sure that the functions your program needs are actually available. There is a machine ID byte at F000:FFFEH whose value is interpreted as follows: ────────────────────────────────────────────────────────────────────────── F8H PS/2 Models 70 and 80 F9H PC Convertible FAH PS/2 Model 30 FBH PC/XT (later models) FCH PC/AT, PC/XT-286, PS/2 Models 50 and 60 FDH PCjr FEH PC/XT (early models) FFH PC "Classic" ────────────────────────────────────────────────────────────────────────── In some cases, submodels can be identified; see Int 15H Function C0H on page 573. Section 3 of this book contains version-dependency information for each ROM BIOS function. When writing your own direct video drivers, you must determine the type and capabilities of the video adapter by a combination of Int 10H calls, reading ports, and inspection of the ROM BIOS data area at 0040:0000H and the memory reserved for the EGA or VGA ROM BIOS, among other things. The techniques required are beyond the scope of this book but are well explained in Programmer's Guide to PC and PS/2 Video Systems (Microsoft Press, 1987). Avoid unstable hardware Some areas of IBM personal computer architecture have remained remarkably stable from the original IBM PC, based on a 4.77 MHz 8088, to today's PS/2 Model 80, based on a 20 MHz 80386. IBM's track record for upward compatibility in its video and serial communications controllers has been excellent; in many cases, the same hardware-dependent code that was written for the original IBM PC runs perfectly well on an IBM PS/2 Model 80. Other areas of relative hardware stability are: ■ Sound control via port 61H ■ The 8253 timer chip's channels 0 and 2 (ports 40H, 42H, and 43H) ■ The game adapter at port 201H ■ Control of the interrupt system via the 8259 PIC's mask register at port 21H However, direct sound generation and manipulation of the 8253 timer or 8259 PIC are quite likely to cause problems if your program is run under a multitasking program manager such as Microsoft Windows or DesqView. Keyboard mapping, the keyboard controller, and the floppy and fixed disk controllers are areas of relative hardware instability. Programs that bypass MS-DOS for keyboard or disk access are much less likely to function properly across the different PC models and are also prone to interfere with each other and with well-behaved applications. OS/2 Compatibility MS-DOS is upwardly compatible in several respects with OS/2, Microsoft's multitasking protected-mode virtual memory operating system for 80286 and 80386 computers. The OS/2 graphical user interface (the Presentation Manager) is nearly identical to Microsoft Windows 2.0. OS/2 versions 1.0 and 1.1 use exactly the same disk formats as MS-DOS so that files may easily be moved between MS-DOS and OS/2 systems. Most important, OS/2 includes a module called the "DOS Compatibility Environment" or "3.x Box," which can run one MS-DOS application at a time alongside protected-mode OS/2 applications. The 3.x Box traps Int 21H function calls and remaps them into OS/2 function calls, emulating an MS-DOS 3.3 environment with the file-sharing module (SHARE.EXE) loaded but returning a major version number of 10 instead of 3 for Int 21H Function 30H. The 3.x Box also supports most ROM BIOS calls, either by emulating their function or by interlocking the device and then calling the original ROM BIOS routine. In addition, the 3.x Box maintains the ROM BIOS data area, provides timer ticks to applications via Int 1CH, and supports certain undocumented MS-DOS services and data structures so that most TSR utilities can function properly. Nevertheless, the 3.x Box's emulation of MS-DOS is not perfect, and you must be aware of certain constraints on MS-DOS applications running under OS/2. The most significant restriction on an MS-DOS application is that it does not receive any CPU cycles when it is in the background. That is, when a protected-mode application has been "selected," so that the user can interact with it, the MS-DOS application is frozen. If the MS-DOS application has captured any interrupt vectors (such as the serial port or timer tick), these interrupts will not be serviced until the application is again selected and in the foreground. OS/2 must freeze MS-DOS applications when they are in the background because they execute in real mode and are thus not subject to hardware memory protection; nothing else ensures that they will not interfere with a protected-mode process that has control of the screen and keyboard. Use of FCBs is restricted in the 3.x Box, as it is under MS-DOS 3 or 4 with SHARE.EXE loaded. A file cannot be opened with an FCB if any other process is using it. The number of FCBs that can be simultaneously opened is limited to 16 or to the number specified in a CONFIG.SYS FCBS= directive. Even when the handle file functions are used, these functions may fail unexpectedly due to the activity of other processes (for example, if a protected-mode process has already opened the file with "deny all" sharing mode); most MS-DOS applications are not written with file sharing in mind, and they do not handle such errors gracefully. Direct writes to a fixed disk using Int 26H or Int 13H are not allowed. This prevents the file system from being corrupted, because protected-mode applications running concurrently with the MS-DOS application may also be writing to the same disk. Imagine the mess if a typical MS-DOS unerase utility were to alter the root directory and FAT at the same time that a protected-mode database program was updating its file and indexes! MS-DOS applications that attempt to reprogram the 8259 to move the interrupt vector table or that modify interrupt vectors already belonging to an OS/2 device driver are terminated by the operating system. MS-DOS applications can change the 8259's interrupt-mask register, disable and reenable interrupts at their discretion, and read or write any I/O port. The obvious corollary is that an MS-DOS program running in the 3.x Box can crash the entire OS/2 system at any time; this is the price for allowing real-mode applications to run at all. Porting MS-DOS Applications to OS/2 The application program interface (API) provided by OS/2 to protected-mode programs is quite different from the familiar Int 21H interface of MS-DOS and the OS/2 3.x Box. However, the OS/2 API is functionally a proper superset of MS-DOS. This makes it easy to convert well-behaved MS-DOS applications to run in OS/2 protected mode, whence they can be enhanced to take advantage of OS/2's virtual memory, multitasking, and interprocess communication capabilities. To give you a feeling for both the nature of the OS/2 API and the practices that should be avoided in MS-DOS programming if portability to OS/2 is desired, I will outline my own strategy for converting existing MS-DOS assembly-language programs to OS/2. For the purposes of discussion, I have divided the conversion process into five steps and have assigned each an easily remembered buzzword: 1. Segmentation 2. Rationalization 3. Encapsulation 4. Conversion 5. Optimization The first three stages can (and should) be performed and tested in the MS-DOS environment; only the last two require OS/2 and the protected-mode programming tools. As you read on, you may notice that an MS-DOS program that follows the compatibility guidelines presented earlier in this chapter requires relatively little work to make it run in protected mode. This is the natural benefit of working with the operating system instead of against it. Segmentation Most of the 80286's protected-mode capabilities revolve around a change in the way memory is addressed. In real mode, the 80286 essentially emulates an 8088/86 processor, and the value in a segment register corresponds directly to a physical memory address. MS-DOS runs on the 80286 in real mode. When an 80286 is running in protected mode, as it does under OS/2, an additional level of indirection is added to memory addressing. A segment register holds a selector, which is an index to a table of descriptors. A descriptor defines the physical address and length of a memory segment, its characteristics (executable, read-only data, or read/write data) and access rights, and whether the segment is currently resident in RAM or has been swapped out to disk. Each time a program loads a segment register or accesses memory, the 80286 hardware checks the associated descriptor and the program's privilege level, generating a fault if the selector or memory operation is not valid. The fault acts like a hardware interrupt, allowing the operating system to regain control and take the appropriate action. This scheme of memory addressing in protected mode has two immediate consequences for application programs. The first is that application programs can no longer perform arithmetic on the contents of segment registers (because selectors are magic numbers and have no direct relationship to physical memory addresses) or use segment registers for storage of temporary values. A program must not load a segment register with anything but a legitimate selector provided by the OS/2 loader or resulting from an OS/2 memory allocation function call. The second consequence is that a program must strictly segregate machine code ("text") from data, placing them in separate segments with distinct selectors (because a selector that is executable is not writable, and vice versa). Accordingly, the first step in converting a program for OS/2 is to turn it into a .EXE-type program that uses the Microsoft segment, class, and group conventions described in Chapter 3. At minimum, the program must have one code segment and one data segment, and should declare a group──with the special name DGROUP──that contains the "near" data segment, stack, and local heap (if any). At the same time, you should remove or rewrite any code that performs direct manipulation of segment values. After restructuring and segmentation, reassemble and link your program and check to be sure it still works as expected under MS-DOS. Changing or adding segmentation often uncovers hidden addressing assumptions in the code, so it is best to track these problems down before making other substantive changes to the program. Rationalization Once you've successfully segmented your program so that it can be linked and executed as a .EXE file under MS-DOS, the next step is to rationalize your code. By rationalization I mean converting your program into a completely well-behaved MS-DOS application. First, you must ruthlessly eliminate any elements that manipulate the peripheral device adapters directly, alter interrupt priorities, edit the system interrupt-vector table, or depend on CPU speed or characteristics (such as timing loops). In protected mode, control of the interrupt system is completely reserved to the operating system and its device drivers, I/O ports may be read or written by an application only under very specific conditions, and timing loops burn up CPU cycles that can be used by other processes. As I mentioned earlier in this chapter, display routines constitute the most common area of hardware dependence in an MS-DOS application. Direct manipulation of the video adapter and its regen buffer poses obvious difficulties in a multitasking, protected-memory environment such as OS/2. For porting purposes, you must convert all routines that write text to the display, modify character attributes, or affect cursor shape or position into Int 21H Function 40H calls using ANSI escape sequences or into ROM BIOS Int 10H calls. Similarly, you must convert all hardware-dependent keyboard operations to Int 21H Function 3FH or ROM BIOS Int 16H calls. Once all hardware dependence has been expunged from your program, your next priority is to make it well-behaved in its use of system memory. Under MS-DOS an application is typically handed all remaining memory in the system to do with as it will; under OS/2 the converse is true: A process is initially allocated only enough memory to hold its code, declared data storage, and stack. You can make the MS-DOS loader behave like the OS/2 loader by linking your application with the /CPARMAXALLOC switch. Alternatively, your program can give up all extra memory during its initialization with Int 21H Function 4AH, as recommended earlier in this chapter. After your program completes its initialization sequence, it should dynamically obtain and release any additional memory it may require for buffers and tables with MS-DOS Int 21H Functions 48H and 49H. To ensure compatibility with protected mode, limit the size of any single allocated block to 65,536 bytes or less, even though MS-DOS allows larger blocks to be allocated. Finally, you must turn your attention to file and device handling. Replace any calls to FCB file functions with their handle-based equivalents, because OS/2 does not support FCBs in protected mode at all. Check pathnames for validity within the application; although MS-DOS and the 3.x Box silently truncate a name or extension, OS/2 refuses to open or create a file in protected mode if the name or extension is too long and returns an error instead. Replace any use of the predefined handles for the standard auxiliary and standard list devices with explicit opens of COM1, PRN, LPT1, and so on, using the resulting handle for read and write operations. OS/2 does not supply processes with standard handles for the serial communications port or printer. Encapsulation When you reach this point, with a well-behaved, segmented MS-DOS application in hand, the worst of a port to OS/2 is behind you. You are now ready to prepare your program for true conversion to protected-mode operation by encapsulating, in individual subroutines, every part of the program that is specific to the host operating system. The objective here is to localize the program's "knowledge" of the environment into small procedures that can be subsequently modified without affecting the remainder of the program. As an example of encapsulation, consider a typical call by an MS-DOS application to write a string to the standard output device (Figure 16-1). In order to facilitate conversion to OS/2, you would replace every instance of such a write to a file or device with a call to a small subroutine that "hides" the mechanics of the actual operating-system function call, as illustrated in Figure 16-2. Another candidate for encapsulation, which does not necessarily involve an operating-system function call, is the application's code to gain access to command-line parameters, environment-block variables, and the name of the file it was loaded from. Under MS-DOS, this information is divided between the program segment prefix (PSP) and the environment block, as we saw in Chapters 3 and 12; under OS/2, there is no such thing as a PSP, and the program filename and command-line information are appended to the environment block. ────────────────────────────────────────────────────────────────────────── stdin equ 0 ; standard input handle stdout equ 1 ; standard output handle stderr equ 2 ; standard error handle msg db 'This is a sample message' msg_len equ $-msg . . . mov dx,seg msg ; DS:DX = message address mov ds,dx mov dx,offset DGROUP:msg mov cx,msg_len ; CX = message length mov bx,stdout ; BX = handle mov ah,40h ; AH = function 40h write int 21h ; transfer to MS-DOS jc error ; jump if error cmp ax,msg_len ; all characters written? jne diskfull ; no, device is full . . . ────────────────────────────────────────────────────────────────────────── Figure 16-1. Typical in-line code for an MS-DOS function call. This particular sequence writes a string to the standard output device. Since the standard output might be redirected to a file without the program's knowledge, it must also check that all of the requested characters were actually written; if the returned length is less than the requested length, this usually indicates that the standard output has been redirected to a disk file and that the disk is full. ────────────────────────────────────────────────────────────────────────── stdin equ 0 ; standard input handle stdout equ 1 ; standard output handle stderr equ 2 ; standard error handle msg db 'This is a sample message' msg_len equ $-msg . . . mov dx,seg msg ; DS:DX = message address mov ds,dx mov dx,offset DGROUP:msg mov cx,msg_len ; CX = message length mov bx,stdout ; BX = handle call write ; perform the write jc error ; jump if error cmp ax,msg_len ; all characters written? jne diskfull ; no, device is full . . . write proc near ; write to file or device ; Call with: ; BX = handle ; CX = length of data ; DS:DX = address of data ; returns: ; if successful, carry clear ; and AX = bytes written ; if error, carry set ; and AX = error code mov ah,40h ; function 40h = write int 21h ; transfer to MS-DOS ret ; return status in CY and AX write endp . . . ────────────────────────────────────────────────────────────────────────── Figure 16-2. Code from Figure 16-1 after "encapsulation." The portion of the code that is operating-system dependent has been isolated inside a subroutine that is called from other points within the application. When you have completed the encapsulation of system services and access to the PSP and environment, subject your program once more to thorough testing under MS-DOS. This is your last chance, while you are still working in a familiar milieu and have access to your favorite debugging tools, to detect any subtle errors you may have introduced during the three conversion steps discussed thus far. Conversion Next, you must rewrite each system-dependent procedure you created during the encapsulation stage to conform to the OS/2 protected-mode API. In contrast to MS-DOS functions, which are actuated through software interrupts and pass parameters in registers, OS/2 API functions are requested through a far call to a named entry point. Parameters are passed on the stack, along with the addresses of variables within the calling program's data segment that will receive any results returned by the function. The status of an operation is returned in register AX──zero if the function succeeded, an error code otherwise. All other registers are preserved. Although it is not my intention here to provide a detailed introduction to OS/2 programming, Figure 16-3 illustrates the final form of our previous example, after conversion for OS/2. Note especially the addition of the extrn statement, the wlen variable, and the simulation of the MS-DOS function status. This code may not be elegant, but it serves the purpose of limiting the necessary changes to a very small portion of the source file. Some OS/2 functions (such as DosOpen) require parameters that have no counterpart under MS-DOS; you can usually select reasonable values for these extra parameters that will make their existence temporarily invisible to the remainder of the application. ────────────────────────────────────────────────────────────────────────── stdin equ 0 ; standard input handle stdout equ 1 ; standard output handle stderr equ 2 ; standard error handle extrn DosWrite:far msg db 'This is a sample message' msg_len equ $-msg wlen dw ? ; receives actual number ; of bytes written . . . mov dx,seg msg ; DS:DX = message address mov ds,dx mov dx,offset DGROUP:msg mov cx,msg_len ; CX = message length mov bx,stdout ; BX = handle call write ; perform the write jc error ; jump if error cmp ax,msg_len ; all characters written? jne diskfull ; no, device is full . . . write proc near ; write to file or device ; call with: ; BX = handle ; CX = length of data ; DS:DX = address of data ; returns: ; if successful, carry clear ; and AX = bytes written ; if error, carry set ; and AX = error code push bx ; handle push ds ; address of data push dx push cx ; length of data push ds ; receives length written mov ax,offset DGROUP:wlen push ax call DosWrite ; transfer to OS/2 or ax,ax ; did write succeed? jnz write1 ; jump, write failed mov ax,wlen ; no error, OR cleared CY ret ; and AX := bytes written write1: stc ; write error, return CY set ret ; and AX = error number write endp . . . ────────────────────────────────────────────────────────────────────────── Figure 16-3. Code from Figure 16-2 after "conversion." The MS-DOS function call has been replaced with the equivalent OS/2 function call. Since the knowledge of the operating system has been hidden inside the subroutine by the previous encapsulation step, the surrounding program's requests for write operations should run unchanged. Note that the OS/2 function had to be declared as an external name with the "far" attribute, and that a variable named wlen was added to the data segment of the application to receive the actual number of bytes written. Figures 16-4, 16-5, and 16-6 list the OS/2 services that are equivalent to selected MS-DOS and ROM BIOS Int 21H, Int 10H, and Int 16H calls. MS-DOS functions related to FCBs and PSPs are not included in these tables because OS/2 does not support either of these structures. The MS-DOS terminate-and-stay-resident functions are also omitted. Because OS/2 is a true multitasking system, a process doesn't need to terminate in order to stay resident while another process is running. ╓┌─┌───────────────────┌────────────────────────────────┌────────────────────╖ MS-DOS Description OS/2 function ────────────────────────────────────────────────────────────────────────── Int 21H Function 0 Terminate process DosExit 1 Character input with echo KbdCharIn 2 Character output VioWrtTTY 3 Auxiliary input DosRead MS-DOS Description OS/2 function ────────────────────────────────────────────────────────────────────────── 3 Auxiliary input DosRead 4 Auxiliary output DosWrite 5 Printer output DosWrite 6 Direct console I/O KbdCharIn, VioWrtTTY 7 Unfiltered input without echo KbdCharIn 8 Character input without echo KbdCharIn 9 Display string VioWrtTTY 0AH (10) Buffered keyboard input KbdStringIn 0BH (11) Check input status KbdPeek 0CH (12) Reset buffer and input KbdFlushBuffer, KbdCharIn 0DH (13) Disk reset DosBufReset 0EH (14) Select disk DosSelectDisk 19H (25) Get current disk DosQCurDisk 1BH (27) Get default drive data DosQFSInfo 1CH (28) Get drive data DosQFSInfo 2AH (42) Get date DosGetDateTime 2BH (43) Set date DosSetDateTime MS-DOS Description OS/2 function ────────────────────────────────────────────────────────────────────────── 2BH (43) Set date DosSetDateTime 2CH (44) Get time DosGetDateTime 2DH (45) Set time DosSetDateTime 2EH (46) Set verify flag DosSetVerify 30H (48) Get MS-DOS version DosGetVersion 36H (54) Get drive allocation DosQFSInfo information 38H (56) Get or set country DosGetCtryInfo information 39H (57) Create directory DosMkdir 3AH (58) Delete directory DosRmdir 3BH (59) Set current directory DosChdir 3CH (60) Create file DosOpen 3DH (61) Open file DosOpen 3EH (62) Close file DosClose 3FH (63) Read file or device DosRead 40H (64) Write file or device DosWrite 41H (65) Delete file DosDelete 42H (66) Set file pointer DosChgFilePtr MS-DOS Description OS/2 function ────────────────────────────────────────────────────────────────────────── 42H (66) Set file pointer DosChgFilePtr 43H (67) Get or set file attributes DosQFileMode, DosSetFileMode 44H (68) I/O control (IOCTL) DosDevIOCtl 45H (69) Duplicate handle DosDupHandle 46H (70) Redirect handle DosDupHandle 47H (71) Get current directory DosQCurDir 48H (72) Allocate memory block DosAllocSeg 49H (73) Release memory block DosFreeSeg 4AH (74) Resize memory block DosReAllocSeg 4BH (75) Execute program DosExecPgm 4CH (76) Terminate process with DosExit return code 4DH (77) Get return code DosCWait 4EH (78) Find first file DosFindFirst 4FH (79) Find next file DosFindNext 54H (84) Get verify flag DosQVerify 56H (86) Rename file DosMove 57H (87) Get or set file date and time DosQFileInfo, MS-DOS Description OS/2 function ────────────────────────────────────────────────────────────────────────── 57H (87) Get or set file date and time DosQFileInfo, DosSetFileInfo 59H (89) Get extended error DosErrClass information 5BH (91) Create new file DosOpen 5CH (92) Lock or unlock file region DosFileLocks 65H (101) Get extended country DosGetCtryInfo information 66H (102) Get or set code page DosGetCp, DosSetCp 67H (103) Set handle count DosSetMaxFH 68H (104) Commit file DosBufReset 6CH (108) Extended open file DosOpen ────────────────────────────────────────────────────────────────────────── Figure 16-4. Table of selected MS-DOS function calls and their OS/2 counterparts. Note that OS/2 functions are typically more powerful and flexible than the corresponding MS-DOS functions, and that this is not a complete list of OS/2 services. ROM BIOS Description OS/2 function ────────────────────────────────────────────────────────────────────────── Int 10H Function 0 Select display mode VioSetMode 1 Set cursor type VioSetCurType 2 Set cursor position VioSetCurPos 3 Get cursor position VioGetCurPos 6 Initialize or scroll window up VioScrollUp 7 Initialize or scroll window down VioScrollDn 8 Read character and attribute VioReadCellStr 9 Write character and attribute VioWrtNCell 0AH (10) Write character VioWrtNChar 0EH (14) Write character in teletype mode VioWrtTTY 0FH (15) Get display mode VioGetMode 10H (16) Set palette, border color, etc. VioSetState 13H (19) Write string in teletype mode VioWrtTTY ────────────────────────────────────────────────────────────────────────── Figure 16-5. Table of ROM BIOS Int 10H video-display driver functions used by MS-DOS applications and their OS/2 equivalents. This is not a complete list of OS/2 video services. ROM BIOS Description OS/2 function ────────────────────────────────────────────────────────────────────────── Int 16H Function 0 Read keyboard character KbdCharIn 1 Get keyboard status KbdPeek 2 Get keyboard flags KbdGetStatus ────────────────────────────────────────────────────────────────────────── Figure 16-6. Table of ROM BIOS Int 16H keyboard driver functions used by MS-DOS applications and their OS/2 equivalents. This is not a complete list of OS/2 keyboard services. Optimization Once your program is running in protected mode, it is time to unravel some of the changes made for purposes of conversion and to introduce various optimizations. Three obvious categories should be considered: 1. Modifying the program's user-interface code for the more powerful OS/2 keyboard and display API functions. 2. Incorporating 80286-specific machine instructions where appropriate. 3. Revamping the application to exploit the OS/2 facilities that are unique to protected mode. (Of course, the application benefits from OS/2's virtual memory capabilities automatically; it can allocate memory until physical memory and disk swapping space are exhausted.) Modifying subroutines that encapsulate user input and output to take advantage of the additional functionality available under OS/2 is straight-forward, and the resulting performance improvements can be quite dramatic. For example, the OS/2 video driver offers a variety of services that are far superior to the screen support in MS-DOS and the ROM BIOS, including high-speed display of strings and attributes at any screen position, "reading back" selected areas of the display into a buffer, and scrolling in all four directions. The 80286-specific machine instructions can be very helpful in reducing code size and increasing execution speed. The most useful instructions are the shifts and rotates by an immediate count other than one, the three-operand multiply where one of the operands is an immediate (literal) value, and the push immediate value instruction (particularly handy for setting up OS/2 function calls). For example, in Figure 16-3, the sequence mov ax,offset DGROUP:wlen push ax could be replaced by the single instruction push offset DGROUP:wlen Restructuring an application to take full advantage of OS/2's protected-mode capabilities requires close study of both the application and the OS/2 API, but such study can pay off with sizable benefits in performance, ease of maintenance, and code sharing. Often, for instance, different parts of an application are concerned with I/O devices of vastly different speeds, such as the keyboard, disk, and video display. It both simplifies and enhances the application to separate these elements into subprocesses (called threads in OS/2) that execute asynchronously, communicate through shared data structures, and synchronize with each other, when necessary, using semaphores. As another example, when several applications are closely related and contain many identical or highly similar procedures, OS/2 allows you to centralize those procedures in a dynamic link library. Routines in a dynamic link library are bound to a program at its load time (rather than by LINK, as in the case of traditional runtime libraries) and are shared by all the processes that need them. This reduces the size of each application .EXE file and allows more efficient use of memory. Best of all, dynamic link libraries drastically simplify code maintenance; the routines in the libraries can be debugged or improved at any time, and the applications that use them will automatically benefit the next time they are executed. ──────────────────────────────────────────────────────────────────────────── SECTION 2 MS-DOS FUNCTIONS REFERENCE ──────────────────────────────────────────────────────────────────────────── Notes to the Reader This section documents the services that the MS-DOS kernel provides to application programs via software interrupts 20H─2FH. Each MS-DOS function is described in the same format: ■ A heading containing the function's name, software interrupt and function number, and an icon indicating the MS-DOS version in which the function was first supported. You can assume that the function is available in all subsequent MS-DOS versions unless explicitly noted otherwise. ■ A synopsis of the actions performed by the function and the circumstances under which it would be used. ■ A summary of the function's arguments. ■ The results and/or error indicators returned by the function. A comprehensive list of error codes can be found in the entry for Int 21H Function 59H. ■ Notes describing special uses or dependencies of the function. ■ A skeleton example of the function's use, written in assembly language. Version icons used in the synopsis, arguments, results, or Notes sections refer to specific minor or major versions, unless they include a + sign to indicate a version and all subsequent versions. For purposes of clarity, the examples may include instructions that would not be necessary if the code were inserted into a working program. For example, most of the examples explicitly set the segment registers when passing the address of a filename or buffer to MS-DOS; in real applications, the segment registers are usually initialized once at entry to the program and left alone thereafter. Int 21H Function Summary by Number ╓┌─┌───────┌────────┌───────────────────────────────────────┌───────┌────────╖ Hex Dec Function name Vers F/H Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 00H 0 Terminate Process 1.0+ 01H 1 Character Input with Echo 1.0+ 02H 2 Character Output 1.0+ 03H 3 Auxiliary Input 1.0+ 04H 4 Auxiliary Output 1.0+ 05H 5 Printer Output 1.0+ 06H 6 Direct Console I/O 1.0+ 07H 7 Unfiltered Character Input Without Echo 1.0+ 08H 8 Character Input Without Echo 1.0+ 09H 9 Display String 1.0+ 0AH 10 Buffered Keyboard Input 1.0+ 0BH 11 Check Input Status 1.0+ 0CH 12 Flush Input Buffer and Then Input 1.0+ 0DH 13 Disk Reset 1.0+ 0EH 14 Select Disk 1.0+ 0FH 15 Open File 1.0+ F 10H 16 Close File 1.0+ F 11H 17 Find First File 1.0+ F 12H 18 Find Next File 1.0+ F Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 12H 18 Find Next File 1.0+ F 13H 19 Delete File 1.0+ F 14H 20 Sequential Read 1.0+ F 15H 21 Sequential Write 1.0+ F 16H 22 Create File 1.0+ F 17H 23 Rename File 1.0+ F 18H 24 Reserved 19H 25 Get Current Disk 1.0+ 1AH 26 Set DTA Address 1.0+ 1BH 27 Get Default Drive Data 1.0+ 1CH 28 Get Drive Data 2.0+ 1DH 29 Reserved 1EH 30 Reserved 1FH 31 Reserved 20H 32 Reserved 21H 33 Random Read 1.0+ F 22H 34 Random Write 1.0+ F 23H 35 Get File Size 1.0+ F 24H 36 Set Relative Record Number 1.0+ F Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 24H 36 Set Relative Record Number 1.0+ F 25H 37 Set Interrupt Vector 1.0+ 26H 38 Create New PSP 1.0+ 27H 39 Random Block Read 1.0+ F 28H 40 Random Block Write 1.0+ F 29H 41 Parse Filename 1.0+ 2AH 42 Get Date 1.0+ 2BH 43 Set Date 1.0+ 2CH 44 Get Time 1.0+ 2DH 45 Set Time 1.0+ 2EH 46 Set Verify Flag 1.0+ 2FH 47 Get DTA Address 2.0+ 30H 48 Get MS-DOS Version Number 2.0+ 31H 49 Terminate and Stay Resident 2.0+ 32H 50 Reserved 33H 51 Get or Set Break Flag, Get Boot Drive 2.0+ 34H 52 Reserved 35H 53 Get Interrupt Vector 2.0+ 36H 54 Get Drive Allocation Information 2.0+ Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 36H 54 Get Drive Allocation Information 2.0+ 37H 55 Reserved 38H 56 Get or Set Country Information 2.0+ 39H 57 Create Directory 2.0+ 3AH 58 Delete Directory 2.0+ 3BH 59 Set Current Directory 2.0+ 3CH 60 Create File 2.0+ H 3DH 61 Open File 2.0+ H 3EH 62 Close File 2.0+ H 3FH 63 Read File or Device 2.0+ H 40H 64 Write File or Device 2.0+ H 41H 65 Delete File 2.0+ H 42H 66 Set File Pointer 2.0+ H 43H 67 Get or Set File Attributes 2.0+ 44H 68 IOCTL (I/O Control) 2.0+ 45H 69 Duplicate Handle 2.0+ 46H 70 Redirect Handle 2.0+ 47H 71 Get Current Directory 2.0+ 48H 72 Allocate Memory Block 2.0+ Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 48H 72 Allocate Memory Block 2.0+ 49H 73 Release Memory Block 2.0+ 4AH 74 Resize Memory Block 2.0+ 4BH 75 Execute Program (EXEC) 2.0+ 4CH 76 Terminate Process with Return Code 2.0+ 4DH 77 Get Return Code 2.0+ 4EH 78 Find First File 2.0+ H 4FH 79 Find Next File 2.0+ H 50H 80 Reserved 51H 81 Reserved 52H 82 Reserved 53H 83 Reserved 54H 84 Get Verify Flag 2.0+ 55H 85 Reserved 56H 86 Rename File 2.0+ 57H 87 Get or Set File Date and Time 2.0+ H 58H 88 Get or Set Allocation Strategy 3.0+ 59H 89 Get Extended Error Information 3.0+ 5AH 90 Create Temporary File 3.0+ H Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 5AH 90 Create Temporary File 3.0+ H 5BH 91 Create New File 3.0+ H 5CH 92 Lock or Unlock File Region 3.0+ H 5DH 93 Reserved 5EH 94 Get Machine Name, Get or Set Printer 3.1+ Setup 5FH 95 Device Redirection 3.1+ 60H 96 Reserved 61H 97 Reserved 62H 98 Get PSP Address 3.0+ 63H 99 Get DBCS Lead Byte Table 2.25 only 64H 100 Reserved 65H 101 Get Extended Country Information 3.3+ 66H 102 Get or Set Code Page 3.3+ 67H 103 Set Handle Count 3.3+ 68H 104 Commit File 3.3+ H 69H 105 Reserved 6AH 106 Reserved Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 6AH 106 Reserved 6BH 107 Reserved 6CH 108 Extended Open File 4.0+ H ────────────────────────────────────────────────────────────────────────── Int 21H Function Summary by Category ╓┌─┌───────┌────────┌───────────────────────────────────────┌───────┌────────╖ Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── Character I/O 01H 1 Character Input with Echo 1.0+ 02H 2 Character Output 1.0+ 03H 3 Auxiliary Input 1.0+ 04H 4 Auxiliary Output 1.0+ 05H 5 Printer Output 1.0+ 06H 6 Direct Console I/O 1.0+ Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 06H 6 Direct Console I/O 1.0+ 07H 7 Unfiltered Character Input Without Echo 1.0+ 08H 8 Character Input Without Echo 1.0+ 09H 9 Display String 1.0+ 0AH 10 Buffered Keyboard Input 1.0+ 0BH 11 Check Input Status 1.0+ 0CH 12 Flush Input Buffer and Then Input 1.0+ File Operations 0FH 15 Open File 1.0+ F 10H 16 Close File 1.0+ F 11H 17 Find First File 1.0+ F 12H 18 Find Next File 1.0+ F 13H 19 Delete File 1.0+ F 16H 22 Create File 1.0+ F 17H 23 Rename File 1.0+ F 23H 35 Get File Size 1.0+ F 29H 41 Parse Filename 1.0+ F 3CH 60 Create File 2.0+ H Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 3CH 60 Create File 2.0+ H 3DH 61 Open File 2.0+ H 3EH 62 Close File 2.0+ H 41H 65 Delete File 2.0+ H 43H 67 Get or Set File Attributes 2.0+ 45H 69 Duplicate Handle 2.0+ 46H 70 Redirect Handle 2.0+ 4EH 78 Find First File 2.0+ H 4FH 79 Find Next File 2.0+ H 56H 86 Rename File 2.0+ 57H 87 Get or Set File Date and Time 2.0+ H 5AH 90 Create Temporary File 3.0+ H 5BH 91 Create New File 3.0+ H 67H 103 Set Handle Count 3.3+ 68H 104 Commit File 3.3+ H 6CH 108 Extended Open File 4.0+ H Record Operations 14H 20 Sequential Read 1.0+ F Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 14H 20 Sequential Read 1.0+ F 15H 21 Sequential Write 1.0+ F 1AH 26 Set DTA Address 1.0+ 21H 33 Random Read 1.0+ F 22H 34 Random Write 1.0+ F 24H 36 Set Relative Record Number 1.0+ F 27H 39 Random Block Read 1.0+ F 28H 40 Random Block Write 1.0+ F 2FH 47 Get DTA Address 2.0+ 3FH 63 Read File or Device 2.0+ H 40H 64 Write File or Device 2.0+ H 42H 66 Set File Pointer 2.0+ H 5CH 92 Lock or Unlock File Region 3.0+ H Directory Operations 39H 57 Create Directory 2.0+ 3AH 58 Delete Directory 2.0+ 3BH 59 Set Current Directory 2.0+ 47H 71 Get Current Directory 2.0+ Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 47H 71 Get Current Directory 2.0+ Disk Management 0DH 13 Disk Reset 1.0+ 0EH 14 Select Disk 1.0+ 19H 25 Get Current Disk 1.0+ 1BH 27 Get Default Drive Data 1.0+ 1CH 28 Get Drive Data 2.0+ 2EH 46 Set Verify Flag 1.0+ 36H 54 Get Drive Allocation Information 2.0+ 54H 84 Get Verify Flag 2.0+ Process Management 00H 0 Terminate Process 1.0+ 26H 38 Create New PSP 1.0+ 31H 49 Terminate and Stay Resident 2.0+ 4BH 75 Execute Program (EXEC) 2.0+ 4CH 76 Terminate Process with Return Code 2.0+ 4DH 77 Get Return Code 2.0+ Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 4DH 77 Get Return Code 2.0+ 62H 98 Get PSP Address 3.0+ Memory Management 48H 72 Allocate Memory Block 2.0+ 49H 73 Release Memory Block 2.0+ 4AH 74 Resize Memory Block 2.0+ 58H 88 Get or Set Allocation Strategy 3.0+ Network Functions 5EH 94 Get Machine Name, Get or Set Printer 3.1+ Setup 5FH 95 Device Redirection 3.1+ Time and Date 2AH 42 Get Date 1.0+ 2BH 43 Set Date 1.0+ 2CH 44 Get Time 1.0+ 2DH 45 Set Time 1.0+ Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 2DH 45 Set Time 1.0+ Miscellaneous System Functions 25H 37 Set Interrupt Vector 1.0+ 30H 48 Get MS-DOS Version Number 2.0+ 33H 51 Get or Set Break Flag, Get Boot Drive 2.0+ 35H 53 Get Interrupt Vector 2.0+ 38H 56 Get or Set Country Information 2.0+ 44H 68 IOCTL (I/O Control) 2.0+ 59H 89 Get Extended Error Information 3.0+ 63H 99 Get Lead Byte Table 2.25 only 65H 101 Get Extended Country Information 3.3+ 66H 102 Get or Set Code Page 3.3+ Reserved Functions 18H 24 Reserved 1DH 29 Reserved 1EH 30 Reserved Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── 1EH 30 Reserved 1FH 31 Reserved 20H 32 Reserved 32H 50 Reserved 34H 52 Reserved 37H 55 Reserved 50H 80 Reserved 51H 81 Reserved 52H 82 Reserved 53H 83 Reserved 55H 85 Reserved 5DH 93 Reserved 60H 96 Reserved 61H 97 Reserved 64H 100 Reserved 69H 105 Reserved 6AH 106 Reserved 6BH 107 Reserved ────────────────────────────────────────────────────────────────────────── Hex Dec Function name Vers F/H ────────────────────────────────────────────────────────────────────────── ────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 20H [1.0] Terminate process ──────────────────────────────────────────────────────────────────────────── Terminates the current process. This is one of several methods that a program can use to perform a final exit. MS-DOS then takes the following actions: ■ All memory belonging to the process is released. ■ File buffers are flushed and any open handles for files or devices owned by the process are closed. ■ The termination handler vector (Int 22H) is restored from PSP:000AH. ■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH. ■ [2.0+] The critical-error handler vector (Int 24H) is restored from PSP:0012H. ■ Control is transferred to the termination handler. If the program is returning to COMMAND.COM, control transfers to the resident portion, and the transient portion is reloaded if necessary. If a batch file is in progress, the next line of the file is fetched and interpreted; otherwise, a prompt is issued for the next user command. Call with: CS = segment address of program segment prefix Returns: Nothing Notes: ■ Any files that have been written to using FCBs should be closed before performing this exit call; otherwise, data may be lost. ■ Other methods of performing a final exit are: ∙ Int 21H Function 00H ∙ Int 21H Function 31H ∙ Int 21H Function 4CH ∙ Int 27H ■ [2.0+] Int 21H Functions 31H and 4CH are the preferred methods for termination, since they allow a return code to be passed to the parent process. ■ [3.0+] If the program is running on a network, it should remove all locks it has placed on file regions before terminating. Example: Terminate the current program, returning control to the program's parent. . . . int 20h ; transfer to MS-DOS ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 00H Terminate process ──────────────────────────────────────────────────────────────────────────── Terminates the current process. This is one of several methods that a program can use to perform a final exit. MS-DOS then takes the following actions: ■ All memory belonging to the process is released. ■ File buffers are flushed and any open handles for files or devices owned by the process are closed. ■ The termination handler vector (Int 22H) is restored from PSP:000AH. ■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH. ■ [2.0+] The critical-error handler vector (Int 24H) is restored from PSP:0012H. ■ Control is transferred to the termination handler. If the program is returning to COMMAND.COM, control transfers to the resident portion, and the transient portion is reloaded if necessary. If a batch file is in progress, the next line of the file is fetched and interpreted; otherwise, a prompt is issued for the next user command. Call with: AH = 00H CS = segment address of program segment prefix Returns: Nothing Notes: ■ Any files that have been written to using FCBs should be closed before performing this exit call; otherwise, data may be lost. ■ Other methods of performing a final exit are: ∙ Int 20H ∙ Int 21H Function 31H ∙ Int 21H Function 4CH<21H4CH> ∙ Int 27H ■ [2.0+] Int 21H Functions 31H and 4CH are the preferred methods for termination, since they allow a return code to be passed to the parent process. ■ [3.0+] If the program is running on a network, it should remove all locks it has placed on file regions before terminating. Example: Terminate the current program, returning control to the program's parent. . . . mov ah,0 ; function number int 21h ; transfer to MS-DOS ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 01H Character input with echo ──────────────────────────────────────────────────────────────────────────── [1] Inputs a character from the keyboard, then echoes it to the display. If no character is ready, waits until one is available. [2.0+] Reads a character from the standard input device and echoes it to the standard output device. If no character is ready, waits until one is available. Input can be redirected. (If input has been redirected, there is no way to detect EOF.) Call with: AH = 01H Returns: AL = 8-bit input data Notes: ■ If the standard input is not redirected, and the character read is a Ctrl-C, an Int 23H is executed. If the standard input is redirected, a Ctrl-C is detected at the console, and BREAK is ON, an Int 23H is executed. ■ To read extended ASCII codes (such as the special function keys F1 to F10) on the IBM PC and compatibles, you must call this function twice. The first call returns the value 00H to signal the presence of an extended code. ■ See also Int 21H Functions 06H, 07H, and 08H, which provide character input with various combinations of echo and/or Ctrl-C sensing. ■ [2.0+] You can also read the keyboard by issuing a read (Int 21H Function 3FH) using the predefined handle for the standard input (0000H), if input has not been redirected, or a handle obtained by opening the logical device CON. Example: Read one character from the keyboard into register AL, echo it to the display, and store it in the variable char. char db 0 ; input character . . . mov ah,1 ; function number int 21h ; transfer to MS-DOS mov char,al ; save character . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 02H Character output ──────────────────────────────────────────────────────────────────────────── [1] Outputs a character to the currently active video display. [2.0+] Outputs a character to the standard output device. Output can be redirected. (If output is redirected, there is no way to detect disk full.) Call with: AH = 02H DL = 8-bit data for output Returns: Nothing Notes: ■ If a Ctrl-C is detected at the keyboard after the requested character is output, an Int 23H is executed. ■ If the standard output has not been redirected, a backspace code (08H) causes the cursor to move left one position. If output has been redirected, the backspace code does not receive any special treatment. ■ [2.0+] You can also send strings to the display by performing a write (Int 21H Function 40H) using the predefined handle for the standard output (0001H), if output has not been redirected, or a handle obtained by opening the logical device CON. Example: Send the character "*" to the standard output device. . . . mov ah,2 ; function number mov dl,'*' ; character to output int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 03H Auxiliary input ──────────────────────────────────────────────────────────────────────────── [1] Reads a character from the first serial port. [2.0+] Reads a character from the standard auxiliary device. The default is the first serial port (COM1). Call with: AH = 03H Returns: AL = 8-bit input data Notes: ■ In most MS-DOS systems, the serial device is unbuffered and is not interrupt-driven. If the auxiliary device sends data faster than your program can process it, characters may be lost. ■ At startup on the IBM PC, PC-DOS initializes the first serial port to 2400 baud, no parity, 1 stop bit, and 8 data bits. Other implementations of MS-DOS may initialize the serial device differently. ■ There is no way for a user program to read the status of the auxiliary device or to detect I/O errors (such as lost characters) through this function call. On the IBM PC, more precise control can be obtained by calling ROM BIOS Int 14H or by driving the communications controller directly. ■ If a Ctrl-C is detected at the keyboard, an Int 23H is executed. ■ [2.0+] You can also input from the auxiliary device by requesting a read (Int 21H Function 3FH) using the predefined handle for the standard auxiliary device (0003H) or using a handle obtained by opening the logical device AUX. Example: Read a character from the standard auxiliary input and store it in the variable char. char db 0 ; input character . . . mov ah,3 ; function number int 21h ; transfer to MS-DOS mov char,al ; save character . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 04H Auxiliary output ──────────────────────────────────────────────────────────────────────────── [1] Outputs a character to the first serial port. [2.0+] Outputs a character to the standard auxiliary device. The default is the first serial port (COM1). Call with: AH = 04H DL = 8-bit data for output Returns: Nothing Notes: ■ If the output device is busy, this function waits until the device is ready to accept a character. ■ There is no way to poll the status of the auxiliary device using this function. On the IBM PC, more precise control can be obtained by calling ROM BIOS Int 14H or by driving the communications controller directly. ■ If a Ctrl-C is detected at the keyboard, an Int 23H is executed. ■ [2.0+] You can also send strings to the auxiliary device by performing a write (Int 21H Function 40H) using the predefined handle for the standard auxiliary device (0003H) or using a handle obtained by opening the logical device AUX. Example: Output a "*'' character to the auxiliary device. . . . mov ah,4 ; function number mov dl,'*' ; character to output int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 05H Printer output ──────────────────────────────────────────────────────────────────────────── [1] Sends a character to the first list device (PRN or LPT1). [2.0+] Sends a character to the standard list device. The default device is the printer on the first parallel port (LPT1), unless explicitly redirected by the user with the MODE command. Call with: AH = 05H DL = 8-bit data for output Returns: Nothing Notes: ■ If the printer is busy, this function waits until the printer is ready to accept the character. ■ There is no standardized way to poll the status of the printer under MS-DOS. ■ If a Ctrl-C is detected at the keyboard, an Int 23H is executed. ■ [2.0+] You can also send strings to the printer by performing a write (Int 21H Function 40H) using the predefined handle for the standard printer device (0004H) or using a handle obtained by opening the logical device PRN or LPT1. Example: Output the character "*'' to the list device. . . . mov ah,5 ; function number mov dl,'*' ; character to output int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 06H Direct console I/O ──────────────────────────────────────────────────────────────────────────── Used by programs that need to read and write all possible characters and control codes without any interference from the operating system. [1] Reads a character from the keyboard or writes a character to the display. [2.0+] Reads a character from the standard input device or writes a character to the standard output device. I/O may be redirected. (If I/O has been redirected, there is no way to detect EOF or disk full.) Call with: AH = 06H DL = function requested 00H─FEH if output request 0FFH if input request Returns: If called with DL = 00H─0FEH Nothing If called with DL = FFH and a character is ready Zero flag = clear AL = 8-bit input data If called with DL = FFH and no character is ready Zero flag = set Notes: ■ No special action is taken upon entry of a Ctrl-C when this service is used. ■ To read extended ASCII codes (such as the special function keys F1 to F10) on the IBM PC and compatibles, you must call this function twice. The first call returns the value 00H to signal the presence of an extended code. ■ See also Int 21H Functions 01H, 07H, and 08H, which provide character input with various combinations of echo and/or Ctrl-C sensing, and Functions 02H and 09H, which may be used to write characters to the standard output. ■ [2.0+] You can also read the keyboard by issuing a read (Int 21H Function 3FH) using the predefined handle for the standard input (0000H), if input has not been redirected, or a handle obtained by opening the logical device CON. ■ [2.0+] You can also send characters to the display by issuing a write (Int 21H Function 40H) using the predefined handle for the standard output (0001H), if output has not been redirected, or a handle obtained by opening the logical device CON. Examples: Send the character "*" to the standard output device. . . . mov ah,6 ; function number mov dl,'*' ; character to output int 21h ; transfer to MS-DOS . . . Read a character from the standard input device and save it in the variable char. If no character is ready, wait until one is available. char db 0 ; input character . . . wait: mov ah,6 ; function number mov dl,0ffh ; parameter for read int 21h ; transfer to MS-DOS jz wait ; wait until char ready mov char,al ; save the character . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 07H Unfiltered character input without echo ──────────────────────────────────────────────────────────────────────────── [1] Reads a character from the keyboard without echoing it to the display. If no character is ready, waits until one is available. [2.0+] Reads a character from the standard input device without echoing it to the standard output device. If no character is ready, waits until one is available. Input may be redirected. (If input has been redirected, there is no way to detect EOF.) Call with: AH = 07H Returns: AL = 8-bit input data Notes: ■ No special action is taken upon entry of a Ctrl-C when this function is used. If Ctrl-C checking is required, use Int 21H Function 08H instead. ■ To read extended ASCII codes (such as the special function keys F1 to F10) on the IBM PC and compatibles, you must call this function twice. The first call returns the value 00H to signal the presence of an extended code. ■ See also Int 21H Functions 01H, 06H, and 08H, which provide character input with various combinations of echo and/or Ctrl-C sensing. ■ [2.0+] You can also read the keyboard by issuing a read (Int 21H Function 3FH) using the predefined handle for the standard input (0000H), if input has not been redirected, or a handle obtained by opening the logical device CON. Example: Read a character from the standard input without echoing it to the display, and store it in the variable char. char db 0 ; input character . . . mov ah,7 ; function number int 21h ; transfer to MS-DOS mov char,al ; save character . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 08H Character input without echo ──────────────────────────────────────────────────────────────────────────── [1] Reads a character from the keyboard without echoing it to the display. If no character is ready, waits until one is available. [2.0+] Reads a character from the standard input device without echoing it to the standard output device. If no character is ready, waits until one is available. Input may be redirected. (If input has been redirected, there is no way to detect EOF.) Call with: AH = 08H Returns: AL = 8-bit input data Notes: ■ If the standard input is not redirected, and the character read is a Ctrl-C, an Int 23H is executed. If the standard input is redirected, a Ctrl-C is detected at the console, and BREAK is ON, an Int 23H is executed. To avoid possible interruption by a Ctrl-C, use Int 21H Function 07H instead. ■ To read extended ASCII codes (such as the special function keys F1 to F10) on the IBM PC and compatibles, you must call this function twice. The first call returns the value 00H to signal the presence of an extended code. ■ See also Int 21H Functions 01H, 06H, and 07H, which provide character input with various combinations of echo and/or Ctrl-C sensing. ■ [2.0+] You can also read the keyboard by issuing a read (Int 21H Function 3FH) using the predefined handle for the standard input (0000H), if input has not been redirected, or a handle obtained by opening the logical device CON. Example: Read a character from the standard input without echoing it to the display, allowing possible detection of Ctrl-C, and store the character in the variable char. char db 0 . . . mov ah,8 ; function number int 21h ; transfer to MS-DOS mov char,al ; save character . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 09H Display string ──────────────────────────────────────────────────────────────────────────── [1] Sends a string of characters to the display. [2.0+] Sends a string of characters to the standard output device. Output may be redirected. (If output has been redirected, there is no way to detect disk full.) Call with: AH = 09H DS:DX = segment:offset of string Returns: Nothing Notes: ■ The string must be terminated with the character $ (24H), which is not transmitted. Any other ASCII codes, including control codes, can be embedded in the string. ■ See Int 21H Functions 02H and 06H for single-character output to the video display or standard output device. ■ If a Ctrl-C is detected at the keyboard, an Int 23H is executed. ■ [2.0+] You can also send strings to the display by performing a write (Int 21H Function 40H) using the predefined handle for the standard output (0001H), if it has not been redirected, or a handle obtained by opening the logical device CON. Example: Send the string Hello World, followed by a carriage return and line feed, to the standard output device. cr equ 0dh lf equ 0ah msg db 'Hello World',cr,lf,'$' . . . mov ah,9 ; function number mov dx,seg msg ; address of string mov ds,dx mov dx,offset msg int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 0AH (10) Buffered keyboard input ──────────────────────────────────────────────────────────────────────────── [1] Reads a line from the keyboard and places it in a user-designated buffer. The characters are echoed to the display. [2.0+] Reads a string of bytes from the standard input device, up to and including an ASCII carriage return (0DH), and places them in a user-designated buffer. The characters are echoed to the standard output device. Input may be redirected. (If input has been redirected, there is no way to detect EOF.) Call with: AH = 0AH DS:DX = segment:offset of buffer Returns: Nothing (data placed in buffer) Notes: ■ The buffer used by this function has the following format: Byte Contents ──────────────────────────────────────────────────────────────────────── 0 maximum number of characters to read, set by program 1 number of characters actually read (excluding carriage return), set by MS-DOS 2+ string read from keyboard or standard input, terminated by a carriage return (0DH) ──────────────────────────────────────────────────────────────────────── ■ If the buffer fills to one fewer than the maximum number of characters it can hold, subsequent input is ignored and the bell is sounded until a carriage return is detected. ■ This input function is buffered with type-ahead capability, and all of the standard keyboard editing commands are active. ■ If the standard input is not redirected, and a Ctrl-C is detected at the console, an Int 23H is executed. If the standard input is redirected, a Ctrl-C is detected at the console, and BREAK is ON, an Int 23H is executed. ■ See Int 21H Functions 01H, 06H, 07H, and 08H for single-character input from the keyboard or standard input device. ■ [2.0+] You can also read strings from the keyboard by performing a read (Int 21H Function 3FH) using the predefined handle for the standard input (0000H), if it has not been redirected, or a handle obtained by opening the logical device CON. Example: Read a string that is a maximum of 80 characters long from the standard input device, placing it in the buffer named buff. buff db 81 ; maximum length of input db 0 ; actual length of input db 81 dup (0) ; actual input placed here . . . mov ah,0ah ; function number mov dx,seg buff ; input buffer address mov ds,dx mov dx,offset buff int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 0BH (11) Check input status ──────────────────────────────────────────────────────────────────────────── [1] Checks whether a character is available from the keyboard. [2.0+] Checks whether a character is available from the standard input device. Input can be redirected. Call with: AH = 0BH Returns: AL = 00H if no character is available FFH if at least one character is available Notes: ■ [1] If a Ctrl-C is detected, an Int 23H is executed. ■ [2.0+] If the standard input is not redirected, and a Ctrl-C is detected at the console, an Int 23H is executed. If the standard input is redirected, a Ctrl-C is detected at the console, and BREAK is ON, an Int 23H is executed. ■ If a character is waiting, this function will continue to return a true flag until the character is consumed with a call to Int 21H Function 01H, 06H, 07H, 08H, 0AH, or 3FH. ■ This function is equivalent to IOCTL Int 21H Function 44H Subfunction 06H. Example: Test whether a character is available from the standard input. . . . mov ah,0bh ; function number int 21h ; transfer to MS-DOS or al,al ; character waiting? jnz ready ; jump if char ready . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 0CH (12) Flush input buffer and then input ──────────────────────────────────────────────────────────────────────────── [1] Clears the type-ahead buffer and then invokes one of the keyboard input functions. [2.0+] Clears the standard input buffer and then invokes one of the character input functions. Input can be redirected. Call with: AH = 0CH AL = number of input function to be invoked after resetting buffer (must be 01H, 06H, 07H, 08H, or 0AH) (if AL = 0AH) DS:DX = segment:offset of input buffer Returns: (if called with AL = 01H, 06H, 07H, or 08H) AL = 8-bit input data (if called with AL = 0AH) Nothing (data placed in buffer) Notes: ■ The function exists to allow a program to defeat MS-DOS's type-ahead feature. It discards any characters that are waiting in MS-DOS's internal type-ahead buffer, forcing the specified input function to wait for a character (usually a keyboard entry) that is truly entered after the program's request. ■ The presence or absence of Ctrl-C checking during execution of this function depends on the function number placed in register AL. ■ A function number in AL other than 01H, 06H, 07H, 08H, or 0AH simply flushes the input buffer and returns control to the calling program. Example: Clear the type-ahead buffer, then wait for a character to be entered, echoing it and then returning it in AL. Store the character in the variable char. char db 0 . . . mov ah,0ch ; function number mov al,1 ; subfunction = input char int 21h ; transfer to MS-DOS mov char,al ; save character . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 0DH (13) Disk reset ──────────────────────────────────────────────────────────────────────────── Flushes all file buffers. All data that has been logically written by user programs, but has been temporarily buffered within MS-DOS, is physically written to the disk. Call with: AH = 0DH Returns: Nothing Notes: ■ This function does not update the disk directory for any files that are still open. If your program fails to properly close all files before the disk is removed, and files have changed size, the data forced out to the disk by this function may still be inaccessible because the directory entries will not be correct. ■ [3.3+] Int 21H Function 68H (Commit File) should be used in preference to this function, since it also updates the disk directory. Example: Flush all MS-DOS internal disk buffers. . . . mov ah,0dh ; function number int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 0EH (14) Select disk ──────────────────────────────────────────────────────────────────────────── Selects the specified drive to be the current, or default, disk drive and returns the total number of logical drives in the system. Call with: AH = 0EH DL = drive code (0 = A, 1 = B, etc.) Returns: AL = number of logical drives in system Notes: ■ [1] 16 drive designators (0 through 0FH) are available. ■ [2] 63 drive designators (0 through 3FH) are available. ■ [3.0+] 26 drive designators (0 through 19H) are available. ■ To preserve upward compatibility, new applications should limit themselves to the drive letters A─Z (0 = A, 1 = B, etc.). ■ Logical drives means the total number of block devices: floppy disks, simulated disk drives (RAMdisks), and hard-disk drives. A single physical hard-disk drive is frequently partitioned into two or more logical drives. ■ [1] [2] In single-drive IBM PC─compatible systems, the value 2 is returned in AL, because PC-DOS supports two logical drives (A: and B:) on the single physical floppy-disk drive. The actual number of physical drives in the system can be determined with ROM BIOS Int 11H. ■ [3.0+] The value returned in AL is either 5 or the drive code corresponding to the LASTDRIVE entry (if any) in CONFIG.SYS, whichever is greater. Example: Make drive B the current (default) disk drive. Save the total number of logical drives in the system in the variable drives. drives db 0 . . . mov ah,0eh ; function number mov dl,1 ; drive 1 = B int 21h ; transfer to MS-DOS mov drives,al ; save total drives . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 0FH (15) Open file ──────────────────────────────────────────────────────────────────────────── Opens a file and makes it available for subsequent read/write operations. Call with: AH = 0FH DS:DX = segment:offset of file control block Returns: If function successful (file found) AL = 00H and FCB filled in by MS-DOS as follows: drive field (offset 00H) = 1 for drive A, 2 for drive B, etc. current block field (offset 0CH) = 00H record size field (offset 0EH) = 0080H [2.0+] size field (offset 10H) = file size from directory [2.0+] date field (offset 14H) = date stamp from directory [2.0+] time field (offset 16H) = time stamp from directory If function unsuccessful (file not found) AL = 0FFH Notes: ■ If your program is going to use a record size other than 128 bytes, it should set the record-size field at FCB offset 0EH after the file is successfully opened and before any other disk operation. ■ If random access is to be performed, the calling program must also set the FCB relative-record field (offset 21H) after successfully opening the file. ■ For format of directory time and date, see Int 21H Function 57H. ■ [2.0+] Int 21H Function 3DH, which allows full access to the hierarchical directory structure, should be used in preference to this function. ■ [3.0+] If the program is running on a network, the file is opened for read/write access in compatibility sharing mode. Example: Attempt to open the file named QUACK.DAT on the default disk drive. myfcb db 0 ; drive = default db 'QUACK ' ; filename, 8 characters db 'DAT' ; extension, 3 characters db 25 dup (0) ; remainder of FCB . . . mov ah,0fh ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if open failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 10H (16) Close file ──────────────────────────────────────────────────────────────────────────── Closes a file, flushes all MS-DOS internal disk buffers associated with the file to disk, and updates the disk directory if the file has been modified or extended. Call with: AH = 10H DS:DX = segment:offset of file control block Returns: If function successful (directory update successful) AL = 00H If function unsuccessful (file not found in directory) AL = FFH Notes: ■ [1] [2] MS-DOS versions 1 and 2 do not reliably detect a floppy-disk change, and an error can occur if the user changes disks while a file is still open on that drive. In the worst case, the directory and file allocation table of the newly inserted disk can be damaged or destroyed. ■ [2.0+] Int 21H Function 3EH should be used in preference to this function. Example: Close the file that was previously opened using the file control block named myfcb. myfcb db 0 ; drive = default db 'QUACK ' ; filename, 8 characters db 'DAT' ; extension, 3 characters db 25 dup (0) ; remainder of FCB . . . mov ah,10h ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if close failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 11H (17) Find first file ──────────────────────────────────────────────────────────────────────────── Searches the current directory on the designated drive for a matching filename. Call with: AH = 11H DS:DX = segment:offset of file control block Returns: If function successful (matching filename found) AL = 00H and buffer at current disk transfer area (DTA) address filled in as an unopened normal FCB or extended FCB, depending on which type of FCB was input to function If function unsuccessful (no matching filename found) AL = FFH Notes: ■ Use Int 21H Function 1AH to set the DTA to point to a buffer of adequate size before calling this function. ■ The wildcard character ? is allowed in the filename in all versions of MS-DOS. In versions 3.0 and later, the wildcard character * may also be used in a filename. If ? or * is used, this function returns the first matching filename. ■ An extended FCB must be used to search for files that have the system, hidden, read-only, directory, or volume-label attributes. ■ If an extended FCB is used, its attribute byte determines the type of search that will be performed. If the attribute byte contains 00H, only ordinary files are found. If the volume-label attribute bit is set, only volume labels will be returned (if any are present). If any other attribute or combination of attributes is set (such as hidden, system, or read-only), those files and all ordinary files will be matched. ■ [2.0+] Int 21H Function 4EH, which allows full access to the hierarchical directory structure, should be used in preference to this function. Example: Search for the first file with the extension .COM in the current directory. buff db 37 dup (0) ; receives search result myfcb db 0 ; drive = default db '????????' ; wildcard filename db 'COM' ; extension = COM db 25 dup (0) ; remainder of FCB . . . ; set DTA address mov ah,1ah ; function number mov dx,seg buff ; buffer address mov ds,dx mov dx,offset buff int 21h ; transfer to MS-DOS ; search for first match mov ah,11h ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if no match . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 12H (18) Find next file ──────────────────────────────────────────────────────────────────────────── Given that a previous call to Int 21H Function 11H has been successful, returns the next matching filename (if any). Call with: AH = 12H DS:DX = segment:offset of file control block Returns: If function successful (matching filename found) AL = 00H and buffer at current disk transfer area (DTA) address set up as an unopened normal FCB or extended FCB, depending on which type of FCB was originally input to Int 21H Function 11H If function unsuccessful (no more matching filenames found) AL = FFH Notes: ■ This function assumes that the FCB used as input has been properly initialized by a previous call to Int 21H Function 11H (and possible subsequent calls to Int 21H Function 12H) and that the filename or extension being searched for contained at least one wildcard character. ■ As with Int 21H Function 11H, it is important to use Int 21H Function 1AH to set the DTA to a buffer of adequate size before calling this function. ■ [2.0+] Int 21H Functions 4EH and 4FH, which allow full access to the hierarchical directory structure, should be used in preference to this function. Example: Assuming a previous successful call to function 11H, search for the next file with the extension .COM in the current directory. If the DTA has not been changed since the previous search, another call to Function 1AH is not necessary. buff db 37 dup (0) ; receives search result my_fcb db 0 ; drive = default db '????????' ; wildcard filename db 'COM' ; extension = COM db 25 dup (0) ; remainder of FCB . . . ; set DTA address mov ah,1ah ; function number mov dx,seg buff ; buffer address mov ds,dx mov dx,offset buff int 21h ; transfer to MS-DOS ; search for next match mov ah,12h ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if no match . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 13H (19) Delete file ──────────────────────────────────────────────────────────────────────────── Deletes all matching files from the current directory on the default or specified disk drive. Call with: AH = 13H DS:DX = segment:offset of file control block Returns: If function successful (file or files deleted) AL = 00H If function unsuccessful (no matching files were found, or at least one matching file was read-only) AL = FFH Notes: ■ The wildcard character ? is allowed in the filename; if ? is present and there is more than one matching filename, all matching files will be deleted. ■ [2.0+] Int 21H Function 41H, which allows full access to the hierarchical directory structure, should be used in preference to this function. ■ [3.0+] If the program is running on a network, the user must have Create rights to the directory containing the file to be deleted. Example: Delete the file MYFILE.DAT from the current disk drive and directory. myfcb db 0 ; drive = default db 'MYFILE ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB . . . mov ah,13h ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump, delete failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 14H (20) Sequential read ──────────────────────────────────────────────────────────────────────────── Reads the next sequential block of data from a file, then increments the file pointer appropriately. Call with: AH = 14H DS:DX = segment:offset of previously opened file control block Returns: AL = 00H if read successful 01H if end of file 02H if segment wrap 03H if partial record read at end of file Notes: ■ The record is read into memory at the current disk transfer area (DTA) address, specified by the most recent call to Int 21H Function 1AH. If the size of the record and the location of the buffer are such that a segment overflow or wraparound would occur, the function fails with a return code of 02H. ■ The number of bytes of data to be read is specified by the record-size field (offset 0EH) of the file control block (FCB). ■ The file location of the data that will be read is specified by the combination of the current block field (offset 0CH) and current record field (offset 20H) of the file control block (FCB). These fields are also automatically incremented by this function. ■ If a partial record is read at the end of file, it is padded to the requested record length with zeros. ■ [3.0+] If the program is running on a network, the user must have Read access rights to the directory containing the file to be read. Example: Read 1024 bytes of data from the file specified by the previously opened file control block myfcb. myfcb db 0 ; drive = default db 'QUACK ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB . . . mov ah,14h ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb ; set record size mov word ptr myfcb+0eH,1024 int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if read failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 15H (21) Sequential write ──────────────────────────────────────────────────────────────────────────── Writes the next sequential block of data into a file, then increments the file pointer appropriately. Call with: AH = 15H DS:DX = segment:offset of previously opened file control block Returns: AL = 00H if write successful 01H if disk is full 02H if segment wrap Notes: ■ The record is written (logically, not necessarily physically) to the disk from memory at the current disk transfer area (DTA) address, specified by the most recent call to Int 21H Function 1AH. If the size of the record and the location of the buffer are such that a segment overflow or wraparound would occur, the function fails with a return code of 02H. ■ The number of bytes of data to be written is specified by the record-size field (offset 0EH) of the file control block (FCB). ■ The file location of the data that will be written is specified by the combination of the current block field (offset 0CH) and current record field (offset 20H) of the file control block (FCB). These fields are also automatically incremented by this function. ■ [3.0+] If the program is running on a network, the user must have Write access rights to the directory containing the file to be written. Example: Write 1024 bytes of data to the file specified by the previously opened file control block myfcb. myfcb db 0 ; drive = default db 'QUACK ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB . . . mov ah,15h ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb ; set record size mov word ptr myfcb+0eh,1024 int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if write failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 16H (22) Create file ──────────────────────────────────────────────────────────────────────────── Creates a new directory entry in the current directory or truncates any existing file with the same name to zero length. Opens the file for subsequent read/write operations. Call with: AH = 16H DS:DX = segment:offset of unopened file control block Returns: If function successful (file was created or truncated) AL = 00H and FCB filled in by MS-DOS as follows: drive field (offset 00H) = 1 for drive A, 2 for drive B, etc. current block field (offset 0CH) = 00H record size field (offset 0EH) = 0080H [2.0+] size field (offset 10H) = file size from directory [2.0+] date field (offset 14H) = date stamp from directory [2.0+] time field (offset 16H) = time stamp from directory If function unsuccessful (directory full) AL = FFH Notes: ■ Since an existing file with the specified name is truncated to zero length (i.e., all data in that file is irretrievably lost), this function must be used with caution. ■ If this function is called with an extended file control block (FCB), the new file may be assigned a special attribute, such as hidden or system, during its creation by setting the appropriate bit in the extended FCB's attribute byte. ■ Since this function also opens the file, a subsequent call to Int 21H Function 0FH is not required. ■ For format of directory time and date, see Int 21H Function 57H. ■ [2.0+] Int 21H Functions 3CH, 5AH, 5BH, and 6CH, which provide full access to the hierarchical directory structure, should be used in preference to this function. ■ [3.0+] If the program is running on a network, the user must have Create rights to the directory that will contain the new file. Example: Create a file in the current directory using the name in the file control block myfcb. myfcb db 0 ; drive = default db 'QUACK ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB . . . mov ah,16h ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if create failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 17H (23) Rename file ──────────────────────────────────────────────────────────────────────────── Alters the name of all matching files in the current directory on the disk in the specified drive. Call with: AH = 17H DS:DX = segment:offset of "special" file control block Returns: If function successful (one or more files renamed) AL = 00H If function unsuccessful (no matching files, or new filename matched an existing file) AL = FFH Notes: ■ The special file control block has a drive code, filename, and extension in the usual position (bytes 0 through 0BH) and a second filename starting 6 bytes after the first (offset 11H). ■ The ? wildcard character can be used in the first filename. Every file matching the first file specification will be renamed to match the second file specification. ■ If the second file specification contains any ? wildcard characters, the corresponding letters in the first filename are left unchanged. ■ The function terminates if the new name to be assigned to a file matches that of an existing file. ■ [2.0+] An extended FCB can be used with this function to rename a directory. ■ [2.0+] Int 21H Function 56H, which allows full access to the hierarchical directory structure, should be used in preference to this function. Example: Rename the file OLDNAME.DAT to NEWNAME.DAT. myfcb db 0 ; drive = default db 'OLDNAME ' ; old file name, 8 chars db 'DAT' ; old extension, 3 chars db 6 dup (0) ; reserved area db 'NEWNAME ' ; new file name, 8 chars db 'DAT' ; new extension, 3 chars db 14 dup (0) ; reserved area . . . mov ah,17h ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if rename failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 18H (24) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 19H (25) Get current disk ──────────────────────────────────────────────────────────────────────────── Returns the drive code of the current, or default, disk drive. Call with: AH = 19H Returns: AL = drive code (0 = A, 1 = B, etc.) Notes: ■ To set the default drive, use Int 21H Function 0EH. ■ Some other Int 21H functions use drive codes beginning at 1 (that is, 1 = A, 2 = B, etc.) and reserve drive code zero for the default drive. Example: Get the current disk drive and save the code in the variable cdrive. cdrive db 0 ; current drive code . . . mov ah,19h ; function number int 21h ; transfer to MS-DOS mov cdrive,al ; save drive code . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 1AH (26) Set DTA address ──────────────────────────────────────────────────────────────────────────── Specifies the address of the disk transfer area (DTA) to be used for subsequent FCB-related function calls. Call with: AH = 1AH DS:DX = segment:offset of disk transfer area Returns: Nothing Notes: ■ If this function is never called by the program, the DTA defaults to a 128-byte buffer at offset 0080H in the program segment prefix. ■ In general, it is the programmer's responsibility to ensure that the buffer area specified is large enough for any disk operation that will use it. The only exception to this is that MS-DOS will detect and abort disk transfers that would cause a segment wrap. ■ Int 21H Function 2FH can be used to determine the current disk transfer address. ■ The only handle-type operations that rely on the DTA address are the directory search functions, Int 21H Functions 4EH and 4FH. Example: Set the current disk transfer area address to the buffer labeled buff. buff db 128 dup (?) . . . mov ah,1ah ; function number mov dx,seg buff ; address of disk mov ds,dx ; transfer area mov dx,offset buff int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 1BH (27) Get default drive data ──────────────────────────────────────────────────────────────────────────── Obtains selected information about the default disk drive and a pointer to the media identification byte from its file allocation table. Call with: AH = 1BH Returns: If function successful AL = sectors per cluster DS:BX = segment:offset of media ID byte CX = size of physical sector (bytes) DX = number of clusters for default drive If function unsuccessful (invalid drive or critical error) AL = FFH Notes: ■ The media ID byte has the following meanings: 0F0H 3.5-inch double-sided, 18 sectors or "other" 0F8H fixed disk 0F9H 5.25-inch double-sided, 15 sectors or 3.5-inch double-sided, 9 sectors 0FCH 5.25-inch single-sided, 9 sectors 0FDH 5.25-inch double-sided, 9 sectors 0FEH 5.25-inch single-sided, 8 sectors 0FFH 5.25-inch double-sided, 8 sectors ■ To obtain information about disks other than the one in the default drive, use Int 21H Function 1CH or 36H. ■ [1] The address returned in DS:BX points to a copy of the first sector of the actual FAT, with the media ID byte in the first byte. ■ [2.0+] The address returned in DS:BX points only to a copy of the media ID byte from the disk's FAT; the memory above that address cannot be assumed to contain the FAT or any other useful information. If direct access to the FAT is required, use Int 25H to read it into memory. Example: Determine whether the current disk drive is fixed or removable. . . . mov ah,1bh ; function number int 21h ; transfer to MS-DOS ; check media ID byte cmp byte ptr [bx],0f8h je fixed ; jump if fixed disk jmp floppy ; else assume floppy . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 1CH (28) Get drive data ──────────────────────────────────────────────────────────────────────────── Obtains allocation information about the specified disk drive and a pointer to the media identification byte from its file allocation table. Call with: AH = 1CH DL = drive code (0 = default, 1 = A, etc.) Returns: If function successful AL = sectors per cluster DS:BX = segment:offset of media ID byte CX = size of physical sector (bytes) DX = number of clusters for default or specified drive If function unsuccessful (invalid drive or critical error) AL = FFH Notes: ■ The media ID byte has the following meanings: 0F0H 3.5-inch double-sided, 18 sectors or "other" 0F8H fixed disk 0F9H 5.25-inch double-sided, 15 sectors or 3.5-inch double-sided, 9 sectors 0FCH 5.25-inch single-sided, 9 sectors 0FDH 5.25-inch double-sided, 9 sectors 0FEH 5.25-inch single-sided, 8 sectors 0FFH 5.25-inch double-sided, 8 sectors ■ In general, this call is identical to Int 21H Function 1BH, except for the ability to designate a specific disk drive. See also Int 21H Function 36H, which returns similar information. ■ [1] The address returned in DS:BX points to a copy of the first sector of the actual FAT, with the media ID byte in the first byte. ■ [2.0+] The address returned in DS:BX points only to a copy of the media ID byte from the disk's FAT; the memory above that address cannot be assumed to contain the FAT or any other useful information. If direct access to the FAT is required, use Int 25H to read it into memory. Example: Determine whether disk drive C is fixed or removable. . . . mov ah,1ch ; function number mov dl,3 ; drive code 3 = C int 21h ; transfer to MS-DOS ; check media ID byte cmp byte ptr ds:[bx],0f8h je fixed ; jump if fixed disk jmp floppy ; else assume floppy . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 1DH (29) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H Function 1EH (30) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H Function 1FH (31) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H Function 20H (32) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 21H (33) Random read ──────────────────────────────────────────────────────────────────────────── Reads a selected record from a file into memory. Call with: AH = 21H DS:DX = segment:offset of previously opened file control block Returns: AL = 00H if read successful 01H if end of file 02H if segment wrap, read canceled 03H if partial record read at end of file Notes: ■ The record is read into memory at the current disk transfer area address, specified by the most recent call to Int 21H Function 1AH. It is the programmer's responsibility to ensure that this area is large enough for any record that will be transferred. If the size and location of the buffer are such that a segment overflow or wraparound would occur, the function fails with a return code of 02H. ■ The file location of the data to be read is determined by the combination of the relative-record field (offset 21H) and the record-size field (offset 0EH) of the FCB. The default record size is 128 bytes. ■ The current block field (offset 0CH) and current record field (offset 20H) are updated to agree with the relative-record field as a side effect of the function. ■ The relative-record field of the FCB is not incremented by this function; it is the responsibility of the application to update the FCB appropriately if it wishes to read successive records. Compare with Int 21H Function 27H, which can read multiple records with one function call and automatically increments the relative-record field. ■ If a partial record is read at end of file, it is padded to the requested record length with zeros. ■ [3.0+] If the program is running on a network, the user must have Read access rights to the directory containing the file to be read. Example: Open the file MYFILE.DAT, set the record length to 1024 bytes, then read record number 4 from the file into the buffer named buff. myfcb db 0 ; drive = default db 'MYFILE ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB buff db 1024 dup (?) ; receives read data . . . ; open the file mov ah,0fh ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check open status jnz error ; jump if no file ; set DTA address mov ah,1ah ; function number mov dx,offset buff ; read buffer address int 21h ; transfer to MS-DOS ; set record size mov word ptr myfcb+0eh,1024 ; set record number mov word ptr myfcb+21h,4 mov word ptr myfcb+23h,0 ; read the record mov ah,21h ; function number mov dx,offset myfcb ; address of FCB int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if read failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 22H (34) Random write ──────────────────────────────────────────────────────────────────────────── Writes data from memory into a selected record in a file. Call with: AH = 22H DS:DX = segment:offset of previously opened file control block Returns: AL = 00H if write successful 01H if disk full 02H if segment wrap, write canceled Notes: ■ The record is written (logically, not necessarily physically) to the file from memory at the current disk transfer address, specified by the most recent call to Int 21H Function 1AH. If the size and location of the buffer are such that a segment overflow or wraparound would occur, the function fails with a return code of 02H. ■ The file location of the data to be written is determined by the combination of the relative-record field (offset 21H) and the record-size field (offset 0EH) of the FCB. The default record size is 128 bytes. ■ The current block field (offset 0CH) and current record field (offset 20H) are updated to agree with the relative-record field as a side effect of the function. ■ The relative-record field of the FCB is not incremented by this function; it is the responsibility of the application to update the FCB appropriately if it wishes to write successive records. Compare with Int 21H Function 28H, which can write multiple records with one function call and automatically increments the relative-record field. ■ If a record is written beyond the current end of file, the space between the old end of file and the new record is allocated but not initialized. ■ [3.0+] If the program is running on a network, the user must have Write access rights to the directory containing the file to be written. Example: Open the file MYFILE.DAT, set the record length to 1024 bytes, write record number 4 into the file from the buffer named buff, then close the file. myfcb db 0 ; drive = default db 'MYFILE ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB buff db 1024 dup (?) ; buffer for write . . . ; open the file mov ah,0fh ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if no file ; set DTA address mov dx,offset buff ; buffer address mov ah,1ah ; function number int 21h ; transfer to MS-DOS ; set record size mov word ptr myfcb+0eh,1024 ; set record number mov word ptr myfcb+21h,4 mov word ptr myfcb+23h,0 ; write the record mov ah,22h ; function number mov dx,offset myfcb ; address of FCB int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if write failed ; close the file mov ah,10h ; function number mov dx,offset myfcb ; address of FCB int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if close failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 23H (35) Get file size ──────────────────────────────────────────────────────────────────────────── Searches for a matching file in the current directory; if one is found, updates the FCB with the file's size in terms of number of records. Call with: AH = 23H DS:DX = segment:offset of unopened file control block Returns: If function successful (matching file found) AL = 00H and FCB relative-record field (offset 21H) set to the number of records in the file, rounded up if necessary to the next complete record If function unsuccessful (no matching file found) AL = FFH Notes: ■ An appropriate value must be placed in the FCB record-size field (offset 0EH) before calling this function. There is no default record size for this function. Compare with the FCB-related open and create functions (Int 21H Functions 0FH and 16H), which initialize the FCB for a default record size of 128 bytes. ■ The record-size field can be set to 1 to find the size of the file in bytes. ■ Because record numbers are zero based, this function can be used to position the FCB's file pointer to the end of file. Example: Determine the size in bytes of the file MYFILE.DAT and leave the result in registers DX:AX. myfcb db 0 ; drive = default db 'MYFILE ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB . . . mov ah,23h ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb ; record size = 1 byte mov word ptr myfcb+0eh,1 int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if no file ; get file size in bytes mov ax,word ptr myfcb+21h mov dx,word ptr myfcb+23h . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 24H (36) Set relative record number ──────────────────────────────────────────────────────────────────────────── Sets the relative-record-number field of a file control block (FCB) to correspond to the current file position as recorded in the opened FCB. Call with: AH = 24H DS:DX = segment:offset of previously opened file control block Returns: AL is destroyed (other registers not affected) FCB relative-record field (offset 21H) updated Notes: ■ This function is used when switching from sequential to random I/O within a file. The contents of the relative-record field (offset 21H) are derived from the record size (offset 0EH), current block (offset 0CH), and current record (offset 20H) fields of the file control block. ■ All four bytes of the FCB relative-record field (offset 21H) should be initialized to zero before calling this function. Example: After a series of sequential record transfers have been performed using the file control block myfcb, obtain the current relative-record position in the file and leave the record number in DX. myfcb db 0 ; drive = default db 'MYFILE ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB . . . mov dx,seg myfcb ; make FCB addressable mov ds,dx ; initialize relative ; record field to zero mov word ptr myfcb+21h,0 mov word ptr myfcb+23h,0 ; now set record number mov ah,24h ; function number mov dx,offset myfcb ; address of FCB int 21h ; transfer to MS-DOS ; load record number in DX mov dx,word ptr myfcb+21h . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 25H (37) Set interrupt vector ──────────────────────────────────────────────────────────────────────────── Initializes a CPU interrupt vector to point to an interrupt handling routine. Call with: AH = 25H AL = interrupt number DS:DX = segment:offset of interrupt handling routine Returns: Nothing Notes: ■ This function should be used in preference to direct editing of the interrupt-vector table by well-behaved applications. ■ Before an interrupt vector is modified, its original value should be obtained with Int 21H Function 35H and saved, so that it can be restored using this function before program termination. Example: Install a new interrupt handler, named zdiv, for "divide by zero" CPU exceptions. . . . mov ah,25h ; function number mov al,0 ; interrupt number mov dx,seg zdiv ; address of handler mov ds,dx mov dx,offset zdiv int 21h ; transfer to MS-DOS . . . zdiv: ; int 00h handler iret ; (does nothing) ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 26H (38) Create new PSP ──────────────────────────────────────────────────────────────────────────── Copies the program segment prefix (PSP) of the currently executing program to a specified segment address in free memory, then updates the new PSP to make it usable by another program. Call with: AH = 26H DX = segment of new program segment prefix Returns: Nothing Notes: ■ After the executing program's PSP is copied into the new segment, the memory size information in the new PSP is updated appropriately and the current contents of the termination (Int 22H), Ctrl-C handler (Int 23H), and critical-error handler (Int 24H) vectors are saved starting at offset 0AH. ■ This function does not load another program or in itself cause one to be executed. ■ [2.0+] Int 21H Function 4BH (EXEC), which can be used to load and execute programs or overlays in either .COM or .EXE format, should be used in preference to this function. Example: Create a new program segment prefix 64 KB above the currently executing program. This example assumes that the running program was loaded as a .COM file so that the CS register points to its PSP throughout its execution. If the running program was loaded as a .EXE file, the address of the PSP must be obtained with Int 21H Function 62H (under MS-DOS 3.0 or later) or by saving the original contents of the DS or ES registers at entry. . . . mov ah,26h ; function number mov dx,cs ; PSP segment of ; this program add dx,1000h ; add 64 KB as ; paragraph address int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 27H (39) Random block read ──────────────────────────────────────────────────────────────────────────── Reads one or more sequential records from a file into memory, starting at a designated file location. Call with: AH = 27H CX = number of records to read DS:DX = segment:offset of previously opened file control block Returns: AL = 00H if all requested records read 01H if end of file 02H if segment wrap 03H if partial record read at end of file CX = actual number of records read Notes: ■ The records are read into memory at the current disk transfer area address, specified by the most recent call to Int 21H Function 1AH. It is the programmer's responsibility to ensure that this area is large enough for the group of records that will be transferred. If the size and location of the buffer are such that a segment overflow or wraparound would occur, the function fails with a return code of 02H. ■ The file location of the data to be read is determined by the combination of the relative-record field (offset 21H) and the record-size field (offset 0EH) of the FCB. The default record size is 128 bytes. ■ After the disk transfer is performed, the current block (offset 0CH), current record (offset 20H), and relative-record (offset 21H) fields of the FCB are updated to point to the next record in the file. ■ If a partial record is read at the end of file, the remainder of the record is padded with zeros. ■ Compare with Int 21H Function 21H, which transfers only one record per function call and does not update the FCB relative-record field. ■ [3.0+] If the program is running on a network, the user must have Read access rights to the directory containing the file to be read. Example: Read four 1024-byte records starting at record number 8 into the buffer named buff, using the file control block myfcb. myfcb db 0 ; drive = default db 'MYFILE ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB buff db 4096 dup (?) ; buffer for data . . . ; set DTA address mov ah,1ah ; function number mov dx,seg buff ; address of buffer mov ds,dx mov dx,offset buff int 21h ; transfer to MS-DOS ; set relative-record number mov word ptr myfcb+21h,8 mov word ptr myfcb+23h,0 ; set record size mov word ptr myfcb+0eh,1024 ; read the records mov ah,27h ; function number mov cx,4 ; number of records mov dx,offset myfcb ; address of FCB int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if read error . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 28H (40) Random block write ──────────────────────────────────────────────────────────────────────────── Writes one or more sequential records from memory to a file, starting at a designated file location. Call with: AH = 28H CX = number of records to write DS:DX = segment:offset of previously opened file control block Returns: AL = 00H if all requested records written 01H if disk full 02H if segment wrap CX = actual number of records written Notes: ■ The records are written (logically, not necessarily physically) to disk from memory at the current disk transfer area address, specified by the most recent call to Int 21H Function 1AH. If the size and location of the buffer are such that a segment overflow or wraparound would occur, the function fails with a return code of 02H. ■ The file location of the data to be written is determined by the combination of the relative-record field (offset 21H) and the record-size field (offset 0EH) of the FCB. The default record size is 128 bytes. ■ After the disk transfer is performed, the current block (offset 0CH), current record (offset 20H), and relative-record (offset 21H) fields of the FCB are updated to point to the next record in the file. ■ If this function is called with CX = 0, no data is written to the disk but the file is extended or truncated to the length specified by combination of the record-size (offset 0EH) and the relative-record (offset 21H) fields of the FCB. ■ Compare with Int 21H Function 22H, which transfers only one record per function call and does not update the FCB relative-record field. ■ [3.0+] If the program is running on a network, the user must have Write access rights to the directory containing the file to be written. Example: Write four 1024-byte records, starting at record number 8, to disk from the buffer named buff, using the file control block myfcb. myfcb db 0 ; drive = default db 'MYFILE ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB buff db 4096 dup (?) ; buffer for data . . . ; set DTA address mov ah,1ah ; function number mov dx,seg buff ; address of buffer mov ds,dx mov dx,offset buff int 21h ; transfer to MS-DOS ; set relative-record number mov word ptr myfcb+21h,8 mov word ptr myfcb+23h,0 ; set record size mov word ptr myfcb+0eh,1024 ; write the records mov ah,28h ; function number mov cx,4 ; number of records mov dx,offset myfcb ; address of FCB int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if write error . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 29H (41) Parse filename ──────────────────────────────────────────────────────────────────────────── Parses a text string into the various fields of a file control block (FCB). Call with: AH = 29H AL = flags to control parsing Bit 3 = 1 if extension field in FCB will be modified only if an extension is specified in the string being parsed. = 0 if extension field in FCB will be modified regardless; if no extension is present in the parsed string, FCB extension is set to ASCII blanks. Bit 2 = 1 if filename field in FCB will be modified only if a filename is specified in the string being parsed. = 0 if filename field in FCB will be modified regardless; if no filename is present in the parsed string, FCB filename is set to ASCII blanks. Bit 1 = 1 if drive ID byte in FCB will be modified only if a drive was specified in the string being parsed. = 0 if the drive ID byte in FCB will be modified regardless; if no drive specifier is present in the parsed string, FCB drive-code field is set to 0 (default). Bit 0 = 1 if leading separators will be scanned off (ignored). = 0 if leading separators will not be scanned off. DS:SI = segment:offset of string ES:DI = segment:offset of file control block Returns: AL = 00H if no wildcard characters encountered 01H if parsed string contained wildcard characters FFH if drive specifier invalid DS:SI = segment:offset of first character after parsed filename ES:DI = segment:offset of formatted unopened file control block Notes: ■ This function regards the following as separator characters: [1] : . ; , = + tab space / " [ ] [2.0+] : . ; , = + tab space ■ This function regards all control characters and the following as terminator characters: : . ; , = + tab space < > | / " [ ] ■ If no valid filename is present in the string to be parsed, upon return ES:DI + 1 points to an ASCII blank. ■ If the * wildcard character occurs in a filename or extension, it and all remaining characters in the corresponding field in the FCB are set to ?. ■ This function (and file control blocks in general) cannot be used with file specifications that include a path. Example: Parse the string fname into the file control block myfcb. fname db 'D:QUACK.DAT',0 ; filename to be parsed myfcb db 37 dup (0) ; becomes file control block . . . mov ah,29h ; function number mov al,01h ; skip leading separators mov si,seg fname ; address of filename mov ds,si mov si,offset fname mov di,seg myfcb ; address of FCB mov es,di mov di,offset myfcb int 21h ; transfer to MS-DOS cmp al,0ffh ; check status je error ; jump, drive invalid . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 2AH (42) Get date ──────────────────────────────────────────────────────────────────────────── Obtains the system day of the month, day of the week, month, and year. Call with: AH = 2AH Returns: CX = year (1980 through 2099) DH = month (1 through 12) DL = day (1 through 31) Under MS-DOS versions 1.1 and later AL = day of the week (0 = Sunday, 1 = Monday, etc.) Notes: ■ This function's register format is the same as that required for Int 21H Function 2BH (Set Date). ■ This function can be used together with Int 21H Function 2BH to find the day of the week for an arbitrary date. The current date is first obtained with Function 2AH and saved. The date of interest is then set with Function 2BH, and the day of the week for that date is obtained with a subsequent call to Function 2AH. Finally, the current date is restored with an additional call to Function 2BH, using the values obtained with the original Function 2AH call. Example: Obtain the current date and save its components in the variables year, day, and month. year dw 0 month db 0 day db 0 . . . mov ah,2ah ; function number int 21h ; transfer to MS-DOS mov year,cx ; save year (word) mov month,dh ; save month (byte) mov day,dl ; save day (byte) . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 2BH (43) Set date ──────────────────────────────────────────────────────────────────────────── Initializes the system clock driver to a specific date. The system time is not affected. Call with: AH = 2BH CX = year (1980 through 2099) DH = month (1 through 12) DL = day (1 through 31) Returns: AL = 00H if date set successfully FFH if date not valid (ignored) Note: ■ This function's register format is the same as that required for Int 21H Function 2AH (Get Date). Example: Set the system date according to the contents of the variables year, day, and month. year dw 0 month db 0 day db 0 . . . mov ah,2bh ; function number mov cx,year ; get year (word) mov dh,month ; get month (byte) mov dl,day ; get day (byte) int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if date invalid . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 2CH (44) Get time ──────────────────────────────────────────────────────────────────────────── Obtains the time of day from the system real-time clock driver, converted to hours, minutes, seconds, and hundredths of seconds. Call with: AH = 2CH Returns: CH = hours (0 through 23) CL = minutes (0 through 59) DH = seconds (0 through 59) DL = hundredths of seconds (0 through 99) Notes: ■ This function's register format is the same as that required for Int 21H Function 2DH (Set Time). ■ On most IBM PC─compatible systems, the real-time clock does not have a resolution of single hundredths of seconds. On such machines, the values returned by this function in register DL are discontinuous. Example: Obtain the current time and save its two major components in the variables hours and minutes. hours db 0 minutes db 0 . . . mov ah,2ch ; function number int 21h ; transfer to MS-DOS mov hours,ch ; save hours (byte) mov minutes,cl ; save minutes (byte) . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 2DH (45) Set time ──────────────────────────────────────────────────────────────────────────── Initializes the system real-time clock to a specified hour, minute, second, and hundredth of second. The system date is not affected. Call with: AH = 2DH CH = hours (0 through 23) CL = minutes (0 through 59) DH = seconds (0 through 59) DL = hundredths of seconds (0 through 99) Returns: AL = 00H if time set successfully FFH if time not valid (ignored) Note: ■ This function's register format is the same as that required for Int 21H Function 2CH (Get Time). Example: Set the system time according to the contents of the variables hours and minutes. Force the current seconds and hundredths of seconds to zero. hours db 0 minutes db 0 . . . mov ah,2dh ; function number mov ch,hours ; get hours (byte) mov cl,minutes ; get minutes (byte) mov dx,0 ; force seconds and ; hundredths to zero int 21h ; transfer to MS-DOS or al,al ; check status jnz error ; jump if time invalid . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [1.0] Function 2EH (46) Set verify flag ──────────────────────────────────────────────────────────────────────────── Turns off or turns on the operating-system flag for automatic read-after-write verification of data. Call with: AH = 2EH AL = 00H if turning off verify flag 01H if turning on verify flag DL = 00H (MS-DOS versions 1 and 2) Returns: Nothing Notes: ■ Because read-after-write verification slows disk operations, the default setting of the verify flag is OFF. ■ If a particular disk unit's device driver does not support read-after-write verification, this function has no effect. ■ The current state of the verify flag can be determined using Int 21H Function 54H. ■ The state of the verify flag is also controlled by the MS-DOS commands VERIFY OFF and VERIFY ON. Example: Save the current state of the system verify flag in the variable vflag, then force all subsequent disk writes to be verified. vflag db 0 ; previous verify flag . . . ; get verify flag mov ah,54h ; function number int 21h ; transfer to MS-DOS mov vflag,al ; save current flag state ; set verify flag mov ah,2eh ; function number mov al,1 ; AL = 1 for verify on mov dl,0 ; DL must be zero int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 2FH (47) Get DTA address ──────────────────────────────────────────────────────────────────────────── Obtains the current address of the disk transfer area (DTA) for FCB file read/write operations. Call with: AH = 2FH Returns: ES:BX = segment:offset of disk transfer area Note: ■ The disk transfer area address is set with Int 21H Function 1AH. The default DTA is a 128-byte buffer at offset 80H in the program segment prefix. Example: Obtain the current disk transfer area address and save it in the variable olddta. olddta dd ? ; save disk transfer address . . . mov ah,2fh ; function number int 21h ; transfer to MS-DOS ; save it as DWORD pointer mov word ptr olddta,bx mov word ptr olddta+2,es . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 30H (48) Get MS-DOS version number ──────────────────────────────────────────────────────────────────────────── Returns the version number of the host MS-DOS operating system. This function is used by application programs to determine the capabilities of their environment. Call with: AH = 30H AL = 00H Returns: If running under MS-DOS version 1 AL = 00H If running under MS-DOS versions 2.0 or later AL = major version number (MS-DOS 3.10 = 3, etc.) AH = minor version number (MS-DOS 3.10 = 0AH, etc.) BH = Original Equipment Manufacturer's (OEM's) serial number (OEM-dependent──usually 00H for IBM's PC-DOS, 0FFH or other values for MS-DOS) BL:CX = 24-bit user serial number (optional, OEM-dependent) Notes: ■ Because this function was not defined under MS-DOS version 1, it should always be called with AL = 00H. In an MS-DOS version 1 environment, AL will be returned unchanged. ■ Care must be taken not to exit in an unacceptable fashion if an MS-DOS version 1 environment is detected. For example, Int 21H Function 4CH (Terminate Process with Return Code), Int 21H Function 40H (Write to File or Device), and the standard error handle are not available in MS-DOS version 1. In such cases a program should display an error message using Int 21H Function 09H and then terminate with Int 20H or Int 21H Function 00H. Example: Get the MS-DOS version number, terminating the current process with an error message if not running under MS-DOS version 2.0 or later. cr equ 0dh ; ASCII carriage return lf equ 0ah ; ASCII line feed msg db cr,lf db 'Wrong MS-DOS version' db cr,lf,'$' . . . mov ax,3000h ; function number int 21h ; transfer to MS-DOS cmp al,2 ; version 2 or later? jae label1 ; yes, jump ; display error message mov ah,09 ; function number mov dx,offset msg ; message address int 21h ; transfer to MS-DOS ; terminate process mov ah,0 ; function number int 21h ; transfer to MS-DOS label1: . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 31H (49) Terminate and stay resident ──────────────────────────────────────────────────────────────────────────── Terminates execution of the currently executing program, passing a return code to the parent process, but reserves part or all of the program's memory so that it will not be overlaid by the next transient program to be loaded. MS-DOS then takes the following actions: ■ File buffers are flushed and any open handles for files or devices owned by the process are closed. ■ The termination handler vector (Int 22H) is restored from PSP:000AH. ■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH. ■ [2.0+] The critical-error handler vector (Int 24H) is restored from PSP:0012H. ■ Control is transferred to the termination handler. If the program is returning to COMMAND.COM, control transfers to the resident portion, and the transient portion is reloaded if necessary. If a batch file is in progress, the next line of the file is fetched and interpreted; otherwise, a prompt is issued for the next user command. Call with: AH = 31H AL = return code DX = amount of memory to reserve (in paragraphs) Returns: Nothing Notes: ■ This function call is typically used to allow user-written utilities, drivers, or interrupt handlers to be loaded as ordinary .COM or .EXE programs and then remain resident. Subsequent entrance to the code is via a hardware or software interrupt. ■ This function attempts to set the initial memory allocation block to the length in paragraphs specified in register DX. If other memory blocks have been requested by the application using Int 21H Function 48H, they will not be released by this function. ■ Other methods of performing a final exit are: ∙ Int 20H ∙ Int 21H Function 00H ∙ Int 21H Function 4CH ∙ Int 27H ■ The return code may be retrieved by a parent process with Int 21H Function 4DH (Get Return Code). It can also be tested in a batch file with an IF ERRORLEVEL statement. By convention, a return code of zero indicates successful execution, and a nonzero return code indicates an error. ■ This function should not be called by .EXE programs that are loaded at the high end of the transient program area (that is, linked with the /HIGH switch) because doing so reserves the memory that is normally used by the transient part of COMMAND.COM. If COMMAND.COM cannot be reloaded, the system will fail. ■ [2.0+] This function should be used in preference to Int 27H because it supports return codes, allows larger amounts of memory to be reserved, and does not require CS to contain the segment of the program segment prefix. ■ [3.0+] If the program is running on a network, it should remove all locks it has placed on file regions before terminating. Example: Exit with a return code of 1 but stay resident, reserving 16 KB of memory starting at the program segment prefix of the process. . . . mov ah,31h ; function number mov al,1 ; return code for parent mov dx,0400h ; paragraphs to reserve int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 32H (50) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 33H (51) Get or set break flag, get boot drive ──────────────────────────────────────────────────────────────────────────── Obtains or changes the status of the operating system's break flag, which influences Ctrl-C checking during function calls. Also returns the system boot drive in version 4.0. Call with: If getting break flag AH = 33H AL = 00H If setting break flag AH = 33H AL = 01H DL = 00H if turning break flag OFF 01H if turning break flag ON [4] If getting boot drive AH = 33H AL = 05H Returns: If called with AL = 00H or 01H DL = 00H break flag is OFF 01H break flag is ON [4] If called with AL = 05H DL = boot drive (1 = A, 2 = B, etc.) Notes: ■ When the system break flag is on, the keyboard is examined for a Ctrl-C entry whenever any operating-system input or output is requested; if Ctrl-C is detected, control is transferred to the Ctrl-C handler (Int 23H). When the break flag is off, MS-DOS only checks for a Ctrl-C entry when executing the traditional character I/O functions (Int 21H Functions 01H through 0CH). ■ The break flag is not part of the local environment of the currently executing program; it affects all programs. An application that alters the flag should first save the flag's original status, then restore the flag before terminating. Example: Save the current state of the system break flag in the variable brkflag, then turn the break flag off to disable Ctrl-C checking during most MS-DOS function calls. brkflag db 0 ; save break flag . . . ; get current break flag mov ah,33h ; function number mov al,0 ; AL = 0 to get flag int 21h ; transfer to MS-DOS mov brkflag,dl ; save current flag ; now set break flag mov ah,33h ; function number mov al,1 ; AL = 1 to set flag mov dl,0 ; set break flag OFF int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 34H (52) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 35H (53) Get interrupt vector ──────────────────────────────────────────────────────────────────────────── Obtains the address of the current interrupt-handler routine for the specified machine interrupt. Call with: AH = 35H AL = interrupt number Returns: ES:BX = segment:offset of interrupt handler Note: ■ Together with Int 21H Function 25H (Set Interrupt Vector), this function is used by well-behaved application programs to modify or inspect the machine interrupt vector table. Example: Obtain the address of the current interrupt handler for hardware interrupt level 0 (divide by zero) and save it in the variable oldint0. oldint0 dd ? ; previous handler address . . . mov ah,35h ; function number mov al,0 ; interrupt level int 21h ; transfer to MS-DOS ; save old handler address mov word ptr oldint0,bx mov word ptr oldint0+2,es . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 36H (54) Get drive allocation information ──────────────────────────────────────────────────────────────────────────── Obtains selected information about a disk drive, from which the drive's capacity and remaining free space can be calculated. Call with: AH = 36H DL = drive code (0 = default, 1 = A, etc.) Returns: If function successful AX = sectors per cluster BX = number of available clusters CX = bytes per sector DX = clusters per drive If function unsuccessful (drive invalid) AX = FFFFH Notes: ■ This function regards "lost" clusters as being in use and does not report them as part of the number of available clusters, even though they are not assigned to a file. ■ Similar information is returned by Int 21H Functions 1BH and 1CH. Example: Calculate the capacity of disk drive C in bytes, leaving the result in the variable drvsize. (This code assumes that the product of sectors/cluster * bytes/sector will not overflow 16 bits.) drvsize dd ? ; drive C size in bytes . . . mov ah,36h ; function number mov dl,3 ; drive C = 3 int 21h ; transfer to MS-DOS mul cx ; sectors/cluster ; * bytes/sector mul dx ; * total clusters ; result now in DX:AX ; store low word mov word ptr drvsize,ax ; store high word mov word ptr drvsize+2,dx . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 37H (55) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 38H (56) Get or set country information ──────────────────────────────────────────────────────────────────────────── [2] Obtains internationalization information for the current country. [3.0+] Obtains internationalization information for the current or specified country or sets the current country code. Call with: If getting country information (MS-DOS version 2) AH = 38H AL = 0 to get "current" country information DS:DX = segment:offset of buffer for returned information If getting country information (MS-DOS versions 3.0 and later) AH = 38H AL = 0 to get "current" country information 1─FEH to get information for countries with code < 255 FFH to get information for countries with code >= 255 BX = country code, if AL = FFH DS:DX = segment:offset of buffer for returned information If setting current country code (MS-DOS versions 3.0 and later) AH = 38H AL = 1─FEH country code for countries with code < 255 FFH for countries with code >= 255 BX = country code, if AL = 0FFH DX = FFFFH Returns: If function successful Carry flag = clear and, if getting internationalization information BX = country code DS:DX = segment:offset of buffer holding internationalization information and buffer filled in as follows: (for PC-DOS 2.0 and 2.1) Byte(s) Contents 00H─01H date format 0 = USA m d y 1 = Europe d m y 2 = Japan y m d 02H─03H ASCIIZ currency symbol 04H─05H ASCIIZ thousands separator 06H─07H ASCIIZ decimal separator 08H─1FH reserved (for MS-DOS versions 2.0 and later, PC-DOS versions 3.0 and later) Byte(s) Contents 00H─01H date format 0 = USA m d y 1 = d m y Europe 2 = Japan y m d 02H─06H ASCIIZ currency symbol string 07H─08H ASCIIZ thousands separator character 09H─0AH ASCIIZ decimal separator character 0BH─0CH ASCIIZ date separator character 0DH─0EH ASCIIZ time separator character 0FH currency format bit 0 = 0 if currency symbol precedes value = 1 if currency symbol follows value bit 1 = 0 if no space between value and currency symbol = 1 if one space between value and currency symbol bit 2 = 0 if currency symbol and decimal are separate = 1 if currency symbol replaces decimal separator 10H number of digits after decimal in currency 11H time format bit 0 = 0 if 12-hour clock = 1 if 24-hour clock 12H─15H case-map call address 16H─17H ASCIIZ data-list separator 18H─21H reserved If function unsuccessful Carry flag = set AX = error code Notes: ■ The default country code is determined by the COUNTRY= directive in CONFIG.SYS or by the KEYBxx keyboard driver file if one is loaded. Otherwise, the default country code is OEM-dependent. ■ The previous contents of register CX may be destroyed by the Get Country Information subfunction. ■ The case-map call address is the segment:offset of a FAR procedure that performs country-specific mapping on character values from 80H through 0FFH. The procedure must be called with the character to be mapped in register AL. If an alternate value exists for that character, it is returned in AL; otherwise, AL is unchanged. In general, lowercase characters are mapped to their uppercase equivalents, and accented or otherwise modified vowels are mapped to their plain vowel equivalents. ■ [3.0+] The value in register DX is used by MS-DOS to select between the Set Country and Get Country Information subfunctions. ■ [3.3+] Int 21H Function 65H (Get Extended Country Information) returns a superset of the information supplied by this function. Examples: Obtain internationalization information for the current country in the buffer ctrybuf. ctrybuf db 34 dup (0) . . . mov ah,38h ; function number mov al,0 ; get current country mov dx,seg ctrybuf ; address of buffer mov ds,dx ; for country information mov dx,offset ctrybuf int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . If the program is running under PC-DOS 3.3 and the current country code is 49 (West Germany), ctrybuf is filled in with the following information: dw 0001h ; date format db 'DM',0,0,0 ; ASCIIZ currency symbol db '.',0 ; ASCIIZ thousands separator db ',',0 ; ASCIIZ decimal separator db '.',0 ; ASCIIZ date separator db '.',0 ; ASCIIZ time separator db 02h ; currency format db 02h ; digits after decimal db 01h ; time format dd 026ah:176ch ; case-map call address db ';',0 ; ASCIIZ data-list separator db 10 dup (0) ; reserved ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 39H (57) Create directory ──────────────────────────────────────────────────────────────────────────── Creates a directory using the specified drive and path. Call with: AH = 39H DS:DX = segment:offset of ASCIIZ pathname Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Note: ■ The function fails if: ∙ any element of the pathname does not exist. ∙ a directory with the same name at the end of the same path already exists. ∙ the parent directory for the new directory is the root directory and is full. ∙ [3.0+] the program is running on a network and the user running the program has insufficient access rights. Example: Create a directory named MYSUB in the root directory on drive C. dname db 'C:\MYSUB',0 . . . mov ah,39h ; function number mov dx,seg dname ; address of pathname mov ds,dx mov dx,offset dname int 21h ; transfer to MS-DOS jc error ; jump if create failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 3AH (58) Delete directory ──────────────────────────────────────────────────────────────────────────── Removes a directory using the specified drive and path. Call with: AH = 3AH DS:DX = segment:offset of ASCIIZ pathname Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Note: ■ The function fails if: ∙ any element of the pathname does not exist. ∙ the specified directory is also the current directory. ∙ the specified directory contains any files. ∙ [3.0+] the program is running on a network and the user running the program has insufficient access rights. Example: Remove the directory named MYSUB in the root directory on drive C. dname db 'C:\MYSUB',0 . . . mov ah,3ah ; function number mov dx,seg dname ; address of pathname mov ds,dx mov dx,offset dname int 21h ; transfer to MS-DOS jc error ; jump if delete failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 3BH (59) Set current directory ──────────────────────────────────────────────────────────────────────────── Sets the current, or default, directory using the specified drive and path. Call with: AH = 3BH DS:DX = segment:offset of ASCIIZ pathname Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ The function fails if any element of the pathname does not exist. ■ Int 21H Function 47H can be used to obtain the name of the current directory before using Int 21H Function 3BH to select another, so that the original directory can be restored later. Example: Change the current directory for drive C to the directory \MYSUB. dname db 'C:\MYSUB',0 . . . mov ah,3bh ; function number mov dx,seg dname ; address of pathname mov ds,dx mov dx,offset dname int 21h ; transfer to MS-DOS jc error ; jump if bad path . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 3CH (60) Create file ──────────────────────────────────────────────────────────────────────────── Given an ASCIIZ pathname, creates a new file in the designated or default directory on the designated or default disk drive. If the specified file already exists, it is truncated to zero length. In either case, the file is opened and a handle is returned that can be used by the program for subsequent access to the file. Call with: AH = 3CH CX = file attribute (bits may be combined) Bit(s) Significance (if set) 0 read-only 1 hidden 2 system 3 volume label 4 reserved (0) 5 archive 6─15 reserved (0) DS:DX = segment:offset of ASCIIZ pathname Returns: If function successful Carry flag = clear AX = handle If function failed Carry flag = set AX = error code Notes: ■ The function fails if: ∙ any element of the pathname does not exist. ∙ the file is being created in the root directory and the root directory is full. ∙ a file with the same name and the read-only attribute already exists in the specified directory. ∙ [3.0+] the program is running on a network and the user running the program has insufficient access rights. ■ A file is usually given a normal (0) attribute when it is created. The file's attribute can subsequently be modified with Int 21H Function 43H. ■ [3.0+] A volume label can be created using an attribute of 0008H, if one does not already exist. When files are created, bit 3 of the attribute parameter should always be clear (0). ■ [3.0+] See the entries for Int 21H Functions 5AH and 5BH, which may also be used to create files. ■ [4.0+] Int 21H Function 6CH combines the services of Functions 3CH, 3DH, and 5BH. Example: Create and open, or truncate to zero length and open, the file C:\MYDIR\MYFILE.DAT, and save the handle for subsequent access to the file. fname db 'C:\MYDIR\MYFILE.DAT',0 fhandle dw ? . . . mov ah,3ch ; function number xor cx,cx ; normal attribute mov dx,seg fname ; address of pathname mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if create failed mov fhandle,ax ; save file handle . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 3DH (61) Open file ──────────────────────────────────────────────────────────────────────────── Given an ASCIIZ pathname, opens the specified file in the designated or default directory on the designated or default disk drive. A handle is returned which can be used by the program for subsequent access to the file. Call with: AH = 3DH AL = access mode Bit(s) Significance 0─2 access mode 000 = read access 001 = write access 010 = read/write access 3 reserved (0) 4─6 sharing mode (MS-DOS versions 3.0 and later) 000 = compatibility mode 001 = deny all 010 = deny write 011 = deny read 100 = deny none 7 inheritance flag (MS-DOS versions 3.0 and later) 0 = child process inherits handle 1 = child does not inherit handle DS:DX = segment:offset of ASCIIZ pathname Returns: If function successful Carry flag = clear AX = handle If function unsuccessful Carry flag = set AX = error code Notes: ■ Any normal, system, or hidden file with a matching name will be opened by this function. If the file is read-only, the success of the operation also depends on the access code in bits 0─2 of register AL. After opening the file, the file read/write pointer is set to offset zero (the first byte of the file). ■ The function fails if: ∙ any element of the pathname does not exist. ∙ the file is opened with an access mode of read/write and the file has the read-only attribute. ∙ [3.0+] SHARE.EXE is loaded and the file has already been opened by one or more other processes in a sharing mode that is incompatible with the current program's request. ■ The file's date and time stamp can be accessed after a successful open call with Int 21H Function 57H. ■ The file's attributes (hidden, system, read-only, or archive) can be obtained with Int 21H Function 43H. ■ When a file handle is inherited by a child process or is duplicated with Int 21H Function 45H or 46H, all sharing and access restrictions are also inherited. ■ [2] Only bits 0─2 of register AL are significant; the remaining bits should be zero for upward compatibility. ■ [3.0+] Bits 4─7 of register AL control access to the file by other programs. (Bits 4─6 have no effect unless SHARE.EXE is loaded.) ■ [3.0+] A file-sharing error causes a critical-error exception (Int 24H) with an error code of 02H. Int 21H Function 59H can be used to obtain information about the sharing violation. ■ [4.0+] Int 21H Function 6CH combines the services of Functions 3CH, 3DH, and 5BH. Example: Open the file C:\MYDIR\MYFILE.DAT for both reading and writing, and save the handle for subsequent access to the file. fname db 'C:\MYDIR\MYFILE.DAT',0 fhandle dw ? . . . mov ah,3dh ; function number mov al,2 ; mode = read/write mov dx,seg fname ; address of pathname mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if open failed mov fhandle,ax ; save file handle . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 3EH (62) Close file ──────────────────────────────────────────────────────────────────────────── Given a handle that was obtained by a previous successful open or create operation, flushes all internal buffers associated with the file to disk, closes the file, and releases the handle for reuse. If the file was modified, the time and date stamp and file size are updated in the file's directory entry. Call with: AH = 3EH BX = handle Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Note: ■ If you accidentally call this function with a zero handle, the standard input device is closed, and the keyboard appears to go dead. Make sure you always call the close function with a valid, nonzero handle. Example: Close the file whose handle is saved in the variable fhandle. fhandle dw 0 . . . mov ah,3eh ; function number mov bx,fhandle ; file handle int 21h ; transfer to MS-DOS jc error ; jump if close failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 3FH (63) Read file or device ──────────────────────────────────────────────────────────────────────────── Given a valid file handle from a previous open or create operation, a buffer address, and a length in bytes, transfers data at the current file-pointer position from the file into the buffer and then updates the file pointer position. Call with: AH = 3FH BX = handle CX = number of bytes to read DS:DX = segment:offset of buffer Returns: If function successful Carry flag = clear AX = bytes transferred If function unsuccessful Carry flag = set AX = error code Notes: ■ If reading from a character device (such as the standard input) in cooked mode, at most one line of input will be read (i.e., up to a carriage return character or the specified length, whichever comes first). ■ If the carry flag is returned clear but AX = 0, then the file pointer was already at end of file when the program requested the read. ■ If the carry flag is returned clear but AX < CX, then a partial record was read at end of file or there is an error. ■ [3.0+] If the program is running on a network, the user must have Read access rights to the directory and file. Example: Using the file handle from a previous open or create operation, read 1024 bytes at the current file pointer into the buffer named buff. buff db 1024 dup (?) ; buffer for read fhandle dw ? ; contains file handle . . . mov ah,3fh ; function number mov dx,seg buff ; buffer address mov ds,dx mov dx,offset buff mov bx,fhandle ; file handle mov cx,1024 ; length to read int 21h ; transfer to MS-DOS jc error ; jump, read failed cmp ax,cx ; check length of read jl done ; jump, end of file . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 40H (64) Write file or device ──────────────────────────────────────────────────────────────────────────── Given a valid file handle from a previous open or create operation, a buffer address, and a length in bytes, transfers data from the buffer into the file and then updates the file pointer position. Call with: AH = 40H BX = handle CX = number of bytes to write DS:DX = segment:offset of buffer Returns: If function successful Carry flag = clear AX = bytes transferred If function unsuccessful Carry flag = set AX = error code Notes: ■ If the carry flag is returned clear but AX < CX, then a partial record was written or there is an error. This can be caused by a Ctrl-Z (1AH) embedded in the data if the destination is a character device in cooked mode or by a disk full condition if the destination is a file. ■ If the function is called with CX = 0, the file is truncated or extended to the current file pointer position. ■ [3.0+] If the program is running on a network, the user must have Write access rights to the directory and file. Example: Using the handle from a previous open or create operation, write 1024 bytes to disk at the current file pointer from the buffer named buff. buff db 1024 dup (?) ; buffer for write fhandle dw ? ; contains file handle . . . mov ah,40h ; function number mov dx,seg buff ; buffer address mov ds,dx mov dx,offset buff mov bx,fhandle ; file handle mov cx,1024 ; length to write int 21h ; transfer to MS-DOS jc error ; jump, write failed cmp ax,1024 ; entire record written? jne error ; no, jump . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 41H (65) Delete file ──────────────────────────────────────────────────────────────────────────── Deletes a file from the specified or default disk and directory. Call with: AH = 41H DS:DX = segment:offset of ASCIIZ pathname Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ This function deletes a file by replacing the first character of its filename in the directory with the character e (E5H) and marking the file's clusters as "free" in the disk's file allocation table. The actual data stored in those clusters is not overwritten. ■ Only one file at a time may be deleted with this function. Unlike the FCB-related Delete File function (Int 21H Function 13H), the * and ? wildcard characters are not allowed in the file specification. ■ The function fails if: ∙ any element of the pathname does not exist. ∙ the designated file exists but has the read-only attribute. (Int 21H Function 43H can be used to examine and modify a file's attribute before attempting to delete it.) ∙ [3.0+] the program is running on a network, and the user running the program has insufficient access rights. Example: Delete the file named MYFILE.DAT from the directory \MYDIR on drive C. fname db 'C:\MYDIR\MYFILE.DAT',0 . . . mov ah,41h ; function number mov dx,seg fname ; filename address mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if delete failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 42H (66) Set file pointer ──────────────────────────────────────────────────────────────────────────── Sets the file location pointer relative to the start of file, end of file, or current file position. Call with: AH = 42H AL = method code 00H absolute offset from start of file 01H signed offset from current file pointer 02H signed offset from end of file BX = handle CX = most significant half of offset DX = least significant half of offset Returns: If function successful Carry flag = clear DX = most significant half of resulting file pointer AX = least significant half of resulting file pointer If function unsuccessful Carry flag = set AX = error code Notes: ■ This function uses a method code and a double-precision (32-bit) value to set the file pointer. The next record read or written in the file will begin at the new file pointer location. No matter what method is used in the call to this function, the file pointer returned in DX:AX is always the resulting absolute byte offset from the start of file. ■ Method 02H may be used to find the size of the file by calling Int 21H Function 42H with an offset of 0 and examining the pointer location that is returned. ■ Using methods 01H or 02H, it is possible to set the file pointer to a location that is before the start of file. If this is done, no error is returned by this function, but an error will be encountered upon a subsequent attempt to read or write the file. Examples: Using the file handle from a previous open or create operation, set the current file pointer position to 1024 bytes after the start of file. fhandle dw ? . . . mov ah,42h ; function number mov al,0 ; method = absolute mov bx,fhandle ; file handle mov cx,0 ; upper half of offset mov dx,1024 ; lower half of offset int 21h ; transfer to MS-DOS jc error ; jump, function failed . . . The following subroutine accepts a record number, record size, and handle and sets the file pointer appropriately. ; call this routine with BX = handle ; AX = record number ; CX = record size ; returns all registers unchanged ; setptr proc near push ax ; save record number push cx ; save record size push dx ; save whatever's in DX mul cx ; size * record number mov cx,ax ; upper part to CX xchg cx,dx ; lower part to DX mov ax,4200h ; function number & method int 21h ; transfer to MS-DOS pop dx ; restore previous DX pop cx ; restore record size pop ax ; restore record number ret ; back to caller setptr endp ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 43H (67) Get or set file attributes ──────────────────────────────────────────────────────────────────────────── Obtains or alters the attributes of a file (read-only, hidden, system, or archive) or directory. Call with: AH = 43H AL = 00H to get attributes 01H to set attributes CX = file attribute, if AL = 01H (bits can be combined) Bit(s) Significance (if set) 0 read-only 1 hidden 2 system 3─4 reserved (0) 5 archive 6─15 reserved (0) DS:DX = segment:offset of ASCIIZ pathname Returns: If function successful Carry flag = clear CX = file attribute Bit(s) Significance (if set) 0 read-only 1 hidden 2 system 3 volume label 4 directory 5 archive 6─15 reserved (0) If function unsuccessful Carry flag = set AX = error code Notes: ■ Bits 3 and 4 of register CX must always be clear (0) when this function is called; in other words, you cannot change an existing file into a directory or volume label. However, you can assign the "hidden" attribute to an existing directory with this function. ■ [3.0+] If the program is running on a network, the user must have Create access rights to the directory containing the file whose attribute is to be modified. Example: Change the attribute of the file D:\MYDIR\MYFILE.DAT to read-only, so that it cannot be accidentally modified or deleted by other application programs. rdonly equ 01h ; file attributes hidden equ 02h system equ 04h volume equ 08h subdir equ 10h archive equ 20h fname db 'D:\MYDIR\MYFILE.DAT',0 . . . mov ah,43h ; function number mov al,01h ; subfunction = modify mov cx,rdonly ; read-only attribute mov dx,seg fname ; filename address mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if modify failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 44H (68) IOCTL (I/O control) ──────────────────────────────────────────────────────────────────────────── Provides a direct path of communication between an application program and a device driver. Allows a program to obtain hardware-dependent information and to request operations that are not supported by other MS-DOS function calls. The IOCTL subfunctions and the MS-DOS versions in which they first became available are: ╓┌─┌──────────────┌──────────────────────────────────────────┌───────────────╖ Subfunction Name MS-DOS version ────────────────────────────────────────────────────────────────────────── 00H Get Device Information 2.0 01H Set Device Information 2.0 02H Receive Control Data from Character 2.0 Device Driver 03H Send Control Data to Character Device 2.0 Driver 04H Receive Control Data from Block Device 2.0 Driver 05H Send Control Data to Block Device Driver 2.0 06H Check Input Status 2.0 07H Check Output Status 2.0 08H Check If Block Device Is Removable 3.0 Subfunction Name MS-DOS version ────────────────────────────────────────────────────────────────────────── 08H Check If Block Device Is Removable 3.0 09H Check If Block Device Is Remote 3.1 0AH (10) Check If Handle Is Remote 3.1 0BH (11) Change Sharing Retry Count 3.1 0CH (12) Generic I/O Control for Character Devices CL = 45H: Set Iteration Count 3.2 CL = 4AH: Select Code Page 3.3 CL = 4CH: Start Code Page Preparation 3.3 CL = 4DH: End Code Page Preparation 3.3 CL = 5FH: Set Display Information 4.0 CL = 65H: Get Iteration Count 3.2 CL = 6AH: Query Selected Code Page 3.3 CL = 6BH: Query Prepare List 3.3 CL = 7FH: Get Display Information 4.0 0DH (13) Generic I/O Control for Block Devices CL = 40H: Set Device Parameters 3.2 CL = 41H: Write Track 3.2 CL = 42H: Format and Verify Track 3.2 CL = 47H: Set Access Flag 4.0 Subfunction Name MS-DOS version ────────────────────────────────────────────────────────────────────────── CL = 47H: Set Access Flag 4.0 CL = 60H: Get Device Parameters 3.2 CL = 61H: Read Track 3.2 CL = 62H: Verify Track 3.2 CL = 67H: Get Access Flag 4.0 0EH (14) Get Logical Drive Map 3.2 0FH (15) Set Logical Drive Map 3.2 ────────────────────────────────────────────────────────────────────────── Only IOCTL Subfunctions 00H, 06H, and 07H may be used for handles associated with files. Subfunctions 00H─08H are not supported on network devices. ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 44H (68) Subfunction 00H IOCTL: get device information ──────────────────────────────────────────────────────────────────────────── Returns a device information word for the file or device associated with the specified handle. Call with: AH = 44H AL = 00H BX = handle Returns: If function successful Carry flag = clear DX = device information word For a file: Bit(s) Significance 0─5 drive number (0 = A, 1 = B, etc.) 6 0 if file has been written 1 if file has not been written 7 0, indicating a file 8─15 reserved For a device: Bit(s) Significance 0 1 if standard input 1 1 if standard output 2 1 if NUL device 3 1 if clock device 4 reserved 5 0 if handle in ASCII mode 1 if handle in binary mode 6 0 if end of file on input 7 1, indicating a device 8─13 reserved 14 0 if IOCTL subfunctions 02H and 03H not supported 1 if IOCTL subfunctions 02H and 03H supported 15 reserved If function unsuccessful Carry flag = set AX = error code Notes: ■ Bits 8─15 of DX correspond to the upper 8 bits of the device-driver attribute word. ■ Bit 5 of the device information word for a handle associated with a character device signifies whether MS-DOS considers that handle to be in binary ("raw") mode or ASCII ("cooked") mode. In ASCII mode, MS-DOS filters the character stream and may take special action when the characters Ctrl-C, Ctrl-S, Ctrl-P, Ctrl-Z, and carriage return are detected. In binary mode, all characters are treated as data, and the exact number of characters requested is always read or written. Example: See Int 21H Function 44H Subfunction 01H. ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 44H (68) Subfunction 01H IOCTL: set device information ──────────────────────────────────────────────────────────────────────────── Sets certain flags for a handle associated with a character device. This subfunction may not be used for a handle that is associated with a file. Call with: AH = 44H AL = 01H BX = handle DX = device information word Bit(s) Significance 0 1 if standard input 1 1 if standard output 2 1 if NUL device 3 1 if clock device 4 reserved (0) 5 0 to select ASCII mode 1 to select binary mode 6 reserved (0) 7 1, indicating a device 8─15 reserved (0) Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ If register DH does not contain 00H, control returns to the program with the carry flag set and error code 0001H (invalid function) in register AX. ■ Bit 5 of the information word for a handle associated with a character device signifies whether MS-DOS considers that handle to be in binary ("raw") or ASCII ("cooked") mode. See Notes for Int 21H Function 44H Subfunction 00H. Example: Place the standard output handle into binary ("raw") mode. This speeds up output by disabling checking for Ctrl-C, Ctrl-S, and Ctrl-P between each character. . . . ; get device information mov ax,4400h ; function & subfunction mov bx,1 ; standard output handle int 21h ; transfer to MS-DOS mov dh,0 ; force DH = 0 or dl,20h ; set binary mode bit ; set device information mov ax,4401h ; function & subfunction int 21h ; transfer to MS-DOS . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 44H (68) Subfunction 02H IOCTL: read control data from character device driver ──────────────────────────────────────────────────────────────────────────── Reads control data from a character-device driver. The length and contents of the data are specific to each device driver and do not follow any standard format. This function does not necessarily result in any input from the physical device. Call with: AH = 44H AL = 02H BX = handle CX = number of bytes to read DS:DX = segment:offset of buffer Returns: If function successful Carry flag = clear AX = bytes read and buffer contains control data from driver If function unsuccessful Carry flag = set AX = error code Notes: ■ If supported by the driver, this subfunction can be used to obtain hardware-dependent status and availability information that is not supported by other MS-DOS function calls. ■ Character-device drivers are not required to support IOCTL Subfunction 02H. A program can test bit 14 of the device information word returned by IOCTL Subfunction 00H to determine whether the driver supports this subfunction. If Subfunction 02H is requested and the driver does not have the ability to process control data, control returns to the program with the carry flag set and error code 0001H (invalid function) in register AX. Example: Read a control string from the standard list driver into the buffer buff. stdprn equ 4 ; standard list handle buflen equ 64 ; length of buffer ctllen dw ? ; length of control string buff db buflen dup (0) ; receives control string . . . mov ax,4402h ; function & subfunction mov bx,stdprn ; standard list handle mov cx,buflen ; buffer length mov dx,seg buff ; buffer address mov ds,dx mov dx,offset buff int 21h ; transfer to MS-DOS jc error ; jump if read failed mov ctllen,ax ; save control string length . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 44H (68) Subfunction 03H IOCTL: write control data to character-device driver ──────────────────────────────────────────────────────────────────────────── Transfers control data from an application to a character-device driver. The length and contents of the data are specific to each device driver and do not follow any standard format. This function does not necessarily result in any output to the physical device. Call with: AH = 44H AL = 03H BX = handle CX = number of bytes to write DS:DX = segment:offset of data Returns: If function successful Carry flag = clear AX = bytes transferred If function unsuccessful Carry flag = set AX = error code Notes: ■ If supported by the driver, this subfunction can be used to request hardware-dependent operations (such as setting baud rate for a serial port) that are not supported by other MS-DOS function calls. ■ Character-device drivers are not required to support IOCTL Subfunction 03H. A program can test bit 14 of the device information word returned by IOCTL Subfunction 00H to determine whether the driver supports this subfunction. If Subfunction 03H is requested and the driver does not have the ability to process control data, control returns to the program with the carry flag set and error code 0001H (invalid function) in register AX. Example: Write a control string from the buffer buff to the standard list device driver. The length of the string is assumed to be in the variable ctllen. stdprn equ 4 ; standard list handle buflen equ 64 ; length of buffer ctllen dw ? ; length of control data buff db buflen dup (?) ; contains control data . . . mov ax,4403h ; function & subfunction mov bx,stdprn ; standard list handle mov dx,seg buff ; buffer address mov ds,dx mov dx,offset buff mov cx,ctllen ; length of control data int 21h ; transfer to MS-DOS jc error ; jump if write failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 44H (68) Subfunction 04H IOCTL: read control data from block-device driver ──────────────────────────────────────────────────────────────────────────── Transfers control data from a block-device driver directly into an application program's buffer. The length and contents of the data are specific to each device driver and do not follow any standard format. This function does not necessarily result in any input from the physical device. Call with: AH = 44H AL = 04H BL = drive code (0 = default, 1 = A, 2 = B, etc.) CX = number of bytes to read DS:DX = segment:offset of buffer Returns: If function successful Carry flag = clear AX = bytes transferred and buffer contains control data from device driver If function unsuccessful Carry flag = set AX = error code Notes: ■ When supported by the driver, this subfunction can be used to obtain hardware-dependent status and availability information that is not provided by other MS-DOS function calls. ■ Block-device drivers are not required to support IOCTL Subfunction 04H. If this subfunction is requested and the driver does not have the ability to process control data, control returns to the program with the carry flag set and error code 0001H (invalid function) in register AX. Example: Read a control string from the block-device driver for drive C into the buffer buff. buflen equ 64 ; length of buffer ctllen dw ? ; length of control string buff db buflen dup (0) ; receives control string . . . mov ax,4404h ; function & subfunction mov bl,3 ; drive C = 3 mov cx,buflen ; buffer length mov dx,seg buff ; buffer address mov ds,dx mov dx,offset buff int 21h ; transfer to MS-DOS jc error ; jump if read failed mov ctllen,ax ; save control string length . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 44H (68) Subfunction 05H IOCTL: write control data to block-device driver ──────────────────────────────────────────────────────────────────────────── Transfers control data from an application program directly to a block-device driver. The length and contents of the control data are specific to each device driver and do not follow any standard format. This function does not necessarily result in any output to the physical device. Call with: AH = 44H AL = 05H BL = drive code (0 = default, 1 = A, 2 = B, etc.) CX = number of bytes to write DS:DX = segment:offset of data Returns: If function successful Carry flag = clear AX = bytes transferred If function unsuccessful Carry flag = set AX = error code Notes: ■ When supported by the driver, this subfunction can be used to request hardware-dependent operations (such as tape rewind or disk eject) that are not provided by other MS-DOS function calls. ■ Block-device drivers are not required to support IOCTL Subfunction 05H. If this subfunction is requested and the driver does not have the ability to process control data, control returns to the program with the carry flag set and error code 0001H (invalid function) in register AX. Example: Write a control string from the buffer buff to the block-device driver for drive C. The length of the string is assumed to be in the variable ctllen. buflen equ 64 ; length of buffer ctllen dw ? ; length of control data buff db buflen dup (?) ; contains control data . . . mov ax,4405h ; function & subfunction mov bl,3 ; drive C = 3 mov dx,seg buff ; buffer address mov ds,dx mov dx,offset buff mov cx,ctllen ; length of control data int 21h ; transfer to MS-DOS jc error ; jump if write failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 44H (68) Subfunction 06H IOCTL: check input status ──────────────────────────────────────────────────────────────────────────── Returns a code indicating whether the device or file associated with a handle is ready for input. Call with: AH = 44H AL = 06H BX = handle Returns: If function successful Carry flag = clear and, for a device: AL = 00H if device not ready FFH if device ready or, for a file: AL = 00H if file pointer at EOF FFH if file pointer not at EOF If function unsuccessful Carry flag = set AX = error code Note: ■ This function can be used to check the status of character devices, such as the serial port, that do not have their own "traditional" MS-DOS status calls. Example: Check whether a character is ready from the standard auxiliary device (usually COM1). stdaux equ 3 ; standard auxiliary handle . . . mov ax,4406h ; function & subfunction mov bx,stdaux ; standard auxiliary handle int 21h ; transfer to MS-DOS jc error ; jump if function failed or al,al ; test status flag jnz ready ; jump if character ready . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 44H (68) Subfunction 07H IOCTL: check output status ──────────────────────────────────────────────────────────────────────────── Returns a code indicating whether the device associated with a handle is ready for output. Call with: AH = 44H AL = 07H BX = handle Returns: If function successful Carry flag = clear and, for a device: AL = 00H if device not ready FFH if device ready or, for a file: AL = FFH If function unsuccessful Carry flag = set AX = error code Note: ■ When used with a handle for a file, this function always returns a ready status, even if the disk is full or no disk is in the drive. Example: Check whether the standard auxiliary device (usually COM1) can accept a character for output. stdaux equ 3 ; standard auxiliary handle . . . mov ax,4407h ; function & subfunction mov bx,stdaux ; standard auxiliary handle int 21h ; transfer to MS-DOS jc error ; jump if function failed or al,al ; test status flag jnz ready ; jump if not busy . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.0] Function 44H (68) Subfunction 08H IOCTL: check if block device is removable ──────────────────────────────────────────────────────────────────────────── Checks whether the specified block device contains a removable storage medium, such as a floppy disk. Call with: AH = 44H AL = 08H BL = drive number (0 = default, 1 = A, 2 = B, etc.) Returns: If function successful Carry flag = clear AL = 00H if medium is removable 01H if medium is not removable If function unsuccessful Carry flag = set AX = error code Notes: ■ If a file is not found as expected on a particular drive, a program can use this subfunction to determine whether the user should be prompted to insert another disk. ■ This subfunction may not be used for a network drive. ■ Block drivers are not required to support Subfunction 08H. If this subfunction is requested and the block device cannot supply the information, control returns to the program with the carry flag set and error code 0001H (invalid function) in register AX. Example: Check whether drive C is removable. . . . mov ax,4408h ; function & subfunction mov bl,3 ; drive 3 = C int 21h ; transfer to MS-DOS jc error ; jump if function failed and al,1 ; test type of medium jnz fixed ; jump if not removable . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.1] Function 44H (68) Subfunction 09H IOCTL: check if block device is remote ──────────────────────────────────────────────────────────────────────────── Checks whether the specified block device is local (attached to the computer running the program) or remote (redirected to a network server). Call with: AH = 44H AL = 09H BL = drive number (0 = default, 1 = A, 2 = B, etc.) Returns: If function successful Carry flag = clear DX = device attribute word bit 12 = 0 if drive is local 1 if drive is remote If function unsuccessful Carry flag = set AX = error code Note: ■ Use of this subfunction should be avoided. Application programs should not distinguish between files on local and remote devices. Example: Check whether drive D is mounted on the machine running the program or is a network drive. . . . mov ax,4409h ; function & subfunction mov bl,4 ; drive 4 = D int 21h ; transfer to MS-DOS jc error ; jump if function failed and dx,1000h ; test local/remote bit jnz remote ; jump if network drive . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.1] Function 44H (68) Subfunction 0AH (10) IOCTL: check if handle is remote ──────────────────────────────────────────────────────────────────────────── Checks whether the specified handle refers to a file or device that is local (located on the PC that is running the program) or remote (located on a network server). Call with: AH = 44H AL = 0AH BX = handle Returns: If function successful Carry flag = clear DX = attribute word for file or device bit 15 = 0 if local 1 if remote If function unsuccessful Carry flag = set AX = error code Notes: ■ Application programs should not ordinarily attempt to distinguish between files on local and remote devices. ■ If the network has not been started, control returns to the calling program with the carry flag set and error code 0001H (invalid function) in register AX. Example: Check if the handle saved in the variable fhandle is associated with a file or device on the machine running the program or on a network server. fhandle dw ? ; device handle . . . mov ax,440ah ; function & subfunction mov bx,fhandle ; file/device handle int 21h ; transfer to MS-DOS jc error ; jump if function failed and dx,8000h ; test local/remote bit jnz remote ; jump if network handle . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.1] Function 44H (68) Subfunction 0BH (11) IOCTL: change sharing retry count ──────────────────────────────────────────────────────────────────────────── Sets the number of times MS-DOS retries a disk operation after a failure caused by a file-sharing violation before it returns an error to the requesting process. This subfunction is not available unless the file-sharing module (SHARE.EXE) is loaded. Call with: AH = 44H AL = 0BH CX = delays per retry (default = 1) DX = number of retries (default = 3) Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ The length of a delay is a machine-dependent value determined by the CPU type and clock speed. Each delay consists of the following instruction sequence: xor cx,cx loop $ which executes 65,536 times before falling out of the loop. ■ The sharing retry count affects the behavior of the system as a whole and is not a local parameter for the process. If a program changes the sharing retry count, it should restore the default values before terminating. Example: Change the number of automatic retries for a file-sharing violation to five. . . . mov ax,440bh ; function & subfunction mov cx,1 ; delays per retry mov dx,5 ; number of retries int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.2] Function 44H (68) Subfunction 0CH (12) IOCTL: generic I/O control for character devices ──────────────────────────────────────────────────────────────────────────── Provides a general-purpose mechanism for communication between application programs and character-device drivers. Call with: AH = 44H AL = 0CH BX = handle CH = category (major) code: 00H = unknown 01H = COM1, COM2, COM3, or COM4 (3.3) 03H = CON (keyboard and display) (3.3) 05H = LPT1, LPT2, or LPT3 (3.2) CL = function (minor) code: 45H = Set Iteration Count (3.2) 4AH = Select Code Page (3.3) 4CH = Start Code Page Preparation (3.3) 4DH = End Code Page Preparation (3.3) 5FH = Set Display Information (4.0) 65H = Get Iteration Count (3.2) 6AH = Query Selected Code Page (3.3) 6BH = Query Prepare List (3.3) 7FH = Get Display Information (4.0) DS:DX = segment:offset of parameter block Returns: If function successful Carry flag = clear and, if called with CL = 65H, 6AH, 6BH, or 7FH DS:DX = segment:offset of parameter block If function unsuccessful Carry flag = set AX = error code Notes: ■ If the minor code is 45H (Set Iteration Count) or 65H (Get Iteration Count), the parameter block is simply a 2-byte buffer containing or receiving the iteration count for the printer. This call is valid only for printer drivers that support Output Until Busy, and determines the number of times the device driver will wait for the device to signal ready before returning from the output call. ■ The parameter block for minor code 4DH (End Code Page Preparation) has the following format: dw 2 ; length of following data dw 0 ; (reserved) ■ For MS-DOS version 3.3, the parameter block for minor codes 4AH (Select Code Page) and 6AH (Query Code Page) has the following format: dw 2 ; length of following data dw ? ; code page ID For MS-DOS version 4.0, minor codes 4AH and 6AH also set or get the double-byte character set (DBCS) lead byte table, and the following format is used: dw (n+2)*2+1 ; length of following data dw ? ; code page ID db start,end ; DBCS lead byte range 1 . . . db start,end ; DBCS lead byte range n db 0,0 ■ The parameter block for minor code 4CH (Start Code Page Preparation) has the following format: dw 0 ; font type ; bit 0 = 0 downloaded ; = 1 cartridge ; bits 1-15 = reserved (0) dw (n+1)*2 ; length of remainder of ; parameter block dw n ; number of code pages in ; the following list dw ? ; code page 1 dw ? ; code page 2 . . . dw ? ; code page n ■ The parameter block for minor code 6BH (Query Prepare List) has the following format, assuming n hardware code pages and m prepared code pages (n <= 12, m <= 12): dw (n+m+2)*2 ; length of following data dw n ; number of hardware code pages dw ? ; hardware code page 1 dw ? ; hardware code page 2 . . . dw ? ; hardware code page n dw m ; number of prepared code pages dw ? ; prepared code page 1 dw ? ; prepared code page 2 . . . dw ? ; prepared code page m ■ After a minor code 4CH (Start Code Page Preparation) call, the data defining the code page font is written to the driver using one or more calls to the IOCTL Write Control Data subfunction (Interrupt 21H, Function 44H, Subfunction 03H). The format of the data is device- and driver-specific. After the font data has been written to the driver, a minor code 4DH (End Code Page Preparation) call must be issued. If no data is written to the driver between the minor code 4CH and 4DH calls, the driver interprets the newly prepared code pages as hardware code pages. ■ A special variation of the minor code 4CH (Start Code Page Preparation) call, called "Refresh," is required to actually load the peripheral device with the prepared code pages. The refresh operation is obtained by requesting minor code 4CH with each code page position in the parameter block set to -1, followed by an immediate call for minor code 4DH (End Code Page Preparation). ■ [4.0+] For minor codes 5FH (Set Display Information) and 7FH (Get Display Information), the parameter block is formatted as follows: db 0 ; level (0 in MS-DOS 4.0) db 0 ; reserved (must be 0) dw 14 ; length of following data dw ? ; control flags ; bit 0 = 0 intensity ; = 1 blink ; bits 1-15 = reserved (0) db ? ; mode type (1 = text, 2 = APA) db 0 ; reserved (must be 0) dw ? ; colors ; 0 = monochrome compatible ; 1 = 2 colors ; 2 = 4 colors ; 4 = 16 colors ; 8 = 256 colors dw ? ; pixel columns dw ? ; pixel rows dw ? ; character columns dw ? ; character rows Example: Get the current code page for the standard list device. stdprn equ 4 ; standard list handle pars dw 2 ; length of data dw ? ; receives code page . . . mov ax,440ch ; function & subfunction mov bx,stdprn ; standard list handle mov ch,5 ; LPTx category mov cl,6ah ; query code page mov dx,seg pars ; parameter block address mov ds,dx mov dx,offset pars int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.2] Function 44H Subfunction 0DH (13) IOCTL: generic I/O control for block devices ──────────────────────────────────────────────────────────────────────────── Provides a general-purpose mechanism for communication between application programs and block-device drivers. Allows a program to inspect or change device parameters for a logical drive and to read, write, format, and verify disk tracks in a hardware-independent manner. Call with: AH = 44H AL = 0DH BL = drive code (0 = default, 1 = A, 2 = B, etc.) CH = category (major) code: 08H = disk drive CL = function (minor) code: 40H = Set Device Parameters 41H = Write Track 42H = Format and Verify Track 47H = Set Access Flag (4.0) 60H = Get Device Parameters 61H = Read Track 62H = Verify Track 67H = Get Access Flag (4.0) DS:DX = segment:offset of parameter block Returns: If function successful Carry flag = clear and, if called with CL = 60H or 61H DS:DX = segment:offset of parameter block If function unsuccessful Carry flag = set AX = error code Notes: ■ The minor code 40H (Set Device Parameters) function must be used before an attempt to write, read, format, or verify a track on a logical drive. In general, the following sequence applies to any of these operations: ∙ Get the current parameters (minor code 60H). Examine and save them. ∙ Set the new parameters (minor code 40H). ∙ Perform the task. ∙ Retrieve the original parameters and restore them with minor code 40H. ■ For minor codes 40H (Set Device Parameters) and 60H (Get Device Parameters), the parameter block is formatted as follows: Special-functions field: offset 00H, length = 1 byte Bit(s) Value Meaning 0 0 device BPB field contains a new default BPB 1 use current BPB 1 0 use all fields in parameter block 1 use track layout field only 2 0 sectors in track may be different sizes (should always be avoided) 1 sectors in track are all same size; sector numbers range from 1 to the total number of sectors in the track (should always be used) 3─7 0 reserved Device type field: offset 01H, length 1 byte Value Meaning 0 320/360 KB, 5.25-inch disk 1 1.2 MB, 5.25-inch disk 2 720 KB, 3.5-inch disk 3 single-density, 8-inch disk 4 double-density, 8-inch disk 5 fixed disk 6 tape drive 7 other type of block device Device attributes field: offset 02H, length 1 word Bit(s) Value Meaning 0 0 removable storage medium 1 nonremovable storage medium 1 0 door lock not supported 1 door lock supported 2─15 0 reserved Number of cylinders field: offset 04H, length 1 word Maximum number of cylinders supported on the block device Media type field: offset 06H, length 1 byte Value Meaning 0 1.2 MB, 5.25-inch disk 1 320/360 KB, 5.25-inch disk Device BPB field: offset 07H, length 31 bytes For format of the device BPB, see separate Note below. If bit 0 = 0 in special-functions field, this field contains the new default BPB for the device. If bit 0 = 1 in special-functions field, the BPB in this field is returned by the device driver in response to subsequent Build BPB requests. Track layout field: offset 26H, variable-length table Length Meaning Word number of sectors in track Word number of first sector in track Word size of first sector in track . . . Word number of last sector in track Word size of last sector in track ■ The device BPB field is a 31-byte data structure that describes the current disk and its control areas. The field is formatted as follows: Byte(s) Meaning 00H─01H bytes per sector 02H sectors per cluster (allocation unit) 03─04H reserved sectors, beginning at sector 0 05H number of file allocation tables (FATs) 06H─07H maximum number of root-directory entries 08H─09H number of sectors 0AH media descriptor 0BH─0CH sectors per FAT 0DH─0EH sectors per track 0FH─10H number of heads 11H─14H number of hidden sectors 15H─18H large number of sectors (if bytes 08H─09H=0) 19H─1EH reserved ■ When minor code 40H (Set Device Parameters) is used, the number of cylinders should not be altered, or some or all of the volume may become inaccessible. ■ For minor codes 41H (Write Track) and 61H (Read Track), the parameter block is formatted as follows: Byte(s) Meaning 00H special-functions field (must be 0) 01H─02H head 03H─04H cylinder 05H─06H starting sector 07H─08H sectors to transfer 09H─0CH transfer buffer address ■ For minor codes 42H (Format and Verify Track) and 62H (Verify Track), the parameter block is formatted as follows: Byte(s) Meaning 00H special-functions field Bit(s) Significance 0 0 = Format/Verify track 1 = Format status call (MS-DOS 4.0 only) 1─7 reserved (0) 01H─02H head 03H─04H cylinder In MS-DOS 4.0, this function may be called with bit 0 of the special-functions field set after a minor code 40H call (Set Device Parameters) to determine whether the driver supports the specified number of tracks and sectors per track. A status is returned in the special-functions field which is interpreted as follows: Value Meaning 0 specified number of tracks and sectors per track supported 1 this function not supported by the ROM BIOS 2 specified number of tracks and sectors per track not supported 3 no disk in drive ■ For minor codes 47H (Set Access Flag) and 67H (Get Access Flag), the parameter block is formatted as follows: Byte Meaning 00H special-functions field (must be 0) 01H disk access flag When the disk access flag is zero, access to the medium is blocked by the driver. The flag is set to zero when the driver detects an unformatted medium or a medium with an invalid boot record. When the access flag is nonzero, read/write operations to the medium are allowed by the driver. A formatting program must clear the disk access flag with minor code 47H before it requests minor code 42H (Format and Verify Track). Example: Get the device parameter block for disk drive C. dbpb db 128 dup (0) ; device parameter block . . . mov ax,440dh ; function & subfunction mov bl,3 ; drive C = 3 mov ch,8 ; disk category mov cl,60h ; get device parameters mov dx,seg dbpb ; buffer address mov ds,dx mov dx,offset dbpb int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.2] Function 44H (68) Subfunction 0EH (14) IOCTL: get logical drive map ──────────────────────────────────────────────────────────────────────────── Returns the logical drive code that was most recently used to access the specified block device. Call with: AH = 44H AL = 0EH BL = drive code (0 = default, 1 = A, 2 = B, etc.) Returns: If function successful Carry flag = clear AL = mapping code 00H if only one logical drive code assigned to the block device 01H─1AH logical drive code (1 = A, 2 = B, etc.) mapped to the block device If function unsuccessful Carry flag = set AX = error code Note: ■ If a drive has not been assigned a logical mapping with Function 44H Subfunction 0FH, the logical and physical drive codes are the same. Example: Check whether drive A has more than one logical drive code. . . . mov ax,440eh ; function & subfunction mov bl,1 ; drive 1 = A int 21h ; transfer to MS-DOS jc error ; jump if function failed or al,al ; test drive code jz label1 ; jump, no drive aliases . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.2] Function 44H (68) Subfunction 0FH (15) IOCTL: set logical drive map ──────────────────────────────────────────────────────────────────────────── Sets the next logical drive code that will be used to reference a block device. Call with: AH = 44H AL = 0FH BL = drive code (0 = default, 1 = A, 2 = B, etc.) Returns: If function successful Carry flag = clear AL = mapping code 00H if only one logical drive code assigned to the block device 01H─1AH logical drive code (1 = A, 2 = B, etc.) mapped to the block device If function unsuccessful Carry flag = set AX = error code Note: ■ When a physical block device is aliased to more than one logical drive code, this function can be used to inform the driver which drive code will next be used to access the device. Example: Notify the floppy-disk driver that the next access will be for logical drive B. . . . mov ax,440fh ; function & subfunction mov bl,2 ; drive 2 = B int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 45H (69) Duplicate handle ──────────────────────────────────────────────────────────────────────────── Given a handle for a currently open device or file, returns a new handle that refers to the same device or file at the same position. Call with: AH = 45H BX = handle to be duplicated Returns: If function successful Carry flag = clear AX = new handle If function unsuccessful Carry flag = set AX = error code Notes: ■ A seek, read, or write operation that moves the file pointer for one of the two handles also moves the file pointer associated with the other. ■ This function can be used to efficiently update the directory for a file that has changed in length, without incurring the overhead of closing and then reopening the file. The handle for the file is simply duplicated with this function and the duplicate is closed, leaving the original handle open for further read/write operations. ■ [3.3] See also Int 21H Function 68H (Commit File). Example: Duplicate the handle stored in the variable fhandle, then close the duplicate. This ensures that all buffered data is physically written to disk and that the directory entry for the corresponding file is updated, but leaves the original handle open for subsequent file operations. fhandle dw 0 ; file handle . . . ; get duplicate handle mov ah,45h ; function number mov bx,fhandle ; original file handle int 21h ; transfer to MS-DOS jc error ; jump if dup failed ; now close dup'd handle mov bx,ax ; put handle into BX mov ah,3eh ; function number int 21h ; transfer to MS-DOS jc error ; jump if close failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 46H (70) Redirect handle ──────────────────────────────────────────────────────────────────────────── Given two handles, makes the second handle refer to the same device or file at the same location as the first handle. The second handle is then said to be redirected. Call with: AH = 46H BX = handle for file or device CX = handle to be redirected Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ If the handle passed in CX already refers to an open file, that file is closed first. ■ A seek, read, or write operation that moves the file pointer for one of the two handles also moves the file pointer associated with the other. ■ This function is commonly used to redirect the standard input and output handles to another file or device before a child process is executed with Int 21H Function 4BH. Example: Redirect the standard output to the list device, so that all output directed to the console will appear on the printer instead. Later, restore the original meaning of the standard output handle. stdin equ 0 stdout equ 1 stderr equ 2 stdaux equ 3 stdprn equ 4 dhandle dw 0 ; duplicate handle . . . ; get dup of stdout mov ah,45h ; function number mov bx,stdout ; standard output handle int 21h ; transfer to MS-DOS jc error ; jump if dup failed mov dhandle,ax ; save dup'd handle ; ; redirect standard output ; to standard list device mov ah,46h ; function number mov bx,stdprn ; standard list handle mov cx,stdout ; standard output handle int 21h ; transfer to MS-DOS jc error ; jump if redirect failed . . . ; restore standard output ; to original meaning mov ah,46h ; function number mov bx,dhandle ; saved duplicate handle mov cx,stdout ; standard output handle int 21h ; transfer to MS-DOS jc error ; jump if redirect failed ; close duplicate handle ; because no longer needed mov ah,3eh ; function number mov bx,dhandle ; saved duplicate handle int 21h ; transfer to MS-DOS jc error ; jump if close failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 47H (71) Get current directory ──────────────────────────────────────────────────────────────────────────── Obtains an ASCIIZ string that describes the path from the root to the current directory, and the name of that directory. Call with: AH = 47H DL = drive code (0 = default, 1 = A, etc.) DS:SI = segment:offset of 64-byte buffer Returns: If function successful Carry flag = clear and buffer is filled in with full pathname from root of current directory. If function unsuccessful Carry flag = set AX = error code Notes: ■ The returned path name does not include the drive identifier or a leading backslash (\). It is terminated with a null (00H) byte. Consequently, if the current directory is the root directory, the first byte in the buffer will contain 00H. ■ The function fails if the drive code is invalid. ■ The current directory may be set with Int 21H Function 3BH. Example: Get the name of the current directory for drive C into the buffer named dbuff. dbuff db 64 dup (0) ; receives path string . . . mov ah,47h ; function number mov dl,03 ; drive C = 3 mov si,seg dbuff ; buffer address mov ds,si mov si,offset dbuff int 21h ; transfer to MS-DOS jc error ; jump if error . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 48H (72) Allocate memory block ──────────────────────────────────────────────────────────────────────────── Allocates a block of memory and returns a pointer to the beginning of the allocated area. Call with: AH = 48H BX = number of paragraphs of memory needed Returns: If function successful Carry flag = clear AX = base segment address of allocated block If function unsuccessful Carry flag = set AX = error code BX = size of largest available block (paragraphs) Notes: ■ If the function succeeds, the base address of the newly allocated block is AX:0000. ■ The default allocation strategy used by MS-DOS is "first fit"; that is, the memory block at the lowest address that is large enough to satisfy the request is allocated. The allocation strategy can be altered with Int 21H Function 58H. ■ When a .COM program is loaded, it ordinarily already "owns" all of the memory in the transient program area, leaving none for dynamic allocation. The amount of memory initially allocated to a .EXE program at load time depends on the MINALLOC and MAXALLOC fields in the .EXE file header. See Int 21H Function 4AH. Example: Request a 64 KB block of memory for use as a buffer. bufseg dw ? ; segment base of new block . . . mov ah,48h ; function number mov bx,1000h ; block size (paragraphs) int 21h ; transfer to MS-DOS jc error ; jump if allocation failed mov bufseg,ax ; save segment of new block . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 49H (73) Release memory block ──────────────────────────────────────────────────────────────────────────── Releases a memory block and makes it available for use by other programs. Call with: AH = 49H ES = segment of block to be released Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ This function assumes that the memory block being released was previously obtained by a successful call to Int 21H Function 48H. ■ The function will fail or can cause unpredictable system errors if: ∙ the program releases a memory block that does not belong to it. ∙ the segment address passed in register ES is not a valid base address for an existing memory block. Example: Release the memory block that was previously allocated in the example for Int 21H Function 48H (page 438). bufseg dw ? ; segment base of block . . . mov ah,49h ; function number mov es,bufseg ; base segment of block int 21h ; transfer to MS-DOS jc error ; jump if release failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 4AH (74) Resize memory block ──────────────────────────────────────────────────────────────────────────── Dynamically shrinks or extends a memory block, according to the needs of an application program. Call with: AH = 4AH BX = desired new block size in paragraphs ES = segment of block to be modified Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code BX = maximum block size available (paragraphs) Notes: ■ This function modifies the size of a memory block that was previously allocated with a call to Int 21H Function 48H. ■ If the program is requesting an increase in the size of an allocated block, and this function fails, the maximum possible size for the specified block is returned in register BX. The program can use this value to determine whether it should terminate, or continue in a degraded fashion with less memory. ■ A program that uses EXEC (Int 21H Function 4BH) to load and execute a child program must call this function first to make memory available for the child, passing the address of its PSP in register ES and the amount of memory needed for its own code, data, and stacks in register BX. Example: Resize the memory block that was allocated in the example for Int 21H Function 48H (page 438), shrinking it to 32 KB. bufseg dw ? ; segment base of block . . . mov ah,4ah ; function number mov bx,0800h ; new size (paragraphs) mov es,bufseg ; segment base of block int 21h ; transfer to MS-DOS jc error ; jump, resize failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 4BH (75) Execute program (EXEC) ──────────────────────────────────────────────────────────────────────────── Allows an application program to run another program, regaining control when it is finished. Can also be used to load overlays, although this use is uncommon. Call with: AH = 4BH AL = subfunction 00H = Load and Execute Program 03H = Load Overlay ES:BX = segment:offset of parameter block DS:DX = segment:offset of ASCIIZ program pathname Returns: If function successful Carry flag = clear [2] all registers except for CS:IP may be destroyed [3.0+] registers are preserved in the usual fashion If function unsuccessful Carry flag = set AX = error code Notes: ■ The parameter block format for Subfunction 00H (Load and Execute Program) is as follows: Bytes Contents 00H─01H segment pointer to environment block 02H─03H offset of command line tail 04H─05H segment of command line tail 06H─07H offset of first FCB to be copied into new PSP + 5CH 08H─09H segment of first FCB 0AH─0BH offset of second FCB to be copied into new PSP + 6CH 0CH─0DH segment of second FCB ■ The parameter block format for Subfunction 03H (Load Overlay) is as follows: Bytes Contents 00H─01H segment address where overlay is to be loaded 02H─03H relocation factor to apply to loaded image ■ The environment block must be paragraph-aligned. It consists of a sequence of ASCIIZ strings in the form: db 'COMSPEC=A:\COMMAND.COM',0 The entire set of strings is terminated by an extra null (00H) byte. ■ The command tail format consists of a count byte, followed by an ASCII string, terminated by a carriage return (which is not included in the count). The first character in the string should be an ASCII space (20H) for compatibility with the command tail passed to programs by COMMAND.COM. For example: db 6,' *.DAT',0dh ■ Before a program uses Int 21H Function 4BH to run another program, it must release all memory it is not actually using with a call to Int 21H Function 4AH, passing the segment address of its own PSP and the number of paragraphs to retain. ■ Ordinarily, all active handles of the parent program are inherited by the child program, although the parent can prevent this in MS-DOS 3.0 and later by setting the inheritance bit when the file or device is opened. Any redirection of the standard input and/or output in the parent process also affects the child process. ■ The environment block can be used to pass information to the child process. If the environment block pointer in the parameter block is zero, the child program inherits an exact copy of the parent's environment. In any case, the segment address of the child's environment is found at offset 002CH in the child's PSP. ■ After return from the EXEC function call, the termination type and return code of the child program may be obtained with Int 21H Function 4DH. Example: See Chapter 12. ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 4CH (76) Terminate process with return code ──────────────────────────────────────────────────────────────────────────── Terminates the current process, passing a return code to the parent process. This is one of several methods that a program can use to perform a final exit. MS-DOS then takes the following actions: ■ All memory belonging to the process is released. ■ File buffers are flushed and any open handles for files or devices owned by the process are closed. ■ The termination handler vector (Int 22H) is restored from PSP:000AH. ■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH. ■ [2.0+] The critical-error handler vector (Int 24H) is restored from PSP:0012H. ■ Control is transferred to the termination handler. If the program is returning to COMMAND.COM, control transfers to the resident portion and the transient portion is reloaded if necessary. If a batch file is in progress, the next line of the file is fetched and interpreted; otherwise, a prompt is issued for the next user command. Call with: AH = 4CH AL = return code Returns: Nothing Notes: ■ [2.0+] This is the preferred method of termination for application programs because it allows a return code to be passed to the parent program and does not rely on the contents of any segment register. Other methods of performing a final exit are: ∙ Int 20H ∙ Int 21H Function 00H ∙ Int 21H Function 31H ∙ Int 27H ■ Any files that have been opened using FCBs and modified by the program should be closed before program termination; otherwise, data may be lost. ■ The return code can be retrieved by the parent process with Int 21H Function 4DH (Get Return Code). It can also be tested in a batch file with an IF ERRORLEVEL statement. By convention, a return code of zero indicates successful execution, and a non-zero return code indicates an error. ■ [3.0+] If the program is running on a network, it should remove all locks it has placed on file regions before terminating. Example: Terminate the current process, passing a return code of 1 to the parent process. . . . mov ah,4ch ; function number mov al,01h ; return code int 21h ; transfer to MS-DOS ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 4DH (77) Get return code ──────────────────────────────────────────────────────────────────────────── Used by a parent process, after the successful execution of an EXEC call (Int 21H Function 4BH), to obtain the return code and termination type of a child process. Call with: AH = 4DH Returns: AH = exit type 00H if normal termination by Int 20H, Int 21H Function 00H, or Int 21H Function 4CH 01H if termination by user's entry of CtrlDC 02H if termination by critical-error handler 03H if termination by Int 21H Function 31H or Int 27H AL = return code passed by child process (0 if child terminated by Int 20H, Int 21H Function 00H, or Int 27H) Notes: ■ This function will yield the return code of a child process only once. A subsequent call without an intervening EXEC (Int 21H Function 4BH) will not necessarily return any valid information. ■ This function does not set the carry flag to indicate an error. If no previous child process has been executed, the values returned in AL and AH are undefined. Example: Get the return code and termination kind of child process that was previously executed with Int 21H Function 4BH (EXEC). retcode dw ? ; return code, termination type . . . mov ah,4dh ; function number int 21h ; transfer to MS-DOS mov retcode,ax ; save child process info . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 4EH (78) Find first file ──────────────────────────────────────────────────────────────────────────── Given a file specification in the form of an ASCIIZ string, searches the default or specified directory on the default or specified drive for the first matching file. Call with: AH = 4EH CX = search attribute (bits may be combined) Bit(s) Significance (if set) 0 read-only 1 hidden 2 system 3 volume label 4 directory 5 archive 6─15 reserved (0) DS:DX = segment:offset of ASCIIZ pathname Returns: If function successful (matching file found) Carry flag = clear and search results returned in current disk transfer area as follows: Byte(s) Description 00H─14H reserved (0) 15H attribute of matched file or directory 16H─17H file time bits 00H─04H = 2-second increments (0─29) bits 05H─0AH = minutes (0─59) bits 0BH─0FH = hours (0─23) 18H─19H file date bits 00H─04H = day (1─31) bits 05H─08H = month (1─12) bits 09H─0FH = year (relative to 1980) 1AH─1DH file size 1EH─2AH ASCIIZ filename and extension If function unsuccessful (no matching files) Carry flag = set AX = error code Notes: ■ This function assumes that the DTA has been previously set by the program with Int 21H Function 1AH to point to a buffer of adequate size. ■ The * and ? wildcard characters are allowed in the filename. If wildcard characters are present, this function returns only the first matching filename. ■ If the attribute is 0, only ordinary files are found. If the volume label attribute bit is set, only volume labels will be returned (if any are present). Any other attribute or combination of attributes (hidden, system, and directory) results in those files and all normal files being matched. Example: Find the first .COM file in the directory \MYDIR on drive C. fname db 'C:\MYDIR\*.COM',0 dbuff db 43 dup (0) ; receives search results . . . ; set DTA address mov ah,1ah ; function number mov dx,seg dbuff ; result buffer address mov ds,dx mov dx,offset dbuff int 21h ; transfer to MS-DOS ; search for first match mov ah,4eh ; function number mov cx,0 ; normal attribute mov dx,seg fname ; address of filename mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if no match . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 4FH (79) Find next file ──────────────────────────────────────────────────────────────────────────── Assuming a previous successful call to Int 21H Function 4EH, finds the next file in the default or specified directory on the default or specified drive that matches the original file specification. Call with: AH = 4FH Assumes DTA points to working buffer used by previous successful Int 21H Function 4EH or 4FH. Returns: If function successful (matching file found) Carry flag = clear and search results returned in current disk transfer area as described for Int 21H Function 4EH If function unsuccessful (no more matching files) Carry flag = set AX = error code Notes: ■ Use of this call assumes that the original file specification passed to Int 21H Function 4EH contained one or more * or ? wildcard characters. ■ When this function is called, the current disk transfer area (DTA) must contain information from a previous successful call to Int 21H Function 4EH or 4FH. Example: Continuing the search operation in the example for Int 21H Function 4EH, find the next .COM file (if any) in the directory \MYDIR on drive C. fname db 'C:\MYDIR\*.COM',0 dbuff db 43 dup (0) ; receives search results . . . ; search for next match mov ah,4fh ; function number int 21h ; transfer to MS-DOS jc error ; jump if no more files . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 50H (80) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H Function 51H (81) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H Function 52H (82) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H Function 53H (83) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 54H (84) Get verify flag ──────────────────────────────────────────────────────────────────────────── Obtains the current value of the system verify (read-after-write) flag. Call with: AH = 54H Returns: AL = current verify flag value 00H if verify off 01H if verify on Notes: ■ Because read-after-write verification slows disk operations, the default state of the system verify flag is OFF. ■ The state of the system verify flag can be changed through a call to Int 21H Function 2EH or by the MS-DOS commands VERIFY ON and VERIFY OFF. Example: Obtain the state of the system verify flag. . . . mov ah,54h ; function number int 21h ; transfer to MS-DOS cmp al,01h ; check verify state je label1 ; jump if verify on ; else assume verify off . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 55H (85) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 56H (86) Rename file ──────────────────────────────────────────────────────────────────────────── Renames a file and/or moves its directory entry to a different directory on the same disk. In MS-DOS version 3.0 and later, this function can also be used to rename directories. Call with: AH = 56H DS:DX = segment:offset of current ASCIIZ pathname ES:DI = segment:offset of new ASCIIZ pathname Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ The function fails if: ∙ any element of the pathname does not exist. ∙ a file with the new pathname already exists. ∙ the current pathname specification contains a different disk drive than does the new pathname. ∙ the file is being moved to the root directory, and the root directory is full. ∙ [3.0+] the program is running on a network and the user has insufficient access rights to either the existing file or the new directory. ■ The * and ? wildcard characters are not allowed in either the current or new pathname specifications. Example: Change the name of the file MYFILE.DAT in the directory \MYDIR on drive C to MYTEXT.DAT. At the same time, move the file to the directory \SYSTEM on the same drive. oldname db 'C:\MYDIR\MYFILE.DAT',0 newname db 'C:\SYSTEM\MYTEXT.DAT',0 . . . mov ah,56h ; function number mov dx,seg oldname ; old filename address mov ds,dx mov dx,offset oldname mov di,seg newname ; new filename address mov es,di mov di,offset newname int 21h ; transfer to MS-DOS jc error ; jump if rename failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.0] Function 57H (87) Get or set file date and time ──────────────────────────────────────────────────────────────────────────── Obtains or modifies the date and time stamp in a file's directory entry. Call with: If getting date and time AH = 57H AL = 00H BX = handle If setting date and time AH = 57H AL = 01H BX = handle CX = time bits 00H─04H = 2-second increments (0─29) bits 05H─0AH = minutes (0─59) bits 0BH─0FH = hours (0─23) DX = date bits 00H─04H = day (1─31) bits 05H─08H = month (1─12) bits 09H─0FH = year (relative to 1980) Returns: If function successful Carry flag = clear and, if called with AL = 00H CX = time DX = date If function unsuccessful Carry flag = set AX = error code Notes: ■ The file must have been previously opened or created via a successful call to Int 21H Function 3CH, 3DH, 5AH, 5BH, or 6CH. ■ If the 16-bit date for a file is set to zero, that file's date and time are not displayed on directory listings. ■ A date and time set with this function will prevail, even if the file is modified afterwards before the handle is closed. Example: Get the date that the file MYFILE.DAT was created or last modified, and then decompose the packed date into its constituent parts in the variables month, day, and year. fname db 'MYFILE.DAT',0 month dw 0 day dw 0 year dw 0 . . . ; first open the file mov ah,3dh ; function number mov al,0 ; read-only mode mov dx,seg fname ; filename address mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if open failed ; get file date/time mov bx,ax ; copy handle to BX mov ah,57h ; function number mov al,0 ; 0 = get subfunction int 21h ; transfer to MS-DOS jc error ; jump if function failed mov day,dx ; decompose date and day,01fh ; isolate day mov cl,5 shr dx,cl mov month,dx ; isolate month and month,0fh mov cl,4 shr dx,cl ; isolate year and dx,03fh ; relative to 1980 add dx,1980 ; correct to real year mov year,dx ; save year ; now close file, ; handle still in BX mov ah,3eh ; function number int 21h ; transfer to MS-DOS jc error ; jump if close failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.0] Function 58H (88) Get or set allocation strategy ──────────────────────────────────────────────────────────────────────────── Obtains or changes the code indicating the current MS-DOS strategy for allocating memory blocks. Call with: If getting strategy code AH = 58H AL = 00H If setting strategy code AH = 58H AL = 01H BX = desired strategy code 00H = first fit 01H = best fit 02H = last fit Returns: If function successful Carry flag = clear and, if called with AL = 00H AX = current strategy code If function unsuccessful Carry flag = set AX = error code Notes: ■ The memory allocation strategies are: ∙ First fit: MS-DOS searches the available memory blocks from low addresses to high addresses, assigning the first one large enough to satisfy the block allocation request. ∙ Best fit: MS-DOS searches all available memory blocks and assigns the smallest available block that will satisfy the request, regardless of its position. ∙ Last fit: MS-DOS searches the available memory blocks from high addresses to low addresses, assigning the highest one large enough to satisfy the block allocation request. ■ The default MS-DOS memory allocation strategy is First Fit (code 0). Example: Save the code indicating the current memory allocation strategy in the variable strat, then change the system's memory allocation strategy to "best fit." strat dw 0 ; previous strategy code . . . ; get current strategy mov ah,58h ; function number mov al,0 ; 0 = get strategy int 21h ; transfer to MS-DOS jc error ; jump if function failed mov strat,ax ; save strategy code ; now set new strategy mov ah,58h ; function number mov al,1 ; 1 = set strategy mov bx,1 ; 1 = best fit int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.0] Function 59H (89) Get extended error information ──────────────────────────────────────────────────────────────────────────── Obtains detailed error information after a previous unsuccessful Int 21H function call, including the recommended remedial action. Call with: AH = 59H BX = 00H Returns: AX = extended error code 01H function number invalid 02H file not found 03H path not found 04H too many open files 05H access denied 06H handle invalid 07H memory control blocks destroyed 08H insufficient memory 09H memory block address invalid 0AH (10) environment invalid 0BH (11) format invalid 0CH (12) access code invalid 0DH (13) data invalid 0EH (14) unknown unit 0FH (15) disk drive invalid 10H (16) attempted to remove current directory 11H (17) not same device 12H (18) no more files 13H (19) disk write-protected 14H (20) unknown unit 15H (21) drive not ready 16H (22) unknown command 17H (23) data error (CRC) 18H (24) bad request structure length 19H (25) seek error 1AH (26) unknown media type 1BH (27) sector not found 1CH (28) printer out of paper 1DH (29) write fault 1EH (30) read fault 1FH (31) general failure 20H (32) sharing violation 21H (33) lock violation 22H (34) disk change invalid 23H (35) FCB unavailable 24H (36) sharing buffer exceeded 25H─31H reserved 32H (50) unsupported network request 33H (51) remote machine not listening 34H (52) duplicate name on network 35H (53) network name not found 36H (54) network busy 37H (55) device no longer exists on network 38H (56) netBIOS command limit exceeded 39H (57) error in network adapter hardware 3AH (58) incorrect response from network 3BH (59) unexpected network error 3CH (60) remote adapter incompatible 3DH (61) print queue full 3EH (62) not enough space for print file 3FH (63) print file canceled 40H (64) network name deleted 41H (65) network access denied 42H (66) incorrect network device type 43H (67) network name not found 44H (68) network name limit exceeded 45H (69) netBIOS session limit exceeded 46H (70) file sharing temporarily paused 47H (71) network request not accepted 48H (72) print or disk redirection paused 49H─4FH reserved 50H (80) file already exists 51H (81) reserved 52H (82) cannot make directory 53H (83) fail on Int 24H (critical error) 54H (84) too many redirections 55H (85) duplicate redirection 56H (86) invalid password 57H (87) invalid parameter 58H (88) network device fault 59H (89) function not supported by network 5AH (90) required system component not installed BH = error class 01H if out of resource (such as storage or handles) 02H if not error, but temporary situation (such as locked region in file) that can be expected to end 03H if authorization problem 04H if internal error in system software 05H if hardware failure 06H if system software failure not the fault of the active process (such as missing configuration files) 07H if application program error 08H if file or item not found 09H if file or item of invalid type or format 0AH (10) if file or item locked 0BH (11) if wrong disk in drive, bad spot on disk, or storage medium problem 0CH (12) if item already exists 0DH (13) unknown error BL = recommended action 01H retry reasonable number of times, then prompt user to select abort or ignore 02H retry reasonable number of times with delay between retries, then prompt user to select abort or ignore 03H get corrected information from user (typically caused by incorrect filename or drive specification) 04H abort application with cleanup (i.e., terminate the program in as orderly a manner as possible: releasing locks, closing files, etc.) 05H perform immediate exit without cleanup 06H ignore error 07H retry after user intervention to remove cause of error CH = error locus 01H unknown 02H block device (disk or disk emulator) 03H network 04H serial device 05H memory and, for MS-DOS 3.0 and later, ES:DI = ASCIIZ volume label of disk to insert, if AX = 0022H (invalid disk change) Notes: ■ This function may be called after any other Int 21H function call that returned an error status, in order to obtain more detailed information about the error type and the recommended action. If the previous Int 21H function call had no error, 0000H is returned in register AX. This function may also be called during the execution of a critical-error (Int 24H) handler. ■ The contents of registers CL, DX, SI, DI, BP, DS, and ES are destroyed by this function. ■ Note that extended error codes 13H─1FH (19─31) and 34 (22H) correspond exactly to the error codes 0─0CH (0─12) and 0FH (15) returned by Int 24H. ■ You should not code your programs to recognize only specific error numbers if you wish to ensure upward compatibility, because new error codes are added in each version of MS-DOS. Example: Attempt to open the file named NOSUCH.DAT using a file control block; if the open request fails, get the extended error code. myfcb db 0 ; drive = default db 'NOSUCH ' ; filename, 8 chars db 'DAT' ; extension, 3 chars db 25 dup (0) ; remainder of FCB . . . label1: ; open the file mov ah,0fh ; function number mov dx,seg myfcb ; address of FCB mov ds,dx mov dx,offset myfcb int 21h ; transfer to MS-DOS or al,al ; check open status jz success ; jump if opened OK ; open failed, get ; extended error info mov ah,59h ; function number xor bx,bx ; BX must = 0 int 21h ; transfer to MS-DOS or ax,ax ; double check for error jz success ; jump if no error cmp bl,2 ; should we retry? jle label1 ; yes, jump jmp error ; no, give up . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.0] Function 5AH (90) Create temporary file ──────────────────────────────────────────────────────────────────────────── Creates a file with a unique name, in the current or specified directory on the default or specified disk drive, and returns a handle that can be used by the program for subsequent access to the file. The name generated for the file is also returned in a buffer specified by the program. Call with: AH = 5AH CX = attribute (bits may be combined) Bit(s) Significance (if set) 0 read-only 1 hidden 2 system 3─4 reserved (0) 5 archive 6─15 reserved (0) DS:DX = segment:offset of ASCIIZ path Returns: If function successful Carry flag = clear AX = handle DS:DX = segment:offset of complete ASCIIZ pathname If function unsuccessful Carry flag = set AX = error code Notes: ■ The ASCIIZ path supplied to this function should be followed by at least 13 additional bytes of buffer space. MS-DOS adds a backslash (\) to the supplied path, if necessary, then appends a null-terminated filename that is a function of the current time. ■ Files created with this function are not automatically deleted when the calling program terminates. ■ The function fails if ∙ any element of the pathname does not exist. ∙ the file is being created in the root directory, and the root directory is full. ■ See also Int 21H Functions 3CH, 5BH, and 6CH, which provide additional facilities for creating files. ■ [3.0+] If the program is running on a network, the file is created and opened for read/write access in compatibility sharing mode. Example: Create a temporary file with a unique name and normal attribute in directory \TEMP of drive C. Note that you must allow room for MS-DOS to append the generated filename to the supplied path. The complete file specification should be used to delete the temporary file before your program terminates. fname db 'C:\TEMP\' ; pathname for temp file db 13 dup (0) ; receives filename fhandle dw ? ; file handle . . . mov ah,5ah ; function number mov cx,0 ; normal attribute mov dx,seg fname ; address of pathname mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if create failed mov fhandle,ax ; save file handle . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.0] Function 5BH (91) Create new file ──────────────────────────────────────────────────────────────────────────── Given an ASCIIZ pathname, creates a file in the designated or default directory on the designated or default drive, and returns a handle that can be used by the program for subsequent access to the file. If a file with the same name already exists, the function fails. Call with: AH = 5BH CX = attribute (bits may be combined) Bit(s) Significance (if set) 0 read-only 1 hidden 2 system 3 volume label 4 reserved (0) 5 archive 6─15 reserved (0) DS:DX = segment:offset of ASCIIZ pathname Returns: If function successful Carry flag = clear AX = handle If function unsuccessful Carry flag = set AX = error code Notes: ■ The function fails if: ∙ any element of the specified path does not exist. ∙ a file with the identical pathname (i.e., the same filename and extension in the same location in the directory structure) already exists. ∙ the file is being created in the root directory, and the root directory is full. ∙ [3.0+] the program is running on a network, and the user has insufficient access rights to the directory that will contain the file. ■ The file is usually given a normal attribute (0) when it is created, and is opened for both read and write operations. The attribute can subsequently be modified with Int 21H Function 43H. ■ See also Int 21H Functions 3CH, 5AH, and 6CH, which provide alternative ways of creating files. ■ This function may be used to implement semaphores in a network or multitasking environment. If the function succeeds, the program has acquired the semaphore. To release the semaphore, the program simply deletes the file. Example: Create and open a file named MYFILE.DAT in the directory \MYDIR on drive C; MS-DOS returns an error if a file with the same name already exists in that location. fname db 'C:\MYDIR\MYFILE.DAT',0 fhandle dw ? ; file handle . . . mov ah,5bh ; function number xor cx,cx ; normal attribute mov dx,seg fname ; filename address mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if create failed mov fhandle,ax ; save file handle . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.0] Function 5CH (92) Lock or unlock file region ──────────────────────────────────────────────────────────────────────────── Locks or unlocks the specified region of a file. This function is not available unless the file-sharing module (SHARE.EXE) is loaded. Call with: AH = 5CH AL = 00H if locking region 01H if unlocking region BX = handle CX = high part of region offset DX = low part of region offset SI = high part of region length DI = low part of region length Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ This function is useful for file and record synchronization in a multitasking environment or network. Access to the file as a whole is controlled by the attribute and file-sharing parameters passed in open or create calls and by the file's attributes, which are stored in its directory entry. ■ The beginning location in the file to be locked or unlocked is supplied as a positive double precision integer, which is a byte offset into the file. The length of the region to be locked or unlocked is similarly supplied as a positive double precision integer. ■ For every call to lock a region of a file, there must be a subsequent unlock call with exactly the same file offset and length. ■ Locking beyond the current end of file is not an error. ■ Duplicate handles created with Int 21H Function 45H, or handles redirected to the file with Int 21H Function 46H, are allowed access to locked regions within the same process. ■ Programs that are loaded with the EXEC call (Int 21H Function 4BH) inherit the handles of their parent but not any active locks. ■ If a process terminates without releasing active locks on a file, the result is undefined. Therefore, programs using this function should install their own Int 23H and Int 24H handlers so that they cannot be terminated unexpectedly. Example: Assume that a file was previously opened and that its handle was saved in the variable fhandle. Lock a 4096 byte region of the file, starting at 32,768 bytes from the beginning of the file, so that it cannot be accessed by other programs. fhandle dw ? ; file handle . . . mov ah,5ch ; function number mov al,0 ; subfunction 0 = lock mov bx,fhandle ; file handle mov cx,0 ; upper part of offset mov dx,32768 ; lower part of offset mov si,0 ; upper part of length mov di,4096 ; lower part of length int 21h ; transfer to MS-DOS jc error ; jump if lock failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 5DH (93) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [3.1] Function 5EH (94) Subfunction 00H Get machine name ──────────────────────────────────────────────────────────────────────────── Returns the address of an ASCIIZ (null-terminated) string identifying the local computer. This function call is only available when Microsoft Networks is running. Call with: AH = 5EH AL = 00H DS:DX = segment:offset of buffer to receive string Returns: If function successful Carry flag = clear CH = 00H if name not defined <> 00H if name defined CL = netBIOS name number (if CH <> 0) DX:DX = segment:offset of identifier (if CH <> 0 ) If function unsuccessful Carry flag = set AX = error code Notes: ■ The computer identifier is a 15-byte string, padded with spaces and terminated with a null (00H) byte. ■ The effect of this call is unpredictable if the file-sharing support module is not loaded. Example: Get the machine name of the local computer into the buffer named mname. mname db 16 dup (?) . . . mov ax,5e00h ; function & subfunction mov dx,seg mname ; address of buffer mov ds,dx mov dx,offset mname int 21h ; transfer to MS-DOS jc error ; jump if function failed or ch,ch ; make sure name exists jz error ; jump if no name defined . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.1] Function 5EH (94) Subfunction 02H Set printer setup string ──────────────────────────────────────────────────────────────────────────── Specifies a string to be sent in front of all files directed to a particular network printer, allowing users at different network nodes to specify individualized operating modes on the same printer. This function call is only available when Microsoft Networks is running. Call with: AH = 5EH AL = 02H BX = redirection list index CX = length of setup string DS:SI = segment:offset of setup string Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ The redirection list index passed in register BX is obtained with Function 5FH Subfunction 02H (Get Redirection List Entry). ■ See also Function 5EH Subfunction 03H, which may be used to obtain the existing setup string for a particular network printer. Example: Initialize the setup string for the printer designated by redirection list index 2 so that the device is put into boldface mode before printing a file requested by this network node. setup db 01bh,045h ; selects boldface mode . . . mov ax,5e02h ; function & subfunction mov bx,2 ; redirection list index 2 mov cx,2 ; length of setup string mov si,seg setup ; address of setup string mov ds,si mov si,offset setup int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.1] Function 5EH (94) Subfunction 03H Get printer setup string ──────────────────────────────────────────────────────────────────────────── Obtains the printer setup string for a particular network printer. This function call is only available when Microsoft Networks is running. Call with: AH = 5EH AL = 03H BX = redirection list index ES:DI = segment:offset of buffer to receive setup string Returns: If function successful Carry flag = clear CX = length of printer setup string ES:DI = address of buffer holding setup string If function unsuccessful Carry flag = set AX = error code Notes: ■ The redirection list index passed in register BX is obtained with Function 5FH Subfunction 02H (Get Redirection List Entry). ■ See also Int 21H Function 5EH Subfunction 02H, which is used to specify a setup string for a network printer. Example: Get the setup string for this network node associated with the printer designated by redirection list index 2. setup db 64 dup (?) ; receives setup string . . . mov ax,5e03h ; function & subfunction mov bx,2 ; redirection list index 2 mov di,seg setup ; address of buffer mov es,di mov di,offset setup int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.1] Function 5FH (95) Subfunction 02H Get redirection list entry ──────────────────────────────────────────────────────────────────────────── Allows inspection of the system redirection list, which associates local logical names with network files, directories, or printers. This function call is only available when Microsoft Networks is running and the file-sharing module (SHARE.EXE) has been loaded. Call with: AH = 5FH AL = 02H BX = redirection list index DS:SI = segment:offset of 16-byte buffer to receive local device name ES:DI = segment:offset of 128-byte buffer to receive network name Returns: If function successful Carry flag = clear BH = device status flag bit 0 = 0 if device valid = 1 if not valid BL = device type 03H if printer 04H if drive CX = stored parameter value DX = destroyed BP = destroyed DS:SI = segment:offset of ASCIIZ local device name ES:DI = segment:offset of ASCIIZ network name If function unsuccessful Carry flag = set AX = error code Note: ■ The parameter returned in CX is a value that was previously passed to MS-DOS in register CX with Int 21H Function 5FH Subfunction 03H (Redirect Device). It represents data that is private to the applications which store and retrieve it and has no meaning to MS-DOS. Example: Get the local and network names for the device specified by the first redirection list entry. local db 16 dup (?) ; receives local device name network db 128 dup (?) ; receives network name . . . mov ax,5f02h ; function & subfunction mov bx,0 ; redirection list entry 0 mov si,seg local ; local name buffer addr mov ds,si mov si,offset local mov di,seg network ; network name buffer addr mov es,di mov di,offset network int 21h ; transfer to MS-DOS jc error ; jump if call failed or bh,bh ; check device status jnz error ; jump if device not valid . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.1] Function 5FH (95) Subfunction 03H Redirect device ──────────────────────────────────────────────────────────────────────────── Establishes redirection across the network by associating a local device name with a network name. This function call is only available when Microsoft Networks is running and the file-sharing module (SHARE.EXE) has been loaded. Call with: AH = 5FH AL = 03H BL = device type 03H if printer 04H if drive CX = parameter to save for caller DS:SI = segment:offset of ASCIIZ local device name ES:DI = segment:offset of ASCIIZ network name, followed by ASCIIZ password Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ The local name can be a drive designator (a letter followed by a colon, such as "D:"), a printer name, or a null string. Printer names must be one of the following: PRN, LPT1, LPT2, or LPT3. If a null string followed by a password is used, MS-DOS attempts to grant access to the network directory with the specified password. ■ The parameter passed in CX can be retrieved by later calls to Int 21H Function 5FH Subfunction 02H. It represents data that is private to the applications which store and retrieve it and has no meaning to MS-DOS. Example: Redirect the local drive E to the directory \FORTH on the server named LMI, using the password FRED. locname db 'E:',0 ; local drive netname db '\\LMI\FORTH',0 db 'FRED',0 . . . mov ax,5f03h ; function & subfunction mov bl,4 ; code 4 = disk drive mov si,seg locname ; address of local name mov ds,si mov si,offset locname mov di,seg netname ; address of network name mov es,di mov di,offset netname int 21h ; transfer to MS-DOS jc error ; jump if redirect failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.1] Function 5FH (95) Subfunction 04H Cancel device redirection ──────────────────────────────────────────────────────────────────────────── Cancels a previous redirection request by removing the association of a local device name with a network name. This function call is only available when Microsoft Networks is running and the file-sharing module (SHARE.EXE) has been loaded. Call with: AH = 5FH AL = 04H DS:SI = segment:offset of ASCIIZ local device name Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Note: ■ The supplied name can be a drive designator (a letter followed by a colon, such as "D:"), a printer name, or a string starting with two backslashes (\\). Printer names must be one of the following: PRN, LPT1, LPT2, or LPT3. If the string with two backslashes is used, the connection between the local machine and the network directory is terminated. Example: Cancel the redirection of local drive E to the network server. locname db 'E:',0 . . . mov ax,5f04h ; function & subfunction mov si,seg locname ; address of local name mov ds,si mov si,offset locname int 21h ; transfer to MS-DOS jc error ; jump if cancel failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 60H (96) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H Function 61H (97) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [3.0] Function 62H (98) Get PSP address ──────────────────────────────────────────────────────────────────────────── Obtains the segment (paragraph) address of the program segment prefix (PSP) for the currently executing program. Call with: AH = 62H Returns: BX = segment address of program segment prefix Notes: ■ Before a program receives control from MS-DOS, its program segment prefix is set up to contain certain vital information, such as: ∙ the segment address of the program's environment block ∙ the command line originally entered by the user ∙ the original contents of the terminate, Ctrl-C, and critical-error handler vectors ∙ the top address of available RAM ■ The segment address of the PSP is normally passed to the program in registers DS and ES when it initially receives control from MS-DOS. This function allows a program to conveniently recover the PSP address at any point during its execution, without having to save it at program entry. Example: Get the segment base of the program segment prefix, then copy the command tail from the PSP into the local buffer named buff. ctail equ 080H ; PSP offset, command tail buff db 80 dup (?) ; copy of command tail . . . ; get PSP address mov ah,62H ; function number int 21h ; transfer to MS-DOS ; copy command tail mov ds,bx ; PSP segment to DS mov si,offset ctail ; offset of command tail mov di,seg buff ; local buffer address mov es,di mov di,offset buff mov cl,[si] ; length of command tail inc cl ; include count byte xor ch,ch cld rep movsb ; copy to local buffer . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [2.25 only] Function 63H (99) Get DBCS lead byte table ──────────────────────────────────────────────────────────────────────────── Obtains the address of the system table of legal lead byte ranges for double-byte character sets (DBCS), or sets or obtains the interim console flag. Int 21H Function 63H is available only in MS-DOS version 2.25; it is not supported in MS-DOS versions 3.0 and later. Call with: AH = 63H AL = subfunction 00H if getting address of DBCS lead byte table 01H if setting or clearing interim console flag 02H if obtaining value of interim console flag If AL = 01H DL = 00H if clearing interim console flag 01H if setting interim console flag Returns: If function successful Carry flag = clear and, if called with AL = 00H DS:SI = segment:offset of DBCS lead byte table or, if called with AL = 02H DL = value of interim console flag If function unsuccessful Carry flag = set AX = error code Notes: ■ The DBCS lead byte table consists of a variable number of two byte entries, terminated by two null (00H) bytes. Each pair defines the beginning and ending value for a range of lead bytes. The value of a legal lead byte is always in the range 80─0FFH. ■ Entries in the lead byte table must be in ascending order. If no legal lead bytes are defined in a given system, the table consists only of the two null bytes. ■ If the interim console flag is set, Int 21H Functions 07H (Unfiltered Character Input), 08H (Character Input without Echo), and 0BH (Keyboard Status) will support interim characters. ■ Unlike most other MS-DOS services, this function call does not necessarily preserve any registers except SS:SP. ■ [4.0] The address of the DBCS lead byte table can also be obtained with Int 21H Function 65H. ──────────────────────────────────────────────────────────────────────────── Int 21H Function 64H (100) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [3.3] Function 65H (101) Get extended country information ──────────────────────────────────────────────────────────────────────────── Obtains information about the specified country and/or code page. Call with: AH = 65H AL = subfunction 01H = Get General Internationalization Information 02H = Get Pointer to Uppercase Table 04H = Get Pointer to Filename Uppercase Table 06H = Get Pointer to Collating Table 07H = Get Pointer to Double-Byte Character Set (DBCS) Vector (MS-DOS versions 4.0 and later) BX = code page of interest (-1 = active CON device) CX = length of buffer to receive information (must be >= 5) DX = country ID (-1 = default) ES:DI = address of buffer to receive information Returns: If function successful Carry flag = clear and requested data placed in calling program's buffer If function unsuccessful Carry flag = set AX = error code Notes: ■ The information returned by this function is a superset of the information returned by Int 21H Function 38H. ■ This function may fail if either the country code or the code page number is invalid, or if the code page does not match the country code. ■ The function fails if the specified buffer length is less than five bytes. If the buffer to receive the information is at least five bytes long but is too short for the requested information, the data is truncated and no error is returned. ■ The format of the data returned by Subfunction 01H is: Byte(s) Contents 00H information ID code (1) 01H─02H length of following buffer 03H─04H country ID 05H─06H code page number 07H─08H date format 0 = USA m d y 1 = Europe d m y 2 = Japan y m d 09H─0DH ASCIIZ currency symbol 0EH─0FH ASCIIZ thousands separator 10H─11H ASCIIZ decimal separator 12H─13H ASCIIZ date separator 14H─15H ASCIIZ time separator 16H currency format flags bit 0 =>0 if currency symbol precedes value =>1 if currency symbol follows value bit 1 =>0 if no space between value and currency symbol =>1 if one space between value and currency symbol bit 2 =>0 if currency symbol and decimal are separate =>1 if currency symbol replaces decimal separator 17H number of digits after decimal in currency 18H time format bit 0 = 0 if 12-hour clock = 1 if 24-hour clock 19H─1CH case-map routine call address 1DH─1EH ASCIIZ data list separator 1FH─28H reserved ■ The format of the data returned by Subfunctions 02H, 04H, 06H, and 07H is: Byte(s) Contents 00H information ID code (2, 4, or 6) 01H─05H double-word pointer to table ■ The uppercase and filename uppercase tables are a maximum of 130 bytes long. The first two bytes contain the size of the table; the following bytes contain the uppercase equivalents, if any, for character codes 80H─FFH. The main use of these tables is to map accented or otherwise modified vowels to their plain vowel equivalents. Text translated with the help of this table can be sent to devices that do not support the IBM graphics character set, or used to create filenames that do not require a special keyboard configuration for entry. ■ The collating table is a maximum of 258 bytes long. The first two bytes contain the table length, and the subsequent bytes contain the values to be used for the corresponding character codes (0─FFH) during a sort operation. This table maps uppercase and lowercase ASCII characters to the same collating codes so that sorts will be case-insensitive, and it maps accented vowels to their plain vowel equivalents. ■ [4.0+] Subfunction 07H returns a pointer to a variable length table of that defines ranges for double-byte character set (DBCS) lead bytes. The table is terminated by a pair of zero bytes, unless it must be truncated to fit in the buffer, and has the following format: dw length db start1,end1 db start2,end2 . . . db 0,0 For example: dw 4 db 81h,9fh db 0e0h,0fch db 0,0 ■ In some cases a truncated translation table may be presented to the program by MS-DOS. Applications should always check the length at the beginning of the table, to make sure it contains a translation code for the particular character of interest. Examples: Obtain the extended country information associated with the default country and code page 437. buffer db 41 dup (0) ; receives country info . . . mov ax,6501h ; function & subfunction mov bx,437 ; code page mov cx,41 ; buffer length mov dx,-1 ; default country mov di,seg buffer ; buffer address mov es,di mov di,offset buffer int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . In this case, MS-DOS filled the following extended country information into the buffer: buffer db 1 ; info ID code dw 38 ; length of following buffer dw 1 ; country ID (USA) dw 437 ; code page number dw 0 ; date format db '$',0,0,0,0 ; currency symbol db ',',0 ; thousands separator db '.',0 ; decimal separator db '-',0 ; date separator db ':',0 ; time separator db 0 ; currency format flags db 2 ; digits in currency db 0 ; time format dd 026ah:176ch ; case map entry point db ',',0 ; data list separator db 10 dup (0) ; reserved Obtain the pointer to the uppercase table associated with the default country and code page 437. buffer db 5 dup (0) ; receives pointer info . . . mov ax,6502h ; function number mov bx,437 ; code page mov cx,5 ; length of buffer mov dx,-1 ; default country mov di,seg buffer ; buffer address mov es,di mov di,offset buffer int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . In this case, MS-DOS filled the following values into the buffer: buffer db 2 ; info ID code dw 0204h ; offset of uppercase table dw 1140h ; segment of uppercase table and the table at 1140:0204H contains the following data: 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDE 1140:0200 80 00 80 9A 45 41 8E 41 8F 80 45 45 ....EA.A..E 1140:0210 45 49 49 49 8E 8F 90 92 92 4F 99 4F 55 55 59 99 EIII.....O.OUUY 1140:0220 9A 9B 9C 9D 9E 9F 41 49 4F 55 A5 A5 A6 A7 A8 A9 ......AIOU..... 1140:0230 AA AB AC AD AE AF B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 ............... 1140:0240 BA BB BC BD BE BF C0 C1 C2 C3 C4 C5 C6 C7 C8 C9 ............... 1140:0250 CA CB CC CD CE CF D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 ............... 1140:0260 DA DB DC DD DE DF E0 E1 E2 E3 E4 E5 E6 E7 E8 E9 ............... 1140:0270 EA EB EC ED EE EF F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 ............... 1140:0280 FA FB FC FD FE FF ...... ──────────────────────────────────────────────────────────────────────────── Int 21H [3.3] Function 66H (102) Get or set code page ──────────────────────────────────────────────────────────────────────────── Obtains or selects the current code page. Call with: AH = 66H AL = subfunction 01H = Get Code Page 02H = Select Code Page BX = code page to select, if AL = 02H Returns: If function successful Carry flag = clear and, if called with AL = 01H BX = active code page DX = default code page If function unsuccessful Carry flag = set AX = error code Note: ■ When the Select Code Page subfunction is used, MS-DOS gets the new code page from the COUNTRY.SYS file. The device must be previously prepared for code page switching with the appropriate DEVICE= directive in the CONFIG.SYS file and NLSFUNC and MODE CP PREPARE commands (placed in the AUTOEXEC.BAT file, usually). Example: Force the active code page to be the same as the system's default code page, that is, restore the code page that was active when the system was first booted. . . . ; get current and ; default code page mov ax,6601h ; function number int 21h ; transfer to MS-DOS jc error ; jump if function failed ; set code page mov bx,dx ; active = default mov ax,6602h ; function number int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.3] Function 67H (103) Set handle count ──────────────────────────────────────────────────────────────────────────── Sets the maximum number of files and devices that may be opened simultaneously using handles by the current process. Call with: AH = 67H BX = number of desired handles Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ This function call controls the size of the table that relates handle numbers for the current process to MS-DOS's internal, global table for all of the open files and devices in the system. The default table is located in the reserved area of the process's PSP and is large enough for 20 handles. ■ The function fails if the requested number of handles is greater than 20 and there is not sufficient free memory in the system to allocate a new block to hold the enlarged table. ■ If the number of handles requested is larger than the available entries in the system's global table for file and device handles (controlled by the FILES entry in CONFIG.SYS), no error is returned. However, a subsequent attempt to open a file or device, or create a new file, will fail if all the entries in the system's global file table are in use, even if the requesting process has not used up all its own handles. Example: Set the maximum handle count for the current process to thirty, so that the process can have as many as 30 files or devices opened simultaneously. (Five of the handles are already assigned to the standard devices when the process starts up.) Note that a FILES=30 (or greater value) entry in the CONFIG.SYS file would also be required for the process to successfully open 30 files or devices. . . . mov ah,67h ; function number mov bx,30 ; maximum number of handles int 21h ; transfer to MS-DOS jc error ; jump if function failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H [3.3] Function 68H (104) Commit file ──────────────────────────────────────────────────────────────────────────── Forces all data in MS-DOS's internal buffers associated with a specified handle to be physically written to the device. If the handle refers to a file, and the file has been modified, the time and date stamp and file size in the file's directory entry are updated. Call with: AH = 68H BX = handle Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code Notes: ■ The effect of this function is equivalent to closing and reopening a file, or to duplicating a handle for the file with Int 21H Function 45H and then closing the duplicate. However, this function has the advantage that it will not fail due to lack of handles, and the application does not risk losing control of the file in multitasking or network environments. ■ If this function is requested for a handle associated with a character device, a success flag is returned, but there is no other effect. Example: Assume that the file MYFILE.DAT has been previously opened and that the handle for that file is stored in the variable fhandle. Call the Commit File function to ensure that any data in MS-DOS's internal buffers associated with the handle is written out to disk and that the directory and file allocation table are up to date. fname db 'MYFILE.DAT',0 ; ASCIIZ filename fhandle dw ? ; file handle . . . mov ah,68h ; function number mov bx,fhandle ; file handle int 21h ; transfer to MS-DOS jc error ; jump if commit failed . . . ──────────────────────────────────────────────────────────────────────────── Int 21H Function 69H (105) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H Function 6AH (106) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H Function 6BH (107) Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 21H [4.0] Function 6CH (108) Extended open file ──────────────────────────────────────────────────────────────────────────── Given an ASCIIZ pathname, opens, creates or replaces a file in the designated or default directory on the designated or default disk drive. Returns a handle that can be used by the program for subsequent access to the file. Call with: AH = 6CH AL = 00H BX = open mode Bit(s) Significance 0─2 access type 000 = read-only 001 = write-only 010 = read/write 3 reserved (0) 4─6 sharing mode 000 = compatibility 001 = deny read/write (deny all) 010 = deny write 011 = deny read 100 = deny none 7 inheritance 0 = child process inherits handle 1 = child does not inherit handle 8─12 reserved (0) 13 critical error handling 0 = execute Int 24H 1 = return error to process 14 write-through 0 = writes may be buffered and deferred 1 = physical write at request time 15 reserved (0) CX = file attribute (bits may be combined; ignored if open) Bit(s) Significance (if set) 0 read-only 1 hidden 2 system 3 volume label 4 reserved (0) 5 archive 6─15 reserved (0) DX = open flag Bits Significance 0─3 action if file exists 0000 = fail 0001 = open file 0010 = replace file 4─7 action if file doesn't exist 0000 = fail 0001 = create file 8─15 reserved (0) DS:SI = segment:offset of ASCIIZ pathname Returns: If function successful Carry flag = clear AX = handle CX = action taken 1 = file existed and was opened 2 = file did not exist and was created 3 = file existed and was replaced If function failed Carry flag = set AX = error code Notes: ■ The function fails if: ∙ any element of the pathname does not exist. ∙ the file is being created in the root directory and the root directory is full. ∙ the file is being created and a file with the same name and the read-only attribute already exists in the specified directory. ∙ the program is running on a network and the user running the program has insufficient access rights. ■ A file is usually given a normal (0) attribute when it is created. The file's attribute can subsequently be modified with Int 21H Function 43H. ■ This function combines the capabilities of Int 21H Functions 3CH, 3DH, and 5BH. It was added to MS-DOS for compatibility with the DosOpen function of OS/2. Example: Create the file MYFILE.DAT, if it does not already exist, in directory \MYDIR on drive C, and save the handle for subsequent access to the file. fname db 'C:\MYDIR\MYFILE.DAT',0 fhandle dw ? . . . mov ax,6c00h ; function number mov bx,4042h ; read/write, deny none, ; write-through mode xor cx,cx ; normal attribute mov dx,0010h ; create if doesn't exist, ; fail if exists mov si,seg fname ; address of pathname mov ds,si mov si,offset fname int 21h ; transfer to MS-DOS jc error ; jump if open failed mov fhandle,ax ; save file handle . . . ──────────────────────────────────────────────────────────────────────────── Int 22H [1.0] Terminate handler address ──────────────────────────────────────────────────────────────────────────── The machine interrupt vector for Int 22H (memory locations 0000:0088H through 0000:008BH) contains the address of the routine that receives control when the currently executing program terminates via Int 20H, Int 27H, or Int 21H Functions 00H, 31H, or 4CH. The address in this vector is also copied into offsets 0AH through 0DH of the program segment prefix (PSP) when a program is loaded but before it begins executing, and is restored from the PSP (in case it was modified by the application) as part of MS-DOS's termination handling. This interrupt should never be issued directly. ──────────────────────────────────────────────────────────────────────────── Int 23H [1.0] Ctrl-C handler address ──────────────────────────────────────────────────────────────────────────── The machine interrupt vector for Int 23H (memory locations 0000:008CH though 0000:008FH) contains the address of the routine which receives control when a Ctrl-C is detected during any character I/O function and, if the Break flag is ON, during most other MS-DOS function calls. The address in this vector is also copied into locations 0EH through 11H of the program segment prefix (PSP) when a program is loaded but before it begins executing, and is restored from the PSP (in case it was modified by the application) as part of MS-DOS's termination handling. This interrupt should never be issued directly. Notes: ■ The initialization code for an application can use Int 21H Function 25H to reset the Interrupt 23H vector to point to its own routine for Ctrl-C handling. In this way, the program can avoid being terminated unexpectedly as the result of the user's entry of a Ctrl-C or Ctrl-Break. ■ When a Ctrl-C is detected and the program's Int 23H handler receives control, all registers are set to their values at the point of the original function call. The handler can then do any of the following: ∙ Set a local flag for later inspection by the application, or take any other appropriate action, and perform an IRET. All registers must be preserved. The MS-DOS function in progress will be restarted from scratch and will proceed to completion, control finally returning to the application in the normal manner. ∙ Take appropriate action and then perform a RET FAR to give control back to MS-DOS. The state of the carry flag is used by MS-DOS to determine what action to take. If the carry flag is set, the application will be terminated; if the carry flag is clear, the application will continue in the normal manner. ∙ Retain control by transferring to an error-handling routine within the application and then resume execution or take other appropriate action, never performing a RET FAR or IRET to end the interrupt-handling sequence. This option will cause no harm to the system. ■ Any MS-DOS function call may be used within the body of an Int 23H handler. Example: See Chapter 5. ──────────────────────────────────────────────────────────────────────────── Int 24H [1.0] Critical-error handler address ──────────────────────────────────────────────────────────────────────────── The machine interrupt vector for Int 24H (memory locations 0000:0090H through 0000:0093H) contains the address of the routine that receives control when a critical error (usually a hardware error) is detected. This address is also copied into locations 12H through 15H of the program segment prefix (PSP) when a program is loaded but before it begins executing, and is restored from the PSP (in case it was modified by the application) as part of MS-DOS's termination handling. This interrupt should never be issued directly. Notes: ■ On entry to the critical-error interrupt handler, bit 7 of register AH is clear (0) if the error was a disk I/O error; otherwise, it is set (1). BP:SI contains the address of a device-driver header from which additional information can be obtained. Interrupts are disabled. The registers will be set up for a retry operation, and an error code will be in the lower half of the DI register, with the upper half undefined. The lower byte of DI contains: 00H write-protect error 01H unknown unit 02H drive not ready 03H unknown command 04H data error (CRC) 05H bad request structure length 06H seek error 07H unknown media type 08H sector not found 09H printer out of paper 0AH write fault 0BH read fault 0CH general failure 0DH reserved 0EH reserved 0FH invalid disk change (MS-DOS version 3 only) Note that these are the same error codes returned by the device driver in the request header. Also, upon entry, the stack is set up as shown in Figure 8-8, page 149. ■ When a disk I/O error occurs, MS-DOS automatically retries the operation before issuing a critical-error Int 24H. The number of retries varies in different versions of MS-DOS, but is typically in the range three to five. ■ Int 24H handlers must preserve the SS, SP, DS, ES, BX, CX, and DX registers. Only Int 21H Functions 01H─0CH and 59H can be used by an Int 24H handler; other function calls will destroy the MS-DOS stack and its ability to retry or ignore an error. ■ When the Int 24H handler issues an IRET, it should return an action code in AL that will be interpreted by DOS as follows: 0 ignore the error 1 retry the operation 2 terminate the program 3 [3.0+] fail the function call in progress ■ If the Int 24H handler returns control directly to the application program rather than to MS-DOS, it must restore the program's registers, removing all but the last three words from the stack, and issue an IRET. Control returns to the instruction immediately following the function call that caused the error. This option leaves MS-DOS in an unstable state until a call to an Int 21H function higher than Function 0CH is made. Example: See Chapter 8. ──────────────────────────────────────────────────────────────────────────── Int 25H [1.0] Absolute disk read ──────────────────────────────────────────────────────────────────────────── Provides a direct linkage to the MS-DOS BIOS module to read data from a logical disk sector into memory. Call with: For access to partitions <= 32 MB AL = drive number (0 = A, 1 = B, etc) CX = number of sectors to read DX = starting sector number DS:BX = segment:offset of buffer For access to partitions > 32 MB (MS-DOS 4.0 and later) AL = drive number (0 = A, 1 = B, etc) CX = -1 DS:BX = segment:offset of parameter block (see Notes) Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code (see Notes) Notes: ■ All registers except the segment registers may be destroyed. ■ When this function returns, the CPU flags originally pushed on the stack by the INT 25H instruction are still on the stack. The stack must be cleared by a POPF or ADD SP,2 to prevent uncontrolled stack growth and to make accessible any other values that were pushed on the stack before the call to INT 25H. ■ Logical sector numbers are obtained by numbering each disk sector sequentially from cylinder 0, head 0, sector 1, and continuing until the last sector on the disk is counted. The head number is incremented before the track number. Logically adjacent sectors may not be physically adjacent, due to interleaving that occurs at the device-adapter level for some disk types. ■ The error code is interpreted as follows: The lower byte (AL) is the same error code that is returned in the lower byte of DI when an Int 24H is issued. The upper byte (AH) contains: 01H if bad command 02H if bad address mark 04H if requested sector not found 08H if direct memory access (DMA) failure 10H if data error (bad CRC) 20H if controller failed 40H if seek operation failed 80H if attachment failed to respond ■ [4.0+] When accessing partitions larger than 32 MB under MS-DOS version 4, this function uses a parameter block with the following format: Bytes Description 00H─03H 32-bit sector number 04H─05H number of sectors to read 06H─07H offset of buffer 08H─09H segment of buffer Example: Read logical sector 1 of drive A into the memory area named buff. (On most MS-DOS floppy disks, this sector contains the beginning of the file allocation table.) buff db 512 dup (?) ; receives data from disk . . . mov al,0 ; drive A mov cx,1 ; number of sectors mov dx,1 ; beginning sector number mov bx,seg buff ; buffer address mov ds,bx mov bx,offset buff int 25h ; request disk read jc error ; jump if read failed add sp,2 ; clear stack . . . ──────────────────────────────────────────────────────────────────────────── Int 26H [1.0] Absolute disk write ──────────────────────────────────────────────────────────────────────────── Provides a direct linkage to the MS-DOS BIOS module to write data from memory to a logical disk sector. Call with: For access to partitions <= 32 MB AL = drive number (0 = A, 1 = B, etc) CX = number of sectors to write DX = starting sector number DS:BX = segment:offset of buffer For access to partitions > 32 MB (MS-DOS 4.0 and later) AL = drive number (0 = A, 1 = B, etc) CX = -1 DS:BX = segment:offset of parameter block (see Notes) Returns: If function successful Carry flag = clear If function unsuccessful Carry flag = set AX = error code (see Notes) Notes: ■ All registers except the segment registers may be destroyed. ■ When this function returns, the CPU flags originally pushed onto the stack by the INT 26H instruction are still on the stack. The stack must be cleared by a POPF or ADD SP,2 to prevent uncontrolled stack growth and to make accessible any other values that were pushed on the stack before the call to INT 26H. ■ Logical sector numbers are obtained by numbering each disk sector sequentially from cylinder 0, head 0, sector 1, and continuing until the last sector on the disk is counted. The head number is incremented before the track number. Logically adjacent sectors may not be physically adjacent, due to interleaving that occurs at the device-adapter level for some disk types. ■ The error code is interpreted as follows: The lower byte (AL) is the same error code that is returned in the lower byte of DI when an Int 24H is issued. The upper byte (AH) contains: 01H if bad command 02H if bad address mark 03H if write-protect fault 04H if requested sector not found 08H if direct memory access (DMA) failure 10H if data error (bad CRC) 20H if controller failed 40H if seek operation failed 80H if attachment failed to respond ■ [4.0+] When accessing partitions larger than 32 MB under MS-DOS version 4, this function uses a parameter block with the following format: Bytes Description 00H─03H 32-bit sector number 04H─05H number of sectors to read 06H─07H offset of buffer 08H─09H segment of buffer Example: Write the contents of the memory area named buff into logical sector 3 of drive C. Warning: Verbatim use of the following code could damage the file system on your fixed disk. There is, unfortunately, no way to provide a really safe example of this function. buff db 512 dup (?) ; contains data for write . . . mov al,2 ; drive C mov cx,1 ; number of sectors mov dx,3 ; beginning sector number mov bx,seg buff ; buffer address mov ds,bx mov bx,offset buff int 26h ; request disk write jc error ; jump if write failed add sp,2 ; clear stack . . . ──────────────────────────────────────────────────────────────────────────── Int 27H [1.0] Terminate and stay resident ──────────────────────────────────────────────────────────────────────────── Terminates execution of the currently executing program, but reserves part or all of its memory so that it will not be overlaid by the next transient program to be loaded. MS-DOS then takes the following actions: ■ File buffers are flushed and any open handles for files or devices owned by the process are closed. ■ The termination handler vector (Int 22H) is restored from PSP:000AH. ■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH. ■ [2.0+] The critical-error handler vector (Int 24H) is restored from PSP:0012H. ■ Control is transferred to the termination handler. If the program is returning to COMMAND.COM, control transfers to the resident portion and the transient portion is reloaded if necessary. If a batch file is in progress, the next line of the file is fetched and interpreted; otherwise a prompt is issued for the next user command. Call with: DX = offset of the last byte plus one (relative to the program segment prefix) of program to be protected CS = segment of program segment prefix Returns: Nothing Notes: ■ This function call is typically used to allow user-written utilities, drivers, or interrupt handlers to be loaded as ordinary .COM or .EXE programs, then remain resident. Subsequent entrance to the code is via a hardware or software interrupt. ■ This function attempts to set the initial memory allocation block to the length in bytes specified in register DX. If other memory blocks have been requested by the application via Int 21H Function 48H, they will not be released by this function. ■ Other methods of performing a final exit are: ∙ Int 20H ∙ Int 21H Function 00H ∙ Int 21H Function 31H ∙ Int 21H Function 4CH ■ This function should not be called by .EXE programs that are loaded at the high end of the transient program area (i.e., linked with the /HIGH switch), because doing so reserves the memory normally used by the transient part of COMMAND.COM. If COMMAND.COM cannot be reloaded, the system will fail. ■ This function does not work correctly when DX contains values in the range 0FFF1H─0FFFFH. In this case, MS-DOS discards the high bit of the value in DX, resulting in the reservation of 32 KB less memory than was requested by the program. ■ [2.0+] Int 21H Function 31H should be used in preference to this function, because it supports return codes, allows larger amounts of memory to be reserved, and does not require CS to contain the segment of the program segment prefix. ■ [3.0+] If the program is running on a network, it should remove all locks it has placed on file regions before terminating. Example: Terminate and stay resident, reserving enough memory to contain the entire program. . . . mov dx,offset pend ; DX = bytes to reserve int 27h ; terminate, stay resident . . . pend equ $ ; offset, end of program end ──────────────────────────────────────────────────────────────────────────── Int 28H Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 29H Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 2AH Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 2BH Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 2CH Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 2DH Reserved ──────────────────────────────────────────────────────────────────────────── ──────────────────────────────────────────────────────────────────────────── Int 2EH Reserved ────────────────────────────────────────────────────────────