Frostbyte's 1980s DOS Shareware Collection/floppyshareware.zip/floppyshareware/GLEN/DOSREF20.ZIP/CHAPTER.001

Text File | 1991-06-17 | 37KB | 678 lines

   **  Programmer's  Technical  Reference  for  MSDOS  and  the  IBM  PC **
                        ┌─────────────────────────────┐
                        │ Shareware Version, 06/17/91 │
                        │  Please Register Your Copy  │
                        └─────────────────────────────┘
                    Copyright (c) 1987, 1991 Dave Williams
                 USA copyright TXG 392-616 ALL RIGHTS RESERVED
                     ISBN 1-878830-02-3 (disk-based text)


                            C H A P T E R    O N E

                              DOS AND THE IBM PC


                                C O N T E N T S

Some History .......................................................... 1**1
What is DOS? .......................................................... 1**2
Other Operating Systems ............................................... 1**3
Specific Versions of MS/PC-DOS ........................................ 1**4
The Operating System Heirarchy ........................................ 1**5
DOS Structure ......................................................... 1**6
DOS Initialization .................................................... 1**7



SOME HISTORY├────────────────────────────────────────────────────────── 1**1

 Development of MSDOS/PCDOS began in October 1980, when IBM began searching
the market for an operating system for the yet-to-be-introduced IBM PC.
Microsoft had no 8086 real operating system to sell, but quickly made a deal
to license Seattle Computer Products' 86-DOS operating system, which had been
written by Tim Paterson earlier in 1980 for use on that company's line of 8086,
S100 bus micros. 86-DOS (also called QDOS, for Quick and Dirty Operating System)
had been written as more or less a 16-bit version of CP/M, since Digital
Research was showing no hurry in introducing CP/M-86.

 This code was quickly polished up and presented to IBM for evaluation. IBM
had originally intended to use Digital Research's CP/M operating system, which
was the industry standard at the time.

 Folklore reports various stories about the rift between DRI and IBM. The most
popular story claims Gary Kildall or DRI snubbed the IBM executives by flying
his airplane when the meeting was scheduled. Another story claims Kildall
didn't want to release the source for CP/M to IBM, which would be odd, since
they released it to other companies. Industry pundit Jerry Pournelle claims
Kildall's wife killed the deal by insisting on various contract changes. I
suspect the deal was killed by the good ol'boy network. It's hard to imagine
a couple of junior IBM executives giving up when ordered to a task as simple
as licensing an operating system from a vendor. Wouldn't look good on their
performance reports. It would be interesting to hear IBM's story...

 IBM found itself left with Microsoft's offering of "Microsoft Disk
Operating System 1.0". An agreement was reached between the two, and IBM agreed
to accept 86-DOS as the main operating system for thir new PC. Microsoft
purchased all rights to 86-DOS in July 1981, and "IBM Personal Computer DOS
1.0" was ready for the introduction of the IBM PC in October 1981. IBM
subjected the operating system to an extensive quality-assurance program,
reportedly found well over 300 bugs, and decided to rewrite the programs. This
is why PC-DOS is copyrighted by both IBM and Microsoft.

 Some early OEM versions of DOS had different names, such as Compaq-DOS, Z-DOS,
Software Bus 86, etc.  By version 2 Microsoft managed to persuade everyone but
IBM to refer to the product as "MS-DOS."

 It is sometimes amusing to reflect on the fact that the IBM PC was not
originally intended to run MSDOS. The target operating system at the end of the
development was for a (not yet in existence) 8086 version of CP/M. On the other
hand, when DOS was originally written the IBM PC did not yet exist! Although
PC-DOS was bundled with the computer, Digital Research's CP/M-86 would probably
have been the main operating system for the PC except for two things - Digital
Research wanted $495 for CP/M-86 (considering PC-DOS was essentially free) and
many software developers found it easier to port existing CP/M software to DOS
than to the new version of CP/M.

 The upgrade from DOS 3.3 to 4.0 was done in-house by IBM.  DOS 4.0 was a
completely IBM product, later licensed back to Microsoft. In early 1990 IBM
announced that it was ceasing development of DOS and all further work would
be done solely by Microsoft.


 Microsoft Press' "MSDOS Encyclopedia" shows a reproduction of a late
DOS 1.25 OEM brochure.  Microsoft was touting future enhancements to
1.25 including Xenix-compatible pipes, process forks, and multitasking,
as well as "graphics and cursor positioning, kanji support, multi-user
and hard disk support, and networking."  Microsoft certainly thought
big, but, alas, the forks, multitasking, and multiuser support never
came about, at least in US versions of DOS.  Oddly, the flyer claims
that

 "MS-DOS has no practical limit on disk size. MS-DOS uses 4-byte XENIX
OS compatible pointers for file and disk capacity up to 4 gigabytes."

 Umm... yeah.  One sort of gets the idea nobody at Microsoft had a hard
disk larger than 32 megabytes...

 For the record they actually delivered:

Xenix-compatible pipes             DOS 2.0  ("|" operator)
process forks, and multitasking   eDOS 4.0  (not delivered in the US)
multi-user                         never delivered
graphics and cursor positioning    DOS 2.0  (ANSI.SYS, more than likely)
kanji support                      DOS 2.01, 2.25 (double-byte char set)
hard disk support                  DOS 2.0  (subdirectories)
networking                         DOS 3.1  (file locking, MS Networks)

 Early Microsoft ads pumped DOS' Xenix-like features and promised Xenix
functionality in future releases.

 We'll probably never know what the real story was behind eDOS/DOS 4/
DOS 5/286DOS/OS2.  Microsoft had announced their intent to build a
multitasking, multiuser version of MSDOS as early as 1982.  They shipped
betas of "DOS 4.0" in '86 and early '87, before 3.3 was even announced.
Microsoft UK announced they had licensed 4.0 to Apricot Computer, and
the French Postal Service was supposed to be running it.  I've never
been able to find out if Apricot ever shipped any 4.0 to end users.

 Despite Gordon Letwin's acid comments about problems with the 80286
processor, I doubt the '286 was the barrier between users and a
multitasking MSDOS.  I also doubt there was any shortage of programming
talent at Microsoft - Digital Research's Concurrent DOS and Software
Link's PC-MOS were developed without undue trouble.


 MSDOS and PC-DOS have been run on more than just the IBM-PC and clones. Some
of the following have been done:

 Hardware PC Emulation:

        Apple II                ->      TransPC 8088 board
        Apple MacIntosh         ->      AST 80286 board
        Atari 400/800           ->      Co-Power 88 board
        Atari ST                ->      PC-Ditto II cartridge
        Commodore Amiga 2000    ->      8088 or A2286D 80286 Bridge Board
        IBM PC/RT               ->      80286 AT adapter
        Kaypro 2                ->      Co-Power Plus board

 Software PC Emulation:

        Apple MacIntosh         ->      SoftPC
        Atari ST                ->      PC-Ditto I
        IBM RS/6000             ->      DOS emulation

 DOS Emulation:

        AIX (IBM RS/6000)       ->      DOS emulation with "PCSIMulator"
        OS/2                    ->      DOS emulation in "Compatibility Box"
        QNX                     ->      DOS window
        SunOS                   ->      DOS window
        Xenix                   ->      DOS emulation with DOSMerge


WHAT IS DOS?├────────────────────────────────────────────────────────── 1**2

 DOS exists as a high-level interface between an application program and the
computer. DOS stands for "Disk Operating System", which reflects the fact that
its main original purpose was to provide an interface between the computer and
its disk drives.

 DOS now lets your programs do simple memory management, I/O from the system
console, and assorted system tasks (time and date, etc) as well as managing
disk operations. Versions 3.1 and up also incorporate basic networking
functions.

 With the introduction of installable device drivers and TSR (terminate but
stay resident) programs in DOS 2.0, the basic DOS functions may be expanded to
cover virtually any scale of operations required.


OTHER OPERATING SYSTEMS├─────────────────────────────────────────────── 1**3

 There are a number of compatible replacements for Microsoft's MSDOS. Some are:

Consortium Technologies MultiDOS     (multitasking, multiuser)
Digital Research Concurrent DOS      (multitasking)
Digital Research Concurrent DOS 386  (for 80386 computers)
Digital Research Concurrent DOS XM   (multitasking, multiuser)
Digital Research DR-DOS 3.31 and 4.0 (PC-DOS clones)
PC-MOS/386                           (multitasking, multiuser)
Wendin-DOS                           (multitasking, multiuser)
VM/386                               (multitasking)


 Various other operating systems are available for the IBM PC. These include:

Digital Research CP/M-86
Digital Research Concurrent CP/M-86 (multitasking)
Minix (multitasking UNIX workalike)
Pick  (database-operating system)
QNX   (multitasking, multiuser)
UNIX  (various systems from IBM itself, Microsoft-SCO, Bell, and various UNIX
       clones, single and multi user) (AIX, Xenix, AT&T System V, etc.)

 "Shell" programs exist which use DOS only for disk management while they more
or less comprise a new operating system. These include:

     DesQview                        Windows                     OmniView
     GEM                             TopView                     TaskView

 Systems using the NEC V-series CPUs can execute Intel 8080/8085 8-bit
instructions as well as the 16-bit 8088-up instructions. They can run standard
Digital Research 8-bit CP/M and MP/M directly, as well as other operating
systems developed for that processor.



SPECIFIC VERSIONS OF MS/PC-DOS├──────────────────────────────────────── 1**4

 DOS 1.x is essentially 86-DOS. DOS 2.x kept the multiple file layout (the two
hidden files and COMMAND.COM) but for all practical purposes is an entirely
different operating system with backwards compatibility with 1.x. I seriously
doubt there has been much code from 1.x retained in 2.x. DOS 3.x is merely an
enhancement of 2.x; there seems little justification for jumping a whole
version number. The disk handling routines were considerably extended in 3.1,
allowing disk access in a "virtual" fashion, independent of whether the drive
was a local or network device. DOS 4.0, originating as it did from outside
Microsoft, can justify a version jump. Unfortunately, 4.x seems to have very
little reason to justify its existence - virtually all of its core features
can be found in one version or another of DOS 3.x. At this time DOS 5 is still
in beta testing. Gordon Letwin of Microsoft says the entire kernel has been
rewritten in hand-optimized assembler and DOS 5 will take advantage of the
'286 and later processors to get extra usable memory.

 DOS version nomenclature: major.minor.minor. The digit to the left of the
decimal point indicates a major DOS version change. 1.0 was the first version.
2.0 added support for subdirectories, 3.0 added support for networking, 4.0
added some minimal support for Lotus-Intel-Microsoft EMS.

 The first minor version indicates customization for a major application. For
example, 2.1 for the PCjr, 3.3 for the PS/2s. The second minor version does not
seem to have any particular meaning.

 The main versions of DOS are:

 86-DOS       February 1981  Paterson's Quick'n'Dirty DOS first runs on PC
 PC-DOS 1.0   August   1981  original IBM release
 PC-DOS 1.05  -------- ----  fixes to BASIC interpreter
 PC-DOS 1.1   June     1982  bugfix, double sided drive support
 MS-DOS 1.25  July     1982  for early compatibles. This is the first non-IBM
                             OEM version
 PC-DOS 2.0   March    1983  for PC/XT, Unix-type subdirectory support,
                             installable device drivers, I/O redirection,
                             subdirectories, hard disk support, handle calls
 PC-DOS 1.85  April    1983  internal IBM - extended 1.1 - not released
 MS-DOS 2.01  -------- 1983  first support for individual country formats, Kanji
 PC-DOS 2.1   October  1983  for IBM PCjr, bugfixes for 2.0. No country support
 MS-DOS 2.11  October  1983  basically a cross of PC-DOS 2.1 and MS-DOS 2.01
 MS-DOS 2.12  -------- ----  special version for TI Professional
 PC-DOS 3.0   August   1984  1.2 meg drive for PC/AT, some new system calls,
                             new external programs, 16-bit FAT, specific support
                             for IBM network
 MS-DOS 3.05  -------- 1984  first OEM version of 3.x
 PC-DOS 3.1   November 1984  bugfix for 3.0, implemented generic network support
 MS-DOS 2.25  October  1985  extended foreign language support
 PC-DOS 3.2   January  1986  720k 3.5 inch drive support, special support for
                             laptops (IBM PC Convertible), XCOPY
 MS-DOS 4.0   April    1986  multitasking (Europe only) - withdrawn from market
 PC-DOS 3.3   April    1987  for PS/2 series, 1.44 meg support, multiple DOS
                             partition support, code page switching, improved
                             foreign language support, some new function calls,
                             support for the AT's CMOS clock.
 MS-DOS 3.31  November 1987  over-32 meg DOS partitions. Different versions
                             from different OEMs (not Microsoft).  Compaq and
                             Wyse are most common.
 PC-DOS 3.4   -------- ----  internal IBM - not released (4.0 development)
 MS-DOS 2.11R -------- 1988  bootable ROM DOS for Tandy machines
 PC-DOS 4.0   August   1988  32mb limit officially broken, minor EMS support,
                             more new function calls, enhanced network support
                             for external commands
 MS-DOS 4.01  January? 1989  Microsoft version with some bugfixes
 MS-DOS 3.21R September1989  DOS in ROM, Flash File System for laptops
 MS-DOS 3.3R  -------- 1990  DOS in ROM, introduced for TI laptops
 MS-DOS 5.0   June     1991  new high memory support, uses up to 8 hard disks,
                             command line editor and aliasing

 IBM's PC-DOS was long considered to be the "standard" version of DOS.  Now
that MS 5.0 is a commercial product most developers will probably write to it.

 Microsoft's policy has been to sell DOS only to OEMs.  Despite this, they
sold small quantities of DOS 3.2, 3.3, and 4.0 without insurmountable
difficulties.  DOS 5.0 was concieved from the beginning as an over-the-counter
retail product.

 Incidentally, IBM refers to its DOS as "The IBM Personal Computer DOS." The
term "PCDOS" is a trademark of IBM's rival DEC.

 Some versions of MS-DOS varied from PC-DOS in the available external commands.
Some OEMs only licensed the basic operating system code (the xxxDOS and xxxBIO
programs, and COMMAND.COM) from Microsoft, and either wrote the rest themselves
or contracted them from outside software houses like Phoenix. Most of the
external programs for DOS 3.x and 4.x are written in "C" while the 1.x and 2.x
utilities were written in assembly language. Other OEMs required customized
versions of DOS for their specific hardware configurations, such as Sanyo 55x
and early Tandy computers, which were unable to exchange their DOS with the IBM
version.

 PC-DOS 3.0 was extremely buggy on release. It does not handle the DOS
environment correctly and there are numerous documented problems with the
batch file parser. The network support code is also nonfunctional in this DOS
version. It is recommended that users upgrade to at least version 3.1.

 DEC MSDOS versions 2.11 for the Rainbow had the ANSI.SYS device driver built
into the main code. The Rainbow also used a unique quad density, single-sided
floppy drive and its DOS had special support for it.

 IBM had a version 1.85 of PC-DOS in April 1983, after the introduction of DOS
2.0. It was evidently for internal use only, supported multiple drive file
searches (a primitive form of PATH), builtin MODE sommands for screen support,
a /P parameter for TYPE for paused screens, an editable command stack like the
public domain DOSEDIT.COM utility, and could be set up to remain completely
resident in RAM instead of a resident/transient part like normal DOS. It is a
pity some of the neat enhancements didn't make it into DOS 2.0. IBM also had
an "internal use only" version 3.4, evidently used while developing DOS 4.0.

 Digital Research's DR-DOS is the first widely available DOS clone. Version
3.4, released in June 1988, was the one first available to the American public.
It was somewhat buggy and its use is not recommended. DR 3.41 is extremely
compatible and its use should pose no problems on any machine.  DR-DOS 5.0
(released May, 1990) is functionally equivalent to MS-DOS 5.0. For all
practical purposes, MS 5.0 is a clone of DR 5.0, since DR beat MS to market by
over a year.  According to Greg Ewald, DRI's DR-DOS product manager, DR-DOS was
developed from Concurrent DOS 386 with the multiuser and multitasking code
stripped out.

 Some versions of DOS used in compatibles do not maintain the 1.x, 2.x, ...
numbering system. Columbia Data Products computers labeled DOS 1.25 as DOS
2.0. Early Compaqs labeled DOS 2.0 as DOS 1.x. Other versions incorporated
special features - Compaq DOS 3.31 and Wyse DOS 3.21 both support >32mb disk
partitions in the same fashion as DOS 4.x.

 AT&T DOS 3.1 differs from generic MSDOS 3.10 in its use of cluster-size and
file allocation table structures. AT&T DOS appears to use rules not from
version 3, but rather those from version 2.

 Epson Equity III and ComputerLand 3.10 DOS's appear to use cluster techniques
that are a cross between versions 2 and 3. On type DOS partitions, these DOS's
use 3.x rules if the partition is larger than 32,680 sectors in total size.
This implies 16 bit FAT entries as well. On partitions below this size, they
will use 2.x rules, including the 12 bit FAT entries.

 Zenith DOS 3.x and Wyse DOS 3.2 have a builtin internal device driver to
handle up to 4 32Mb DOS partitions on a single hard disk. Wyse DOS 3.31 will
handle single partitions up to 512Mb with a 32-bit FAT.

 According to PC Week Magazine, July 4, 1988, Arabic versions of MSDOS are
shipping with a hardware copy-protection system from Rainbow Technologies.
This is similar to the short-lived system used by AutoCAD 2.52 and a very few
other MSDOS programs, where an adapter block is plugged into the parallel port
and software makes use of coded bytes within the block. This type of copy
protection has been common on Commodore products for several years, where it is
called a "dongle."
 The AutoCAD dongle was defeated by a small program written within weeks of
version 2.52's debut. Version 2.62 was released 3 months later, without the
dongle. The DOS dongle will, however, prevent the system from booting at all
unless it is found.
 This makes the Arabic version of MSDOS the first copy-protected operating
system, a dubious distinction at best. The modifications to the operating
system to support the dongle are not known at this time. Frankly, it would
seem that burning the operating system into ROMs would be cheaper and simpler.

 Versions of DOS sold in Great Britain are either newer than those sold in the
US or use a different numbering system. DOS 3.4, 4.0, 4.1, 4.2, and 4.3 had
been released there between the US releases of 3.3 and 4.0.
 MSDOS 4.0 (eDOS) was introduced in mid-1987 in Europe (at SICOB in Paris and
sometime earlier by Apricot Computer in the UK). It offered multitasking
provided applications were specially written for it.
 David Fraser (Microsoft UK Managing Director) is on record saying that "DOS
4.0 is unlikely to set the world alight and is of interest only to specific
OEMs who want its features for networking and communications."  Standard DOS
applications will run under DOS 4.x as a foreground task according to uncertain
information. It differs from earlier versions only in allowing background tasks
to run. For further information, see Chapter 4.

 Microsoft changed their OEM licensing agreements between DOS versions 2.x and
3.x. OEM versions of DOS 3.x must maintain certain data areas and undocumented
functions in order to provide compatibility with the networking features of the
operating system. For this reason, resident programs will be much more reliable
when operating under DOS 3.x.

 Several versions of DOS have been modified to be run out of ROM. The Sharp
PC5000 had MSDOS 1.25 in ROM, and the Toshiba 1000 and some Tandy 1000 models
have MSDOS 2.11 in ROM. In mid September 1989 Microsoft introduced 3.21R ROMs
for laptops, and in early '90 Texas Instruments laptops were the first to get
the 3.3R ROMs. Digital Research has also announced its DR-DOS 3.41 and 5.0 is
available in a ROM version and Award Software is marketing DOS cards to OEMs
as a plug-in to ISA-bus machines.

 IBM's release of DOS 4.0 (and the immediate subsequent release of a bugfix)
was a dubious step "forward." DOS 4.0 was the first version of DOS to come with
a warranty; the catch is that IBM warranted it only for a very slim list of
IBM-packaged software. 4.0 had some minor EMS support, support for large hard
disks, and not much else. With its voracious RAM requirements and lack of
compatibility with previous versions of DOS (many major software packages
crash under DOS 4.0), plus the increase in price to a cool $150, there was
no great rush to go to that version of DOS.

 Microsoft undertook development of MSDOS 5.0 in early 1990, soliciting
input from Usenet, BIX, and Compuserve among others.  5.0 is a functional
clone of Digital Research's DR-DOS 5.0.  5.0's compatibility was assured
by what has been claimed as the largest beta-test program in history -
in his address to the Boston Computer Society, Bill Gates announced over
7,500 testers were involved.




THE OPERATING SYSTEM HIERARCHY├──────────────────────────────────────── 1**5


 The Disk Operating System (DOS) and the ROM BIOS serve as an insulating layer
between the application program and the machine, and as a source of services
to the application program.

  As the term 'system' might imply, DOS is not one program but a collection
of programs designed to work together to allow the user access to programs
and data.  Thus, DOS consists of several layers of "control"programs and a set
of "utility" programs.

 The system hierarchy may be thought of as a tree, with the lowest level being
the actual hardware. The 8088 or V20 processor sees the computer's address
space as a ladder one byte wide and one million bytes long. Parts of this
ladder are in ROM, parts in RAM, and parts are not assigned. There are also
65,536 "ports" that the processor can use to control devices.

 The hardware is normally addressed by the ROM BIOS, which will always know
where everything is in its particular system. The chips may usually also be
written to directly, by telling the processor to write to a specific address or
port. This sometimes does not work as the chips may not always be at the same
addresses or have the same functions from machine to machine.



DOS STRUCTURE├───────────────────────────────────────────────────────── 1**6

DOS consists of four components:

 * The boot record
 * The ROM BIOS interface  (IBMBIO.COM, DRBIOS.SYS, or IO.SYS)
 * The DOS program file    (IBMDOS.COM, DRBDOS.SYS, or MSDOS.SYS)
 * The command processor   (COMMAND.COM or aftermarket replacement)


* The Boot Record

 The boot record begins on track 0, sector 1, side 0 of every diskette formatted
by the DOS FORMAT command. The boot record is placed on diskettes to produce an
error message if you try to start up the system with a nonsystem diskette in
drive A. For hard disks, the boot record resides on the first sector of the DOS
partition. All media supported by DOS use one sector for the boot record.


* Read Only Memory (ROM) BIOS Interface and Extensions

 The file IBMBIO.COM or IO.SYS is the interface module to the ROM BIOS.
This file provides a low-level interface to the ROM BIOS device routines and
may contain extensions or changes to the system board ROMs. Some compatibles do
not have a ROM BIOS to extend, and load the entire BIOS from disk. (Sanyo 55x,
Viasyn machines). Some versions of MSDOS, such as those supplied to Tandy, are
named IBMBIO.COM but are not IBM files.

 These low-level interface routines include the instructions for performing
operations such as displaying information on the screen, reading the keyboard,
sending data out to the printer, operating the disk drives, and so on. It is
the operating system's means of controlling the hardware. IBMBIO.COM contains
any modifications or updates to the ROM BIOS that are needed to correct any
bugs or add support for other types of hardware such as new disk drives. By
using IBMBIO.COM to update the ROM BIOS on the fly when the user turns on their
computer, IBM does not need to replace the ROM BIOS chip itself, but makes any
corrections through the cheaper and easier method of modifying the IBMBIO.COM
file instead.

 IBMBIO.COM also keeps track of hardware operations on an internal stack or
"scratch pad" area for the operating system to save information such as
addresses it will need, etc. An example of the use for this stack can be seen
when running a program such as a word processor. If you have told the word
processor to save your letter, it will write the data to your disk. During this
time, if you start typing some more information, the keyboard generates a
hardware interrupt. Since you don't want the process of writing the information
to the disk to be interrupted, DOS allocates a slot in the stack for the
keyboard's hardware interrupt and when it gets a chance, (probably after the
data has been written to the disk), it can process that interrupt and pick up
the characters you may have been typing. The STACKS= command in DOS 3.2+'s
CONFIG.SYS file controls the number of stack frames availible for this
purpose.

 IBMBIO.COM also reads your CONFIG.SYS file and installs any device drivers
(i.e. DEVICE=ANSI.SYS) or configuration commands it may find there.



* The DOS Program

 The actual DOS program is the file IBMDOS.COM or MSDOS.SYS. It provides a high-
level interface for user (application) programs. This program consists of file
management routines, data blocking/deblocking for the disk routines, and a
variety of built-in functions easily accessible by user programs.

 When a user program calls these function routines, they accept high-level
information by way of register and control block contents. When a user program
calls DOS to perform an operation, these functions translate the requirement
into one or more calls to IBMBIO.COM, MSDOS.SYS or system hardware to complete
the request.

 This section is often referred to as the "kernel" by systems programmers.



* The Command Interpreter

 The command interpreter, COMMAND.COM, is the part you interact with on the
command line. COMMAND.COM has three parts. IBM calls them the "resident
portion", the "initialization portion" and the "transient portion".

 IBM's original documentation spoke of installing alternate command
interpreters (programs other than COMMAND.COM) with the SHELL= statement in
CONFIG.SYS. Unfortunately, IBM chose not to document much of the interaction
between IBMDOS.COM and IBMBIO.COM. By the time much of the interaction was
widely understood, many commercial software programs had been written to use
peculiarities of COMMAND.COM itself.

 Two programs exist that perform as actual "shells" by completely replacing
COMMAND.COM and substituting their own command interpreter to use with the
hidden DOS files. These are Command Plus, a commercial package, and the very
interesting shareware 4DOS package. Both supply greatly enhanced batch
language and editing capabilities.

NOTE: DOS 3.3+ checks for the presence of a hard disk, and will default to
      COMSPEC=C:\. Previous versions default to COMSPEC=A:\. Under some DOS
      versions, if COMMAND.COM is not immediately availible for reloading
      (i.e., swapping to a floppy with COMMAND.COM on it) DOS may crash.


Resident Portion:

 The resident portion resides in memory immediately following IBMDOS.COM and its
data area. This portion contains routines to process interrupts 22h (Terminate
Address), 23h (Ctrl-Break Handler), and 24h (Critical Error Handler), as well as
a routine to reload the transient portion if needed. For DOS 3.x, this portion
also contains a routine to load and execute external commands, such as files
with exensions of COM or EXE.

 When a program terminates, a checksum is used to determine if the application
program overlaid the transient portion of COMMAND.COM. If so, the resident
portion will reload the transient portion from the area designated by COMSPEC=
in the DOS environment. If COMMAND.COM cannot be found, the system will halt.

 All standard DOS error handling is done within the resident portion of
COMMAND.COM.  This includes displaying error messages and interpreting the
replies to the "Abort, Retry, Ignore, Fail?" message.

 Since the transient portion of COMMAND.COM is so large (containing the
internal commands and all those error messages), and it is not needed when the
user is running an application it can be overlaid that program if that
application needs the room.  When the application is through, the resident
portion of COMMAND.COM brings the transient portion back into memory to show
the prompt.  This is why you will sometimes see the message "Insert disk with
COMMAND.COM". It needs to get the transient portion off the disk since it was
overlaid with the application program.

 The initialization portion of COMMAND.COM follows the resident portion and is
given control during the bootup procedure. This section actually processes the
AUTOEXEC.BAT file. It also decides where to load the user's programs when they
are executed. Since this code is only needed during startup, it is overlaid by
the first program which COMMAND.COM loads.

 The transient portion is loaded at the high end of memory and it is the
command processor itself. It interprets whatever the user types in at the
keyboard, hence messages such as "Bad command or file name" for when the user
misspells a command. This portion contains all the internal commands (i.e.
COPY, DIR, RENAME, ERASE), the batch file processor (to run .BAT files) and
a routine to load and execute external commands which are either .COM or
.EXE files.

 The transient portion of COMMAND.COM produces the system prompt, (C>), and
reads what the user types in from the keyboard and tries to do something with
it. For any .COM or .EXE files, it builds a command line and issues an EXEC
function call to load the program and transfer control to it.



DOS INITIALIZATION├──────────────────────────────────────────────────── 1**7

 The system is initialized by a software reset (Ctrl-Alt-Del), a hardware reset
(reset button), or by turning the computer on. The Intel 80x8x series processors
always look for their first instruction at the end of their address space
(0FFFF0h) when powered up or reset. This address contains a jump to the first
instruction for the ROM BIOS.

 Built-in ROM programs (Power-On Self-Test, or POST, in the IBM) check machine
status and run inspection programs of various sorts. Some machines set up a
reserved RAM area with bytes indicating installed equipment (AT and PCjr).

 When the ROM BIOS finds a ROM on an adapter card, it lets that ROM take
control of the system so that it may perform any set up necessary to use the
hardware or software controlled by that ROM. The ROM BIOS searches absolute
addresses C8000h through E0000h in 2K increments in search of a valid ROM.
A valid ROM is determined by the first few bytes in the ROM. The ROM will have
the bytes 55h, AAh, a length indicator and then the assembly language
instruction to CALL FAR (to bring in a "FAR" routine). A checksum is done on
the ROM to verify its integrity, then the BIOS performs the CALL FAR to bring
in the executible code. The adapter's ROM then performs its initialization
tasks and hopefully returns control of the computer back to the ROM BIOS so it
can continue with the booting process.

 The ROM BIOS routines then look for a disk drive at A: or an option ROM
(usually a hard disk) at absolute address C:800h. If no floppy drive or option
ROM is found, the BIOS calls int 19h (ROM BASIC if it is an IBM) or displays
an error message.

 If a bootable disk is found, the ROM BIOS loads the first sector of information
from the disk and then jumps into the RAM location holding that code. This code
normally is a routine to load the rest of the code off the disk, or to "boot"
the system.

 The following actions occur after a system initialization:

 1.  The boot record is read into memory and given control.

 2.  The boot record then checks the root directory to assure that the first
     two files are IBMBIO.COM and IBMDOS.COM. These two files must be the
     first two files, and they must be in that order (IBMBIO.COM first, with
     its sectors in contiguous order).
     NOTE: IBMDOS.COM need not be contiguous in version 3.x+.

 3.  The boot record loads IBMBIO.COM into memory.

 4.  The initialization code in IBMBIO.COM loads IBMDOS.COM, determines
     equipment status, resets the disk system, initializes the attached
     devices, sets the system parameters and loads any installable device
     drivers according to the CONFIG.SYS file in the root directory (if
     present), sets the low-numbered interrupt vectors, relocates IBMDOS.COM
     downward, and calls the first byte of DOS.
     NOTE: CONFIG.SYS may be a hidden file.

 5.  DOS initializes its internal working tables, initializes the interrupt
     vectors for interrupts 20h through 27h, and builds a Program Segment
     Prefix for COMMAND.COM at the lowest available segment. For DOS versions
     3.10 up, DOS also initializes the vectors for interrupts 0Fh through 3Fh.
     An initialization routine is included in the resident portion and assumes
     control during startup. This routine contains the AUTOEXEC.BAT file
     handler and determines the segment address where user application programs
     may be loaded. The initialization routine is then no longer needed and is
     overlaid by the first program COMMAND.COM loads.
     NOTE: AUTOEXEC.BAT may be a hidden file.

 6.  IBMDOS.COM uses the EXEC function call to load and start the top-level
     command processor. The default command processor is COMMAND.COM in the
     root directory of the boot drive. If COMMAND.COM is in a subdirectory
     or another command processor is to be used, it must be specified by a
     SHELL= statement in the CONFIG.SYS file.
     A transient portion is loaded at the high end of memory. This is the
     command processor itself, containing all of the internal command
     processors and the batch file processor. For DOS 2.x, this portion also
     contains a routine to load and execute external commands, such as files
     with extensions of COM or EXE.
      This portion of COMMAND.COM also produces the DOS prompt (such as "A>"),
     reads the command from the standard input device (usually the keyboard or
     a batch file), and executes the command. For external commands, it builds
     a command line and issues an EXEC function call to load and transfer
     control to the program.

note 1) COMMAND.COM may be a hidden file.
     2) For IBM DOS 2.x, the transient portion of the command processor
        contains the EXEC routine that loads and executes external commands.
        For MSDOS 2.x+ and IBM DOS 3.x+, the resident portion of the command
        processor contains the EXEC routine.
     3) IBMDOS only checks for a file named "COMMAND.COM". It will load
        any file of that name if no SHELL= command is used.


 That pretty much covers the bootup process. After the command processor is
loaded, it runs the AUTOEXEC.BAT file and then the user gets their prompt to
begin working.