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** Programmer's Technical Reference for MSDOS and the IBM PC ** USA copyright TXG 392-616 ALL RIGHTS RESERVED ───────────────────────────────┤ DOSREF (tm) ├──────────────────────────────── ISBN 1-878830-02-3 (disk-based text) Copyright (c) 1987, 1991 Dave Williams ┌─────────────────────────────┐ │ Shareware Version, 12/20/91 │ │ Please Register Your Copy │ └─────────────────────────────┘ C H A P T E R F O U R DOS INTERRUPTS AND FUNCTION CALLS note: The registered version of this chapter is twice this size. DOS REGISTERS├───────────────────────────────────────────────────────────────── DOS uses the following registers, pointers, and flags when it executes interrupts and function calls: ┌───────────────────┬──────────┬───────────────────────────────────────────────┐ │GENERAL REGISTERS │ register │ definition │ │ ├──────────┼───────────────────────────────────────────────┤ │ │ AX │ accumulator (16 bit) │ │ │ AH │ accumulator high-order byte (8 bit) │ │ │ AL │ accumulator low order byte (8 bit) │ │ │ BX │ base (16 bit) │ │ │ BH │ base high-order byte (8 bit) │ │ │ BL │ base low-order byte (8 bit) │ │ │ CX │ count (16 bit) │ │ │ CH │ count high order byte (8 bit) │ │ │ CL │ count low order byte (8 bit) │ │ │ DX │ data (16 bit) │ │ │ DH │ date high order byte (8 bit) │ │ │ DL │ data low order byte (8 bit) │ ├───────────────────┼──────────┼───────────────────────────────────────────────┤ │SEGMENT REGISTERS │ register │ definition │ │ ├──────────┼───────────────────────────────────────────────┤ │ │ CS │ code segment (16 bit) │ │ │ DS │ data segment (16 bit) │ │ │ SS │ stack segment (16 bit) │ │ │ ES │ extra segment (16 bit) │ ├───────────────────┼──────────┼───────────────────────────────────────────────┤ │INDEX REGISTERS │ register │ definition │ │ ├──────────┼───────────────────────────────────────────────┤ │ │ DI │ destination index (16 bit) │ │ │ SI │ stack index (16 bit) │ ├───────────────────┼──────────┼───────────────────────────────────────────────┤ │SEGMENT REGISTERS │ register │ definition │ │ ├──────────┼───────────────────────────────────────────────┤ │ │ CS │ code segment (16 bit) │ │ │ DS │ data segment (16 bit) │ │ │ SS │ stack segment (16 bit) │ │ │ ES │ extra segment (16 bit) │ ├───────────────────┼──────────┼───────────────────────────────────────────────┤ │INDEX REGISTERS │ register │ definition │ │ ├──────────┼───────────────────────────────────────────────┤ │ │ DI │ destination index (16 bit) │ │ │ SI │ stack index (16 bit) │ ├───────────────────┼──────────┼───────────────────────────────────────────────┤ │POINTERS │ register │ definition │ │ ├──────────┼───────────────────────────────────────────────┤ │ │ SP │ stack pointer (16 bit) │ │ │ BP │ base pointer (16 bit) │ │ │ IP │ instruction pointer (16 bit) │ ├───────────────────┴──────────┴───────────────────────────────────────────────┤ │FLAGS AF, CF, DF, IF, OF, PF, SF, TF, ZF │ └──────────────────────────────────────────────────────────────────────────────┘ These registers, pointers, and flags are "lowest common denominator" 8088-8086 CPU oriented. DOS makes no attempt to use any of the special or enhanced instructions availible on the later CPUs which will execute 8088 code, such as the 80186, 80286, 80386, or NEV V20, V30, V40, or V50. When DOS takes control after a function call, it switches to an internal stack. Registers which are not used to return information (other than AX) are preserved. The calling program's stack must be large enough to accomodate the interrupt system - at least 128 bytes in addition to other interrupts. DOS actually maintains three stacks - stack 1: 384 bytes (in DOS 3.1) for functions 00h and for 0Dh and up, and for ints 25h and 26h. stack 2: 384 bytes (in DOS 3.1) for function calls 01h through 0Ch. stack 3: 48 bytes (in DOS 3.1) for functions 0Dh and above. This stack is the initial stack used by the int 21h handler before it decides which of the other two to use. It is also used by function 59h (get extended error), and 01h to 0Ch if they are called during an int 24h (critical error) handler. Functions 33h (get/set break flag), 50h (set process ID), 51h (get process ID) and 62h (get PSP address) do not use any DOS stack under DOS 3.x (under 2.x, 50h and 51h use stack number 2). IBM and Microsoft made a change back in DOS 3.0 or 3.1 to reduce the size of DOS. They reduced the space allocated for scratch areas when interrupts are being processed. The default seems to vary with the DOS version and the machine, but 8 stack frames seems to ring a bell. That means that if you get more than 8 interrupts at the same time, clock, disk, printer spooler, keyboard, com port, etc., the system will crash. It seems to happen usually on a network. STACKS=16,256 means allow 16 interrupts to interrupt each other and allow 256 bytes for each for scratch area. Eight is marginal. DOS 3.2 does some different stack switching than previous versions. The interrupts which are switched are 02h, 08h, 09h, 0Ah, 0Bh, 0Ch, 0Dh, 0Eh, 70h, 72h, 73h, 74h, 75h, 76h, and 77h. DOS 3.2 has a special check in the initialization code for a PCjr and don't enable stack switching on that machine. INTERRUPTS├──────────────────────────────────────────────────────────────────── Microsoft recommends that a program wishing to examine or set the contents of any interrupt vector use the DOS function calls 35h and 25h provided for those purposes and avoid referencing the interrupt vector locations directly. DOS reserves interrupt numbers 20h to 3Fh for its own use. This means absolute memory locations 80h to 0FFh are reserved by DOS. The defined interrupts are as follows with all values in hexadecimal. ┌─────────────────────────────────────────────────────────────────────────────┐ │Interrupt 21h Function Call Request │ └─────────────────────────────────────────────────────────────────────────────┘ (0:0084h) DOS provides a wide variety of function calls for character device I/O, file management, memory management, date and time functions,execution of other programs, and more. They are grouped as follows: call description 00h program terminate 01h-0Ch character device I/O, CP/M compatibility format 0Dh-24h file management, CP/M compatibility format 25h-26h nondevice functions, CP/M compatibility format 27h-29h file management, CP/M compatibility format 2Ah-2Eh nondevice functions, CP/M compatibility format 2Fh-38h extended functions 39h-3Bh directory group 3Ch-46h extended file management 47h directory group 48h-4Bh extended memory management 54h-57h extended functions 5Eh-5Fh networking 60h-62h extended functions 63h-66h enhanced foreign language support List of DOS services: * = undocumented 00h terminate program 01h get keyboard input 02h display character to STDIO 03h get character from STDAUX 04h output character to STDAUX 05h output character to STDPRN 06h direct console I/O - keyboard to screen 07h get char from std I/O without echo 08h get char from std I/O without echo, checks for ^C 09h display a string to STDOUT 0Ah buffered keyboard input 0Bh check STDIN status 0Ch clear keyboard buffer and invoke keyboard function 0Dh flush all disk buffers 0Eh select disk 0Fh open file with File Control Block 10h close file opened with File Control Block 11h search for first matching file entry 12h search for next matching file entry 13h delete file specified by File Control Block 14h sequential read from file specified by File Control Block 15h sequential write to file specified by File Control Block 16h find or create firectory entry for file 17h rename file specified by file control block 18h* unknown 19h return current disk drive 1Ah set disk transfer area (DTA) 1Bh get current disk drive FAT 1Ch get disk FAT for any drive 1Dh* unknown 1Eh* unknown 1Fh* read DOS disk block, default drive 20h* unknown 21h random read from file specified by FCB 22h random write to file specified by FCB 23h return number of records in file specified by FCB 24h set relative file record size field for file specified by FCB 25h set interrupt vector 26h create new Program Segment Prefix (PSP) 27h random file block read from file specified by FCB 28h random file block write to file specified by FCB 29h parse the command line for file name 2Ah get the system date 2Bh set the system date 2Ch get the system time 2Dh set the system time 2Eh set/clear disk write VERIFY 2Fh get the Disk Transfer Address (DTA) 30h get DOS version number 31h TSR, files opened remain open 32h* read DOS Disk Block 33h get or set Ctrl-Break 34h* INDOS Critical Section Flag 35h get segment and offset address for an interrupt 36h get free disk space 37h* get/set option marking character (SWITCHAR) 38h return country-dependent information 39h create subdirectory 3Ah remove subdirectory 3Bh change current directory 3Ch create and return file handle 3Dh open file and return file handle 3Eh close file referenced by file handle 3Fh read from file referenced by file handle 40h write to file referenced by file handle 41h delete file 42h move file pointer (move read-write pointer for file) 43h set/return file attributes 44h device IOCTL (I/O control) info 45h duplicate file handle 46h force a duplicate file handle 47h get current directory 48h allocate memory 49h release allocated memory 4Ah modify allocated memory 4Bh load or execute a program 4Ch terminate prog and return to DOS 4Dh get return code of subprocess created by 4Bh 4Eh find first matching file 4Fh find next matching file 50h* set new current Program Segment Prefix (PSP) 51h* puts current PSP into BX 52h* pointer to the DOS list of lists 53h* translates BPB (Bios Parameter Block, see below) 54h get disk verification status (VERIFY) 55h* create PSP: similar to function 26h 56h rename a file 57h get/set file date and time 58h get/set allocation strategy (DOS 3.x) 59h get extended error information 5Ah create a unique filename 5Bh create a DOS file 5Ch lock/unlock file contents 5Dh* network 5Eh* network printer 5Fh* network redirection 60h* parse pathname 61h* unknown 62h get program segment prefix (PSP) 63h* get lead byte table (DOS 2.25) 64h* unknown 65h get extended country information (DOS 3.3) 66h get/set global code page table (DOS 3.3) 67h set handle count (DOS 3.3) 68h commit file (DOS 3.3) 69h disk serial number (DOS 4.0) 6Ah unknown 6Bh unknown 6Ch extended open/create (DOS 4.0) CALLING THE DOS SERVICES├────────────────────────────────────────────────────── The DOS services are invoked by placing the number of the desired function in register AH, subfunction in AL, setting the other registers to any specific requirements of the function, and invoking int 21h. On return, the requested service will be performed if possible. Most codes will return an error; some return more information. Details are contained in the listings for the individual functions. Extended error return may be obtained by calling function 59h (see 59h). Register settings listed are the ones used by DOS. Some functions will return with garbage values in unused registers. Do not test for values in unspecified registers; your program may exhibit odd behavior. DS:DX pointers are the data segment register (DS) indexed to the DH and DL registers (DX). DX always contains the offset address, DS contains the segment address. The File Control Block services (FCB services) were part of DOS 1.0. Since the release of DOS 2.0, Microsoft has recommended that these services not be used. A set of considerably more enhanced services (handle services) were introduced with DOS 2.0. The handle services provide support for wildcards and subdirectories, and enhanced error detection via function 59h. The data for the following calls was compiled from various Intel, Microsoft, IBM, and other publications. There are many subtle differences between MSDOS and PCDOS and between the individual versions. Differences between the versions are noted as they occur. There are various ways of calling the DOS functions. For all methods, the function number is loaded into register AH, subfunctions and/or parameters are loaded into AL or other registers, and call int 21 by one of the following methods: A) call interrupt 21h directly (the recommended procedure) B) perform a long call to offset 50h in the program's PSP. 1) This method will not work under DOS 1.x 2) Though recommended by Microsoft for DOS 2.0, this method takes more time and is no longer recommended. C) place the function number in CL and perform an intrasegment call to location 05h in the current code segment. This location contains a long call to the DOS function dispatcher. 1) IBM recommends this method be used only when using existing programs written for different calling conventions. (such as converting CP/M programs). This method should be avoided unless you have some specific use for it. 2) AX is always destroyed by this method. 3) This method is valid only for functions 00h-24h. There are also various ways of exiting from a program. (assuming it is not intended to be a TSR). All methods except call 4Ch must ensure that the segment register contains the segment address of the PSP. A) Interrupt 21h, function 4Ch (Terminate with Result Code). This is the "official" recommended method of returning to DOS. B) Interrupt 21h, function 00h (Exit Program). This is the early style int 21 function call. It simply calls int 20h. C) Interrupt 20h (Exit). D) A JMP instruction to offset 00h (int 20h vector) in the Program Segment Prefix. This is just a roundabout method to call int 20h. This method was set up in DOS 1.0 for ease of conversion for CP/M programs. It is no longer recommended for use. E) A JMP instruction to offset 05h (int 21 vector) in the Program Segment Prefix, with AH set to 00h or 4Ch. This is another CP/M type function. INT 21H DOS services Function (hex) * Indicates Functions not documented in the IBM DOS Technical Reference. Note some functions have been documented in other Microsoft or licensed OEM documentation. Function 00h Terminate Program Ends program, updates, FAT, flushes buffers, restores registers entry AH 00h CS segment address of PSP return none note 1) Program must place the segment address of the PSP control block in CS before calling this function. 2) The terminate, ctrl-break,and critical error exit addresses (0Ah, 0Eh, 12h) are restored to the values they had on entry to the terminating program, from the values saved in the program segment prefix at locations PSP:000Ah, PSP:000Eh, and PSP:0012h. 3) All file buffers are flushed and the handles opened by the process are closed. 4) Any files that have changed in length and are not closed are not recorded properly in the directory. 5) Control transfers to the terminate address. 6) This call performs exactly the same function as int 20h. 7) All memory used by the program is returned to DOS. Function 01h Get Keyboard Input Waits for char at STDIN (if nescessary), echoes to STDOUT entry AH 01h return AL ASCII character from STDIN (8 bits) note 1) Checks char for Ctrl-C, if char is Ctrl-C, executes int 23h. 2) For function call 06h, extended ASCII codes require two function calls. The first call returns 00h as an indicator that the next call will be an extended ASCII code. 3) Input and output are redirectable. If redirected, there is no way to detect EOF. Function 02h Display Output Outputs char in DL to STDOUT entry AH 02h DL 8 bit data (usually ASCII character) return none note 1) If char is 08 (backspace) the cursor is moved 1 char to the left (nondestructive backspace). 2) If Ctrl-C is detected after input, int 23h is executed. 3) Input and output are redirectable. If redirected, there is no way to detect disk full. Function 03h Auxiliary Input Get (or wait until) character from STDAUX entry AH 03h return AL char from auxiliary device note 1) AUX, COM1, COM2 is unbuffered and not interrupt driven 2) This function call does not return status or error codes. For greater control it is recommended that you use ROM BIOS routine (int 14h) or write an AUX device driver and use IOCTL. 3) At startup, PC-DOS initializes the first auxiliary port (COM1) to 2400 baud, no parity, one stop bit, and an 8-bit word. MSDOS may differ. 4) If Ctrl-C is has been entered from STDIN, int 23h is executed. Function 04h Auxiliary Output Write character to STDAUX entry AH 04h DL char to send to AUX return none note 1) This function call does not return status or error codes. For greater control it is recommended that you use ROM BIOS routine (int 14h) or write an AUX device driver and use IOCTL. 2) If Ctrl-C is has been entered from STDIN, int 23h is executed. 3) Default is COM1 unless redirected by DOS. 4) If the device is busy, this function will wait until it is ready. Function 05h Printer Output Write character to STDPRN entry AL 05h DL character to send return none note 1) If Ctrl-C is has been entered from STDIN, int 23h is executed. 2) Default is PRN or LPT1 unless redirected with the MODE command. 3) If the printer is busy, this function will wait until it is ready. Function 06h Direct Console I/O Get character from STDIN; echo character to STDOUT entry AH 06h DL 0FFh for console input, or 00h-0FEh for console output return ZF set (1) = no character clear (0) = character recieved AL character note 1) Extended ASCII codes require two function calls. The first call returns 00h to indicate the next call will return an extended code. 2) If DL is not 0FFh, DL is assumed to have a valid character that is output to STDOUT. 3) This function does not check for Ctrl-C or Ctrl-PrtSc. 4) Does not echo input to screen 5) If I/O is redirected, EOF or disk full cannot be detected. Function 07h Direct Console Input Without Echo (does not check BREAK) Get or wait for char at STDIN, returns char in AL entry AH 07h return AL character from standard input device note 1) Extended ASCII codes require two function calls. The first call returns 00h to indicate the next call will return an extended code. 2) No checking for Ctrl-C or Ctrl-PrtSc is done. 3) Input is redirectable. Function 08h Console Input Without Echo (checks BREAK) Get or Wait for char at STDIN, return char in AL entry AH 08h return AL char from standard input device note 1) Char is checked for ctrl-C. If ctrl-C is detected, executes int 23h. 2) For function call 08h, extended ASCII characters require two function calls. The first call returns 00h to signify an extended ASCII code. The next call returns the actual code. 3) Input is redirectable. If redirected, there is no way to check EOF. Function 09h Print String Outputs Characters in the Print String to the STDOUT entry AH 09h DS:DX pointer to the Character String to be displayed return none note 1) The character string in memory must be terminated by a $ (24h) The $ is not displayed. 2) Output to STDOUT is the same as function call 02h. Function 0Ah Buffered Keyboard Input Reads characters from STDIN and places them in the buffer beginning at the third byte. entry AH 0Ah DS:DX pointer to an input buffer return none note 1) Min buffer size = 1, max = 255 2) Char is checked for ctrl-C. If ctrl-C is detected, executes int 23h. 3) Format of buffer DX: byte contents 1 Maximum number of chars the buffer will take, including CR. Reading STDIN and filling the buffer continues until a carriage return (<Enter> or 0Dh) is read. If the buffer fills to one less than the maximum number the buffer can hold, each additional number read is ignored and ASCII 7 (BEL) is output to the display until a carriage return is read. (you must set this value) 2 Actual number of characters received, excluding the carriage return, which is always the last character. (the function sets this value) 3-n Characters received are placed into the buffer starting here. Buffer must be at least as long as the number in byte 1. 4) Input is redirectable. If redirected, there is no way to check EOF. 5) The string may be edited with the standard DOS editing commands as it is being entered. 6) Extended ASCII characters are stored as 2 bytes, the first byte being zero. Function 0Bh Check Standard Input (STDIN) status Checks for character availible at STDIN entry AH 0Bh return AL 0FFh if a character is availible from STDIN 00h if no character is availible from STDIN note 1) Checks for Ctrl-C. If Ctrl-C is detected, int 23h is executed 2) Input can be redirected. 3) Checks for character only, it is not read into the application 4) IBM reports that this call does not work properly under the DOSSHELL program in DOS 4.00 and 4.01. DOSSHELL will return all zeroes. This function works correctly from the command line or application. Function 0Ch Clear Keyboard Buffer & Invoke a Keyboard Function (FCB) Dumps buffer, executes function in AL (01h,06h,07h,08h,0Ah only) entry AH 0Ch AL function number (must be 01h, 06h, 07h, 08h, or 0Ah) return AL 00h buffer was flushed, no other processing performed other any other value has no meaning note 1) Forces system to wait until a character is typed. 2) Flushes all typeahead input, then executes function specified by AL (by moving it to AH and repeating the int 21 call). 3) If AL contains a value not in the list above, the keyboard buffer is flushed and no other action is taken. Function 0Dh Disk Reset Flushes all currently open file buffers to disk entry AH 0Dh return none note 1) Does not close files. Does not update directory entries; files changed in size but not closed are not properly recorded in the directory 2) Sets DTA address to DS:0080h 3) Should be used before a disk change, Ctrl-C handlers, and to flush the buffers to disk. Function 0Eh Select Disk Sets the drive specified in DL (if valid) as the default drive entry AL 0Eh DL new default drive number (0=A:,1=B:,2=C:,etc.) return AL total number of logical drives (not nescessarily physical) note 1) For DOS 1.x and 2.x, the minimum value for AL is 2. 2) For DOS 3.x and 4.x, the minimum value for AL is 5. 3) The drive number returned is not nescessarily a valid drive. 4) For DOS 1.x: 16 logical drives are availible, A-P. For DOS 2.x: 63 logical drives are availible. (Letters are only used for the first 26 drives. If more than 26 logical drives are used, further drive letters will be other ASCII characters ie {,], etc. For DOS 3.x: 26 logical drives are availible, A-Z. For DOS 4.x: 26 logical drives are availible, A-Z. Function 0Fh Open Disk File (FCB) Searches current directory for specified filename and opens it entry AH 0Fh DS:DX pointer to an unopened FCB return AL 00h if file found 0FFh if file not not found note 1) If the drive code was 0 (default drive) it is changed to the actual drive used (1=A:,2=B:,3=C:, etc). This allows changing the default drive without interfering with subsequent operations on this file. 2) The current block field (FCB bytes C-D, offset 0Ch) is set to zero. 3) The size of the record to be worked with (FCB bytes E-F, offset 0Eh) is set to the system default of 80h. The size of the file (offset 10h) and the date (offset 14h) are set from information obtained in the root directory. You can change the default value for the record size (FCB bytes E-F) or set the random record size and/or current record field. Perform these actions after the open but before any disk operations. 4) The file is opened in compatibility mode. 5) Microsoft recommends handle function call 3Dh be used instead. 6) This call is also used by the APPEND command in DOS 3.2+ 7) Before performing a sequential disk operation on the file, you must set the Current Record field (offset 20h). Before performing a random disk operation on the file, you must set the Relative Record field (offset 21h). If the default record size of 128 bytes is incorrect, set it to the correct value. Function 10h Close File (FCB) Closes a File After a File Write entry AH 10h DS:DX pointer to an opened FCB return AL 00h if the file is found and closed 0FFh if the file is not found in the current directory note 1) This function call must be done on open files that are no longer needed, and after file writes to insure all directory information is updated. 2) If the file is not found in its correct position in the current directory, it is assumed that the diskette was changed and AL returns 0FFh. This error return is reportedly not completely reliable with DOS version 2.x. 3) If found, the directory is updated to reflect the status in the FCB, the buffers to that file are flushed, and AL returns 00h. Function 11h Search For First Matching Entry (FCB) Searches current disk & directory for first matching filename entry AH 11h DS:DX pointer to address of FCB return AL 00h successful match 0FFh no matching filename found note 1) The FCB may contain the wildcard character ? under Dos 2.x, and ? or * under 3.x and 4.x. 2) The original FCB at DS:DX contains information to continue the search with function 12h, and should not be modified. 3) If a matching filename is found, AL returns 00h and the locations at the Disk Transfer Address are set as follows: a) If the FCB provided for searching was an extended FCB, then the first byte at the disk transfer address is set to 0FFh followed by 5 bytes of zeroes, then the attribute byte from the search FCB, then the drive number used (1=A, 2=B, etc) then the 32 bytes of the directory entry. Thus, the disk transfer address contains a valid unopened FCB with the same search attributes as the search FCB. b) If the FCB provided for searching was a standard FCB, then the first byte is set to the drive number used (1=A,2=b,etc), and the next 32 bytes contain the matching directory entry. Thus, the disk transfer address contains a valid unopened normal FCB. 4) If an extended FCB is used, the following search pattern is used: a) If the FCB attribute byte is zero, only normal file entries are found. Entries for volume label, subdirectories, hidden or system files, are not returned. b) If the attribute byte is set for hidden or system files, or subdirectory entries, it is to be considered as an inclusive search. All normal file entries plus all entries matching the specified attributes are returned. To look at all directory entries except the volume label, the attribute byte may be set to hidden + system + directory (all 3 bits on). c) If the attribute field is set for the volume label, it is considered an exclusive search, and ONLY the volume label entry is returned. 5) This call is also used by the APPEND command in DOS 3.2+ Function 12h Search For Next Entry Using FCB (FCB) Search for next matching filename entry AH 12h DS:DX pointer to the unopened FCB specified from the previous Search First (11h) or Search Next (12h) return AL 00h if matching filename found 0FFh if matching filename was not found note 1) After a matching filename has been found using function call 11h, function 12h may be called to find the next match to an ambiguous request. For DOS 2.x, ?'s are allowed in the filename. For DOS 3.x and 4.x, global (*) filename characters are allowed. 2) The DTA contains info from the previous Search First or Search Next. 3) All of the FCB except for the name/extension field is used to keep information nescessary for continuing the search, so no disk operations may be performed with this FCB between a previous function 11h or 12h call and this one. 4) If the file is found, an FCB is created at the DTA address and set up to open or delete it. Function 13h Delete File Via FCB (FCB) Deletes file specified in FCB from current directory entry AH 13h DS:DX pointer to address of FCB return AL 00h file deleted 0FFh if file not found or was read-only note 1) All matching current directory entries are deleted. The global filename character "?" is allowed in the filename. 2) Will not delete files with read-only attribute set 3) Close open files before deleting them. 4) Requires Network Access Rights Function 14h Sequential Disk File Read (FCB) Reads record sequentially from disk via FCB entry AH 14h DS:DX pointer to an opened FCB return AL 00h successful read 01h end of file (no data read) 02h Data Transfer Area too small for record size specified or segment overflow 03h partial record read, EOF found note 1) The record size is set to the value at offset 0Eh in the FCB. 2) The record pointed to by the Current Block (offset 0Ch) and the Current Record (offset 20h) fields is loaded at the DTA, then the Current Block and Current Record fields are incremented. 3) The record is read into memory at the current DTA address as specified by the most recent call to function 1Ah. If the size of the record and location of the DTA are such that a segment overflow or wraparound would occur, the error return is set to AL=02h 4) If a partial record is read at the end of the file, it is passed to the requested size with zeroes and the error return is set to AL=03h. Function 15h Sequential Disk Write (FCB) Writes record specified by FCB sequentially to disk entry AH 15h DS:DX pointer to address of FCB return AL 00h successful write 01h diskette full, write canceled 02h disk transfer area (DTA) too small or segment wrap note 1) The data to write is obtained from the disk transfer area 2) The record size is set to the value at offset 0Eh in the FCB. 3) This service cannot write to files set as read-only 4) The record pointed to by the Current Block (offset 0Ch) and the Current Record (offset 20h) fields is loaded at the DTA, then the Current Block and Current Record fields are incremented. 5) If the record size is less than a sector, the data in the DTA is written to a buffer; the buffer is written to disk when it contains a full sector of data, the file is closed, or a Reset Disk (function 0Dh) is issued. 6) The record is written to disk at the current DTA address as specified by the most recent call to function 1Ah. If the size of the record and location of the DTA are such that a segment overflow or wraparound would occur, the error return is set to AL=02h Function 16h Create A Disk File (FCB) Search and open or create directory entry for file entry AH 16h DS:DX pointer to an FCB return AL 00h successful creation 0FFh no room in directory note 1) If a matching directory entry is found, the file is truncated to zero bytes. 2) If there is no matching filename, a filename is created. 3) This function calls function 0Fh (Open File) after creating or truncating a file. 4) A hidden file can be created by using an extended FCB with the attribute byte (offset FCB-1) set to 2. Function 17h Rename File Specified by File Control Block (FCB) Renames file in current directory entry AH 17h DS:DX pointer to an FCB (see note 4) return AL 00h successfully renamed 0FFh file not found or filename already exists note 1) This service cannot rename read-only files 2) The "?" wildcard may be used. 3) If the "?" wildcard is used in the second filename, the corresponding letters in the filename of the directory entry are not changed. 4) The FCB must have a drive number, filename, and extension in the usual position, and a second filename starting 6 bytes after the first, at offset 11h. 5) The two filenames cannot have the same name. 6) FCB contains new name starting at byte 17h. Function 18h Internal to DOS * Unknown entry AH 18h return AL 0 Function 19h Get Current Disk Drive Return designation of current default disk drive entry AH 19h return AL current default drive (0=A, 1=B,etc.) note Some other DOS functions use 0 for default, 1=A, 2=B, etc. Function 1Ah Set Disk Transfer Area Address (DTA) Sets DTA address to the address specified in DS:DX entry AH 1Ah DS:DX pointer to buffer return none note 1) The default DTA is 128 bytes at offset 80h in the PSP. DOS uses the DTA for all file I/O. 2) Registers are unchanged. 3) No error codes are returned. 2) Disk transfers cannot wrap around from the end of the segment to the beginning or overflow into another segment. Function 1Bh Get Current Drive File Allocation Table Information Returns information from the FAT on the current drive entry AH 1Bh exit AL number of sectors per allocation unit (cluster) DS:BX address of the current drive's media descriptor byte CX number of bytes per sector DX number of allocation units (clusters) for default drive note 1) Save DS before calling this function. 2) This call returned a pointer to the FAT in DOS 1.x. Beginning with DOS 2.00, it returns a pointer only to the table's ID byte. 3) IBM recommends programmers avoid this call and use int 25h instead. Function 1Ch Get File Allocation Table Information for Specific Device Returns information on specified drive entry AH 1Ch DL drive number (1=A, 2=B, 3=C, etc) return AL number of sectors per allocation unit (cluster) DS:BX address of media descriptor byte for drive in DL CX sector size in bytes DX number of allocation units (clusters) note 1) DL = 0 for default. 2) Save DS before calling this function. 3) Format of media-descriptor byte: bits: 0 0 (clear) not double sided 1 (set) double sided 1 0 (clear) not 8 sector 1 (set) 8 sector 2 0 (clear) nonremovable device 1 (set) removable device 3-7 always set (1) 4) This call returned a pointer to the FAT in DOS 1.x. Beginning with DOS 2.00, it returns a pointer only to the table's ID byte. 5) IBM recommends programmers avoid this call and use int 25h instead. Function 1Dh Not Documented by Microsoft * Unknown entry AH 1Dh return AL 0 Function 1Eh Not Documented by Microsoft * Unknown entry AH 1Eh return AL 0 note Apparently does nothing Function 1Fh Get Default Drive Parameter Block * Same as function call 32h (below), except that the table is accessed from the default drive entry AH 1Fh other registers unknown return AL 00h no error 0FFh error DS:BX points to DOS Disk Parameter Block for default drive. note 1) Unknown vector returned in ES:BX. 2) For DOS 2.x and 3.x, this just invokes function 32h (undocumented, Read DOS Disk Block) with DL=0 Function 20h Unknown * Internal - does nothing? entry AH 20h return AL 0 Function 21h Random Read from File Specified by File Control Block (FCB) Reads one record as specified in the FCB into the current DTA. entry AH 21h DS:DX address of the opened FCB return AL 00h successful read operation 01h end of file (EOF), no data read 02h DTA too small for the record size specified 03h end of file (EOF), partial data read note 1) The current block and current record fields are set to agree with the random record field. Then the record addressed by these fields is read into memory at the current Disk Transfer Address. 2) The current file pointers are NOT incremented this function. 3) If the DTA is larger than the file, the file is padded to the requested length with zeroes. Function 22h Random Write to File Specified by FCB (FCB) Writes one record as specified in the FCB to the current DTA entry AH 22h DS:DX address of the opened FCB return AL 00h successful write operation 01h disk full; no data written (write was canceled) 02h DTA too small for the record size specified (write was canceled) note 1) This service cannot write to read-only files. 2) The record pointed to by the Current Block (offset 0Ch) and the Current Record (offset 20h) fields is loaded at the DTA, then the Current Block and Current Record fields are incremented. 3) If the record size is less than a sector, the data in the DTA is written to a buffer; the buffer is written to disk when it contains a full sector of data, the file is closed, or a Reset Disk (function 0Dh) is issued. 4) The current file pointers are NOT incremented this function. 5) The record is written to disk at the current DTA address as specified by the most recent call to function 1Ah. If the size of the record and location of the DTA are such that a segment overflow or wraparound would occur, the error return is set to AL=02h Function 23h Get File Size (FCB) Searches current subdirectory for matching file, returns size in FCB entry AH 23h DS:DX address of an unopened FCB return AL 00h file found 0FFh file not found note 1) Record size field (offset 0Eh) must be set before invoking this function 2) The disk directory is searched for the matching entry. If a matching entry is found, the random record field is set to the number of records in the file. If the value of the Record Size field is not an even divisor of the file size, the value set in the relative record field is rounded up. This gives a returned value larger than the actual file size 3) This call is used by the APPEND command in DOS 3.2+ Function 24h Set Relative Record Field (FCB) Set random record field specified by an FCB entry AH 24h DS:DX address of an opened FCB return Random Record Field of FCB is set to be same as Current Block and Current Record. note 1) You must invoke this function before performing random file access. 2) The relative record field of FCB (offset 21h) is set to be same as the Current Block (offset 0Ch) and Current Record (offset 20h). 3) No error codes are returned. 4) The FCB must already be opened. Function 25h Set Interrupt Vector Sets the address of the code DOS is to perform each time the specified interrupt is invoked. entry AH 25h AL int number to reassign the handler to DS:DX address of new interrupt vector return none note 1) Registers are unchanged. 2) No error codes are returned. 3) The interrupt vector table for the interrupt number specified in AL is set to the address contained in DS:DX. Use function 35h (Get Vector) to get the contents of the interrupt vector and save it for later use. 4) When you use function 25 to set an interrupt vector, DOS 3.2 doesn't point the actual interrupt vector to what you requested. Instead, it sets the interrupt vector to point to a routine inside DOS, which does this: 1. Save old stack pointer 2. Switch to new stack pointer allocated from DOS's stack pool 3. Call your routine 4. Restore old stack pointer The purpose for this was to avoid possible stack overflows when there are a large number of active interrupts. IBM was concerned (this was an IBM change, not Microsoft) that on a Token Ring network there would be a lot of interrupts going on, and applications that hadn't allocated very much stack space would get clobbered. Function 26h Create New Program Segment Prefix (PSP) This service copies the current program-segment prefix to a new memory location for the creation of a new program or overlay. Once the new PSP is in place, a DOS program can read a DOS COM or overlay file into the memory location immediately following the new PSP and pass control to it. entry AH 26h DX segment number for the new PSP return none note 1) Microsoft recommends you use the newer DOS service 4Bh (EXEC) instead. 2) The entire 100h area at location 0 in the current PSP is copied into location 0 of the new PSP. The memory size information at location 6 in the new segment is updated and the current termination, ctrl-break, and critical error addresses from interrupt vector table entries for ints 22h, 23h, and 24 are saved in the new program segment starting at 0Ah. They are restored from this area when the program terminates. 3) Current PSP is copied to specified segment Function 27h Random Block Read From File Specified by FCB Similar to 21h (Random Read) except allows multiple files to be read. entry AH 27h CX number of records to be read DS:DX address of an opened FCB return AL 00h successful read 01h end of file, no data read 02h DTA too small for record size specified (read canceled) 03h end of file CX actual number of records read (includes partial if AL=03h) note 1) The record size is specified in the FCB. The service updates the Current Block (offset 0Ch) and Current Record (offset 20h) fields to the next record not read. 2) If CX contained 0 on entry, this is a NOP. 3) If the DTA is larger than the file, the file is padded to the requested length with zeroes. 4) This function assumes that the FCB record size field (0Eh) is correctly set. If not set by the user, the default is 128 bytes. 5) The record is written to disk at the current DTA address as specified by the most recent call to function 1Ah. If the size of the record and location of the DTA are such that a segment overflow or wraparound would occur, the error return is set to AL=02h Function 28h Random Block Write to File Specified in FCB Similar to 27h (Random Write) except allows multiple files to be read. entry AH 28h CX number of records to write DS:DX address of an opened FCB return AL 00h successful write 01h disk full, no data written 02h DTA too small for record size specified (write canceled) CX number of records written note 1) The record size is specified in the FCB. 2) This service allocates disk clusters as required. 3) This function assumes that the FCB Record Size field (offset 0Eh) is correctly set. If not set by the user, the default is 128 bytes. 4) The record size is specified in the FCB. The service updates the Current Block (offset 0Ch) and Current Record (offset 20h) fields to the next record not read. 5) The record is written to disk at the current DTA address as specified by the most recent call to function 1Ah. If the size of the record and location of the DTA are such that a segment overflow or wraparound would occur, the error return is set to AL=02h 6) If called with CX=0, no records are written, but the FCB's File Size entry (offset 1Ch) is set to the size specified by the FCB's Relative Record field (offset 21h). Function 29h Parse the Command Line for Filename Parses a text string into the fields of a File Control Block entry AH 29h DS:SI pointer to string to parse ES:DI pointer to memory buffer to fill with unopened FCB AL bit mask to control parsing bit 0 = 0: parsing stops if file seperator found 1: causes service to scan past leading chars such as blanks. Otherwise assumes the filename begins in the first byte 1 = 0: drive number in FCB set to default (0) if string contains no drive number 1: drive number in FCB not changed 2 = 0: filename in FCB set to 8 blanks if no filename in string 1: filename in FCB not changed if string does not contain a filename 3 = 0: extension in FCB set to 3 blanks if no extension in string 1: extension left unchanged 4-7 must be zero return AL 00h no wildcards in name or extension 01h wildcards appeared in name or extension 0FFh invalid drive specifier DS:SI pointer to the first byte after the parsed string ES:DI pointer to a buffer filled with the unopened FCB note 1) If the * wildcard characters are found in the command line, this service will replace all subsequent chars in the FCB with question marks. 2) This service uses the characters as filename separators DOS 1 : ; . , + / [ ] = " TAB SPACE DOS 2,3 : ; . , + = TAB SPACE 3) This service uses the characters : ; . , + < > | / \ [ ] = " TAB SPACE or any control characters as valid filename separators 4) A filename cannot contain a filename terminator. If one is encountered, all processing stops. The handle functions will allow use of some of these characters. 5) If no valid filename was found on the command line, ES:DI +1 points to a blank (ASCII 32). 6) This function cannot be used with filespecs which include a path 7) Parsing is in the form D:FILENAME.EXT. If one is found, a corresponding unopened FCB is built at ES:DI Function 2Ah Get Date Returns day of the week, year, month, and date entry AH 2Ah return CX year (1980-2099) DH month (1-12) DL day (1-31) AL weekday 00h Sunday 01h Monday 02h Tuesday 03h Wednesday 04h Thursday 05h Friday 06h Saturday note 1) Date is adjusted automatically if clock rolls over to the next day, and takes leap years and number of days in each month into account. 2) Although DOS cannot set an invalid date, it can read one, such as 1/32/80, etc. 3) DesQview also accepts CX = 4445h and DX = 5351h, i.e. 'DESQ' as valid 4) DOS will accept CH=0 (midnight) as a valid time, but if a file's time is set to exactly midnight the time will not be displayed by the DIR command. Function 2Bh Set Date set current system date entry AH 2Bh CX year (1980-2099) DH month (1-12) DL day (1-31) return AL 00h no error (valid date) 0FFh invalid date specified note 1) On entry, CX:DX must have a valid date in the same format as returned by function call 2Ah 2) DOS 3.3 also sets CMOS clock Function 2Ch Get Time Get current system time from CLOCK$ driver entry AH 2Ch return CH hours (0-23) CL minutes (0-59) DH seconds (0-59) DL hundredths of a second (0-99) note 1) Time is updated every 5/100 second. 2) The date and time are in binary format Function 2Dh Set Time Sets current system time entry AH 2Dh CH hours (0-23) CL minutes (0-59) DH seconds (0-59) DL hundredths of seconds (0-99) return AL 00h if no error 0FFh if bad value sent to routine note 1) DOS 3.3 also sets CMOS clock 2) CX and DX must contain a valid time in binary Function 2Eh Set/Reset Verify Switch Set verify flag entry AH 2Eh AL 00 to turn verify off (default) 01 to turn verify on return none note 1) This is the call invoked by the DOS VERIFY command 2) Setting of the verify switch can be obtained by calling call 54h 3) This call is not supported on network drives 4) DOS checks this flag each time it accesses a disk Function 2Fh Get Disk Transfer Address (DTA) Returns current disk transfer address used by all DOS read/write operations entry AH 2Fh return ES:BX address of DTA note 1) The DTA is set by function call 1Ah 2) Default DTA address is a 128 byte buffer at offset 80h in that program's Program Segment Prefix Function 30h Get DOS Version Number Return DOS version and/or user number entry AH 30h return AH minor version number (i.e., DOS 2.10 returns AX = 0A02h) AL major version number BH OEM ID number 00h IBM 16h DEC (others not known) BL:CX 24-bit user serial number note 1) If AL returns a major version number of zero, the DOS version is below 1.28 for MSDOS and below 2.00 for PCDOS. 2) IBM PC-DOS always returns 0000h in BX and CX. 3) OS/2 v1.0 Compatibility Box returns a value of 10 for major version. 4) Due to the OS/2 return and the fact that some European versions of DOS carry higher version numbers than IBM's DOS, utilities which check for a DOS version should not abort if a higher version than required is found unless some specific problems are known. Function 31h Terminate Process and Stay Resident KEEP, or TSR entry AH 31h AL exit code DX program memory requirement in 16 byte paragraphs return AX return code (retrieveable by function 4Dh) note 1) Files opened by the application are not closed when this call is made 2) Memory can be used more efficiently if the block containing the copy of the DOS environment is deallocated before terminating. This can be done by loading ES with the segment contained in 2Ch of the PSP and issuing function call 49h (Free Allocated Memory). 3) Unlike int 27h, more than 64k may be made resident with this call Function 32h Read DOS Disk Block * Retrieve the pointer to the drive parameter block for a drive entry AH 32h DL drive (0=default, 1=A:, etc.). return AL 00h if drive is valid 0FFh if drive is not valid DS:BX pointer to DOS Drive Parameter Table. Format of block: Bytes Type Value 00h byte Drive: 0=A:, 1=B:, etc. 01h byte Unit within drive (0, 1, 2, etc.) 02h-03h word Bytes per sector 04h byte Sectors per cluster - 1 05h byte Cluster to sector shift (i.e., how far to shift- left the bytes/sector to get bytes/cluster) 06h-07h word Number of reserved (boot) sectors 08h byte Number of FATs 09h-0Ah word Number of root directory entries 0Bh-0Ch word Sector # of 1st data. Should be same as # of sectors/track. 0Dh-0Eh word # of clusters + 1 (=last cluster #) 0Fh byte Sectors for FAT 10h-11h word First sector of root directory 12h-15h dword Address of device driver header for this drive 16h byte Media Descriptor Byte for this drive 17h byte 0FFh indicates block must be rebuilt (DOS 3.x) 00h indicates block device has been accessed 18h-1Bh dword address of next DOS Disk Block (0FFFFh means last in chain) 22h byte Current Working Directory (2.0 only) (64 bytes) note 1) Use [BX+0D] to find no. of clusters (>1000H, 16-bit FAT; if not, 12-bit (exact dividing line is probably a little below 1000h to allow for bad sectors, EOF markers, etc.) 2) Short article by C.Petzold, PC Magazine Vol.5,no.8, and the article "Finding Disk Parameters" in the May 1986 issue of PC Tech Journal. 3) This call is mostly supported in OS/2 1.0's DOS Compatibility Box. The dword at 12h will not return the address of the next device driver when in the Compatibility Box. 4) used by CHKDSK Function 33h Control-Break Check Get or set control-break checking at CON entry AH 33h AL 00h to test for break checking 01h to set break checking DL 00h to disable break checking 01h to enable break checking 02h internal, called by PRINT.COM (DOS 3.1) 03h unknown 04h unknown 05h boot drive (DOS 4.0+) return DL 00h if break=off 01h if break=on (if AL=05h) boot drive, A=1, B=2, etc) AL 0FFh error Function 34h Return INDOS Flag * Returns ES:BX pointing to Critical Section Flag, byte indicating whether it is safe to interrupt DOS. entry AH 34h return ES:BX points to DOS "critical section flag" note 1) If byte is 0, it is safe to interrupt DOS. This was mentioned in some documentation by Microsoft on a TSR standard, and PC Magazine reports it functions reliably under DOS versions 2.0 through 3.3. Chris Dunford (of CED fame) and a number of anonymous messages on the BBSs indicate it may not be totally reliable. 2) The byte at ES:BX+1 is used by the Print program for this same purpose, so it's probably safer to check the WORD at ES:BX. 3) Reportedly, examination of DOS 2.10 code in this area indicates that the byte immediately following this "critical section flag" must be 00h to permit the PRINT.COM interrupt to be called. For DOS 3.0 and 3.1 (except Compaq DOS 3.0), the byte before the "critical section flag" must be zero; for Compaq DOS 3.0, the byte 01AAh before it must be zero. 4) In DOS 3.10 this reportedly changed to word value, with preceding byte. 5) This call is supported in OS/2 1.0's DOS Compatibility Box 6) Gordon Letwin of Microsoft discussed this call on ARPAnet in 1984. He stated: a) this is not supported under any version of the DOS b) it usually works under DOS 2, but there may be circumstances when it doesn't (general disclaimer, don't know of a specific circumstance) c) it will usually not work under DOS 3 and DOS 3.1; the DOS is considerably restructured and this flag takes on additional meanings and uses d) it will fail catastrophically under DOS 4.0 and forward. Obviously this information is incorrect since the call works fine through DOS 3.3. Microsoft glasnost? Function 35h Get Vector Get interrupt vector entry AH 35h AL interrupt number (hexadecimal) return ES:BX address of interrupt vector note Use function call 25h to set the interrupt vectors Function 36h Get Disk Free Space get information on specified drive entry AH 36h DL drive number (0=default, 1=A:, 2=B:, etc) return AX number of sectors per cluster 0FFFFh means drive specified in DL is invalid BX number of availible clusters CX bytes per sector DX clusters per drive note 1) Mult AX * CX * BX for free space on disk 2) Mult AX * CX * DX for total disk space 3) Function 36h returns an incorrect value after an ASSIGN command. Prior to ASSIGN, the DX register contains 0943h on return, which is the free space in clusters on the HC diskette. After ASSIGN, even with no parameters, 0901h is returned in the DX register; this is an incorrect value. Similar results occur with DD diskettes on a PC-XT or a PC-AT. This occurs only when the disk is not the default drive. Results are as expected when the drive is the default drive. Therefore, the circumvention is to make the desired drive the default drive prior to issuing this function call. 4) Int 21h, function call 36h returns an incorrect value after an ASSIGN command. Prior to ASSIGN, the DX register contains 0943h on return, which is the free space in clusters on the HC diskette. After ASSIGN, even with no parameters, 0901h is returned in the DX register; this is an incorrect value. Similar results occur with DD diskettes on a PC-XT or a PC-AT. This occurs only when the disk is not the default drive. Results are as expected when the drive is the default drive. Therefore, the circumvention is to make the desired drive the default drive prior to issuing this function call. 5) This function supercedes functions 1Bh and 1Ch. Function 37h SWITCHAR / AVAILDEV * Get/set option marking character (is usually "/"), and device type entry AH 37h AL 00h read switch character (returns current character in DL) 01h set character in DL as new switch character (DOS 2.x) 02h read device availability (as set by function AL=3) into DL. A 0 means devices that devices must be accessed in file I/O calls by /dev/device. A non-zero value means that devices are accessible at every level of the directory tree (e.g., PRN is the printer and not a file PRN). AL=2 to return flag in DL, AL=3 to set from DL (0 = set, 1 = not set). (DOS 2.x) 03h get device availability, where: DL 00h means /dev/ must precede device names 01h means /dev/ need not precede device names return DL switch character (if AL=0 or 1) device availability flag (if AL=2 or 3) AL 0FFh the value in AL was not in the range 0-3. note 1) Functions 2 & 3 appear not to be implemented for DOS 3.x. 2) It is documented on page 4.324 of the MS-DOS (version 2) Programmer's Utility Pack (Microsoft - published by Zenith). 3) Works on all versions of IBM PC-DOS from 2.0 through 3.3.1. 4) The SWITCHAR is the character used for "switches" in DOS command arguments (defaults to '/', as in "DIR/P"). '-' is popular to make a system look more like UNIX; if the SWITCHAR is anything other than '/', then '/' may be used instead of '\' for pathnames 5) Ignored by XCOPY, PKARC, LIST 6) SWITCHAR may not be set to any character used in a filename 7) In DOS 3.x you can still read the "AVAILDEV" byte with subfunction 02h but it always returns 0FFh even if you try to change it to 0 with subfunction 03h. 8) AVAILDEV=0 means that devices must be referenced in an imaginary subdirectory "\dev" (similar to UNIX's /dev/*); a filename "PRN.DAT" can be created on disk and manipulated like any other. If AVAILDEV != 0 then device names are recognized anywhere (this is the default): "PRN.DAT" is synonymous with "PRN:". 9) These functions reportedly are not supported in the same fashion in various implementations of DOS. 10) used by DOS 3.3 CHKDSK, BASIC, DEBUG Function 38h Return Country Dependent Information (PCDOS 2.0, 2.1, MSDOS 2.00 only) entry AH 38h AL function code (must be 0 in DOS 2.x) DS:DX pointer to 32 byte memory buffer for returned information return CF set on error AX error code (02h) BX country code DS:DX pointer to buffer filled with country information: bytes 0,1 date/time format 0 USA standard H:M:S M/D/Y 1 European standard H:M:S D/M/Y 2 Japanese standard H:M:S D:M:Y byte2 ASCIIZ string currency symbol byte3 zeroes byte4 ASCIIZ string thousands separator byte5 zeroes byte6 ASCIIZ string decimal separator byte7 zeroes bytes 8,1Fh 24 bytes reserved Function 38h Get Country Dependent Information (PCDOS 3.x+, MSDOS 2.01+) entry AH 38h AL function code 00h to get current country information 01h-0FEh country code to get information for, for countries with codes less than 255 0FFh to get country information for countries with a code greater than 255 BX 16 bit country code if AL=0FFh DS:DX pointer to the memory buffer where the data will be returned DX 0FFFFh if setting country code rather than getting info return CF 0 (clear) function completed 1 (set) error AX error code 02h invalid country code (no table for it) (if DX <> 0FFFFh) BX country code (usually international telephone code) DS:DX pointer to country data buffer bytes 0,1 date/time format 0 USA standard H:M:S M/D/Y 1 European standard H:M:S D/M/Y 2 Japanese standard H:M:S D:M:Y bytes 2-6 currency symbol null terminated byte 07h thousands separator null terminated byte 08h zeroes byte 09h decimal separator null terminated byte 0Ah zeroes byte 0Bh date separator null terminated byte 0Ch zeroes byte 0Dh time separator null terminated byte 0Eh zeroes byte 0Fh bit field currency format bit 0 = 0 if currency symbol precedes the value 1 if currency symbol is after the value bit 1 = 0 no spaces between value and currency symbol 1 one space between value and currency symbol bit 2 = 1 set if currency symbol replaces decimal pt bits 3-7 not defined by Microsoft byte 10h number of significant decimal digits in currency (number of places to right of decimal point) byte 11h time format bit 0 = 0 12 hour clock 1 24 hour clock bits 1-7 unknown, probably not used bytes 12h-15h address of case map routine (FAR CALL, AL = char) entry AL ASCII code of character to be converted to uppercase return AL ASCII code of the uppercase input character byte 16h data-list separator character byte 17h zeroes bytes 18h-21h 5 words reserved note 1) When an alternate keyboard handler is invoked, the keyboard routine is loaded into user memory starting at the lowest portion of availible user memory. The BIOS interrupt vector that services the keyboard is redirected to the memory area where the new routine resides. Each new routine takes up about 1.6K of memory and has lookup tables that return values unique to each language. (KEYBxx in the DOS book) Once the keyboard interrupt vector is changed by the DOS keyboard routine, the new routine services all calls unless the system is returned to the US format by the ctrl-alt-F1 keystroke combination. This does not change the interrupt vector back to the BIOS location; it merely passes the table lookup to the ROM locations. 2) Ctrl-Alt-F1 will only change systems with US ROMS to the US layout. Some systems are delivered with non-US keyboard handler routines in ROM 3) Case mapping call: the segment/offset of a FAR procedure that performs country-specific lower-to-upper case mapping on ASCII characters 80h to 0FFh. It is called with the character to be mapped in AL. If there is an uppercase code for the letter, it is returned in AL, if there is no code or the function was called with a value of less than 80h AL is returned unchanged. 4) This call is fully implemented in MS-DOS version 2.01 and higher. It is in version 2.00 but not fully implemented (according to Microsoft) Function 38h Set Country Dependent Information entry AH 38h AL code country code to set information for, for countries with codes less than 255 0FFh to set country information for countries with a code greater than 255 BX 16 bit country code if AL=0FFh DX 0FFFFh return CF clear successful set if error AX error code (02h) Function 39h Create Subdirectory (MKDIR) Makes a subdirectory along the indicated path entry AH 39h DS:DX address of ASCIIZ pathname string return flag CF 0 successful 1 error AX error code if any (3, 5) note 1) The ASCIIZ string may contain drive and subdirectory. 2) Drive may be any valid drive (not nescessarily current drive) 3) The pathname cannot exceed 64 characters Function 3Ah Remove Subdirectory (RMDIR) entry AH 3Ah DS:DX address of ASCIIZ pathname string return CF clear successful set AX error code if any (3, 5, 16) note 1) The ASCIIZ string may contain drive and subdirectory. 2) Drive may be any valid drive (not nescessarily current drive) 3) The pathname cannot exceed 64 characters Function 3Bh Change Current Directory (CHDIR) entry AH 3Bh DS:DX address of ASCIIZ string return flag CF 0 successful 1 error AX error code if any (3) note 1) The pathname cannot exceed 64 characters 2) The ASCIIZ string may contain drive and subdirectory. 3) Drive may be any valid drive (not nescessarily current drive) Function 3Ch Create A File (CREAT) Create a file with handle entry AH 3Ch CX attributes for file 00h normal 01h read only 02h hidden 03h system DS:DX address of ASCIIZ filename string return flag CF 0 successful creation 1 error AX 16 bit file handle or error code (3, 4, 5) note 1) The ASCIIZ string may contain drive and subdirectory. 2) Drive may be any valid drive (not nescessarily current drive) 3) If the volume label or subdirectory bits are set in CX, they are ignored 4) The file is opened in read/write mode 5) If the file does not exist, it is created. If one of the same name exists, it is truncated to a length of 0. 6) Good practice is to attempt to open a file with fn 3Dh and jump to an error routine if successful, create file if 3Dh fails. That way an existing file will not be truncated and overwritten. Function 3Dh Open A File Open disk file with handle entry AH 3Dh AL access code byte (DOS 2.x) bits 0-2 file attribute 000 read only 001 write only 010 read/write bits 3-7 should be set to zero (DOS 3.x) bits 0-2 file attribute 000 read only 001 write only 010 read/write bit 3 reserved 0 should be set to zero bits 4-6 sharing mode (network) 000 compatibility mode (the way FCBs open files) 001 read/write access denied (exclusive) 010 write access denied 011 read access denied 100 full access permitted bit 7 inheritance flag 0 file inherited by child process 1 file private to child process DS:DX address of ASCIIZ pathname string return flag CF set on error AX error code 1 error AX 16 bit file handle or error code (1, 2, 4, 5, 0Ch) note 1) Opens any normal, system, or hidden file 2) Files that end in a colon are not opened 3) The rear/write pointer is set at the first byte of the file and the record size of the file is 1 byte (the read/write pointer can be changed through function call 42h). The returned file handle must be used for all subsequent input and output to the file. 4) If the file handle was inherited from a parent process or was duplicated by DUP or FORCEDUP, all sharing and access restrictions are also inherited. 5) A file sharing error (error 1) causes an int 24h to execute with an error code of 2 Function 3Eh Close A File Handle Close a file and release handle for reuse entry AH 3Eh BX file handle return flag CF 0 successful close 1 error AX error code if error (6) note 1) When executed, the file is closed, the directory is updated, and all buffers for that file are flushed. If the file was changed, the time and date stamps are changed to current 2) If called with the handle 00000, it will close STDIN (normally the keyboard). Function 3Fh Read From A File Or Device Read from file with handle entry AH 3Fh BX file handle CX number of bytes to read DS:DX address of buffer return flag CF 0 successful read 1 error AX 0 pointer was already at end of file or number of bytes read or error code (5, 6) note 1) This function attempts to transfer the number of bytes specified in CX to a buffer location. It is not guaranteed that all bytes will be read. If AX < CX a partial record was read. 2) If performed from STDIN (file handle 0000), the input can be redirected 3) If used to read the keyboard, it will only read to the first CR 4) The file pointer is incremented to the last byte read. Function 40h Write To A File Or Device Write to file with handle entry AH 40h BX file handle CX number of bytes to write DS:DX address of buffer return flag CF 0 successful write 1 error AX number of bytes written or error code (5, 6) note 1) This call attempts to transfer the number of bytes indicated in CX from a buffer to a file. If CX and AX do not match after the write, an error has taken place; however no error code will be returned for this problem. This is usually caused by a full disk. 2) If the write is performed to STDOUT (handle 0001), it may be redirected 3) To truncate the file at the current position of the file pointer, set the number of bytes in CX to zero before calling int 21h. The pointer can be moved to any desired position with function 42h. 4) This function will not write to a file or device marked read-only. 5) May also be used to display strings to CON instead of fn 09h. This function will write CX bytes and stop; fn 09h will continue to write until a $ character is found. 6) This is the call that DOS actually uses to write to the screen in DOS 2.x and above. Function 41h Delete A File From A Specified Subdirectory (UNLINK) entry AH 41h DS:DX pointer to ASCIIZ filespec to delete return CF 0 successful 1 error AX error code if any (2, 5) note 1) This function will not work on a file marked read-only 2) Wildcards are not accepted Function 42h Move a File Read/Write Pointer (LSEEK) entry AH 42h AL method code 00h offset from beginning of file 01h offset from present location 02h offset from end of file BX file handle CX most significant half of offset DX least significant half of offset return AX low offset of new file pointer DX high offset of new file pointer CF 0 successful move 1 error AX error code (1, 6) note 1) If pointer is at end of file, reflects file size in bytes. 2) The value in DX:AX is the absolute 32 bit byte offset from the beginning of the file Function 43h Get/Set file attributes (CHMOD) entry AH 43h AL 00h get file attributes 01h set file attributes CX file attributes to set bit 0 read only 1 hidden file 2 system file 3 volume label 4 subdirectory 5 written since backup DS:DX pointer to full ASCIIZ file name return CF set if error AX error code (1, 2, 3, 5) CX file attributes on get attributes: 01h read only 02h hidden 04h system 0FFh archive note 1) This call will not change the volume label or directory bits Function 44h I/O Control for Devices (IOCTL) Get or Set Device Information entry AH 44h AL 00h Get Device Information (from DX) BX file or device handle return DX device info If bit 7 set: (character device) bit 0: console input device 1: console output device 2: NUL device 3: CLOCK$ device 4: device is special 5: binary (raw) mode 6: not EOF 12: network device (DOS 3.x) 14: can process IOCTL control strings (func 2-5) If bit 7 clear: (file) bits 0-5: block device number 6: file has not been written 12: Network device (DOS 3.x) 15: file is remote (DOS 3.x) 01h Set Device Information (DH must be zero for this call) DX bits: 0 1 console input device 1 1 console output device 2 1 null device 3 1 clock device 4 1 reserved 5 0 binary mode - don't check for control chars 1 cooked mode - check for control chars 6 0 EOF - End Of File on input 7 device is character device if set, if not, EOF is 0 if channel has been written, bits 0-5 are block device number 12 network device 14 1 can process control strings (AL 2-5, can only be read, cannot be set) 15 n reserved 02h Read CX bytes to device in DS:DX from BX control chan 03h Write Device Control String BX device handle CX number of bytes to write DS:DX pointer to buffer return AX number of bytes written 04h Read From Block Device (drive number in BL) BL drive number (0=default) CX number of bytes to read DS:DX pointer to buffer return AX number of bytes read 05h Write to Block Device (drive number in BL) AX number of bytes transfered 06h Get Input Handle Status BX file or device handle return AL 0FFh device ready 00h device not ready 07h Get Output Handle Status return AL 00h not ready 0FFh ready note: for DOS 2.x, files are always ready for output 08h Removable Media Bit (DOS 3.x+) return AX 00h device is removable 01h device is nonremovable 0Fh invalid drive specification 09h Test whether Local or Network Device in BL (DOS 3.x+) BL drive number (0=default) return DX attribute word, bit 12 set if device is remote 0Ah Is Handle in BX Local or Remote? (DOS 3.x+) BX file handle return DX (attribute word) bit 15 set if file is remote 0Bh Change Sharing Retry Count to DX (default=3), (DOS 3.x+) CX delay (default=1) DX retry count (default=3) 0Ch General IOCTL (DOS 3.3 [3.2?]) allows a device driver to prepare, select, refresh, and query Code Pages 0Dh Block Device Request (DOS 3.3+) BL drive number (0=default) CH major subfunction CL minor subfunction 40h set device parameters 41h write logical device track 42h format and verify logical device track 60h get device parameters 61h read logical device track 62h verify logical device track DS:DX pointer to parameter block 0Eh Get Logical Device (DOS 3.3+) BL drive number (0=default) return AL=0 block device has only one logical drive assigned 1..n the last letter used to reference the device (1=A:,etc) 0Fh Set Logical Device (DOS 3.3+) BL drive number: 0=default, 1=A:, 2=B:, etc. BX file handle CX number of bytes to read or write DS:DX data or buffer DX data return AX number of bytes transferred or error code (call function 59h for extended error codes) or status 00h not ready 0FFh ready CF set if error Function 45h Duplicate a File Handle (DUP) entry AH 45h BX file handle to duplicate return CF clear AX duplicate handle set AX error code (4, 6) note 1) If you move the pointed of one handle, the pointer of the other will also be moved. 2) The handle in BX must be open Function 46h Force Duplicate of a Handle (FORCEDUP or CDUP) Forces handle in CX to refer to the same file at the same position as BX entry AH 46h BX existing file handle CX new file handle return CF clear both handles now refer to existing file set error AX error code (4, 6) note 1) If CX was an open file, it is closed first 2) If you move the read/write pointer of either file, both will move 3) The handle in BX must be open Function 47h Get Current Directory Places full pathname of current directory/drive into a buffer entry AH 47h DL drive (0=default, 1=A:, etc.) DS:SI points to 64-byte buffer area return CF clear DS:DI pointer to ASCIIZ pathname of current directory set AX error code (0Fh) note String does not begin with a drive identifier or a backslash Function 48h Allocate Memory Allocates requested number of 16-byte paragraphs of memory entry AH 48h BX number of 16-byte paragraphs desired return CF clear AX segment address of allocated space BX maximum number paragraphs available set AX error code (7, 8) note BX indicates maximum memory availible only if allocation fails Function 49h Free Allocated Memory Frees specified memory blocks entry AH 49h ES segment address of area to be freed return CF clear successful set AX error code (7, 9) note 1) This call is only valid when freeing memory obtained by function 48h. 2) A program should not try to release memory not belonging to it. Function 4Ah Modify Allocated Memory Blocks (SETBLOCK) Expand or shrink memory for a program entry AH 4AH BX new size in 16 byte paragraphs ES segment address of block to change return CF clear nothing set AX error code (7, 8, 9) or BX max number paragraphs available note 1) Max number paragraphs availible is returned only if the call fails 2) Memory can be expanded only if there is memory availible Function 4Bh Load or Execute a Program (EXEC) entry AH 4Bh AL 00h load and execute program. A PSP is built for the program the ctrl-break and terminate addresses are set to the new PSP. *01h load but don't execute (note 1) *01h load but don't execute (internal, DOS 3.x & DESQview) *02h load but do not execute (internal, DOS 2.x only) 03h load overlay (do not create PSP, do not begin execution) DS:DX points to the ASCIIZ string with the drive, path, and filename to be loaded ES:BX points to a parameter block for the load (AL=00h) word segment address of environment string to be passed dword pointer to the command line to be placed at PSP+80h dword pointer to default FCB to be passed at PSP+5Ch dword pointer to default FCB to be passed at PSP+6Ch (*AL=01h) word segment of environment (0 = use current) dword pointer to command line dword pointer to FCB 1 dword pointer to FCB 2 dword will hold SS:SP on return dword will hold program entry point (CS:IP) on return (*AL=02h) word segment of environment (0 = use current) dword pointer to command line dword pointer to FCB 1 dword pointer to FCB 2 (AL=03h) word segment address where file will be loaded word relocation factor to be applied to the image return CF set error AX error code (1, 2, 8, 0Ah, 0Bh) note 1) If you make this call with AL=1 the program will be loaded as if you made the call with AL=0 except that the program will not be executed. Additionally, with AL=1 the stack segment and pointer along with the program's CS:IP entry point are returned to the program which made the 4B01h call. These values are put in the four words at ES:BX+0Eh. On entry to the call ES:BX points to the environment address, the command line and the two default FCBs. This form of EXEC is used by DEBUG.COM. 2) Application programs may invoke a secondary copy of the command processor (normally COMMAND.COM) by using the EXEC function. Your program may pass a DOS command as a parameter that the secondary command processor will execute as though it had been entered from the standard input device. The procedure is: A. Assure that adequate free memory (17k for 2.x and 3.0, 23k for 3.1 up) exists to contain the second copy of the command processor and the command it is to execute. This is accomplished by executing function call 4Ah to shrink memory allocated to that of your current requirements. Next, execute function call 48h with BX=0FFFFh. This returns the amount of memory availible. B. Build a parameter string for the secondary command processor in the form: 1 byte length of parameter string xx bytes parameter string 1 byte 0Dh (carriage return) For example, the assembly language statement below would build the string to cause execution of the command FOO.EXE: DB 19,"/C C:FOO",13 C. Use the EXEC function call (4Bh), function value 0 to cause execution of the secondary copy of the command processor. (The drive, directory, and name of the command processor can be gotten from the COMSPEC variable in the DOS environment passed to you at PSP+2Ch.) D. Remember to set offset 2 of the EXEC control block to point to the string built above. 3) All open files of a process are duplicated in the newly created process after an EXEC, except for files originally opened with the inheritance bit set to 1. 4) The environment is a copy of the original command processor's environment. Changes to the EXECed environment are not passed back to the original. The environment is followed by a copy of the DS:DX filename passed to the child process. A zero value will cause the child process to inherit the environment of the calling process. The segment address of the environment is placed at offset 2Ch of the PSP of the program being invoked. 5) This function uses the same resident part of COMMAND.COM, but makes a duplicate of the transient part. 6) How EXEC knows where to return to: Basically the vector for int 22h holds the terminate address for the current process. When a process gets started, the previous contents of int 22h get tucked away in the PSP for that process, then int 22h gets modified. So if Process A EXECs process B, while Process B is running, the vector for int 22h holds the address to return to in Process A, while the save location in Process B's PSP holds the address that process A will return to when *it* terminates. When Process B terminates by one of the usual legal means, the contents of int 22h are (surmising) shoved onto the stack, the old terminate vector contents are copied back to int 22h vector from Process B's PSP, then a RETF or equivalent is executed to return control to process A. 7) To load an overlay file with 4B: first, don't de-allocate the memory that the overlay will load into. With the other 4Bh functions, the opposite is true--you have to free the memory first, with function 4Ah. Second, the "segment address where the file will be loaded" (first item in the parameter block for sub-function 03) should be a paragraph boundary within your currently-allocated memory. Third, if the procedures within the overlay are FAR procs (while they execute, CS will be equal to the segment address of the overlay area), the relocation factor should be set to zero. On the other hand, if the CS register will be different from the overlay area's segment address, the relocation factor should be set to represent the difference. You determine where in memory the overlay file will load by using the segment address mentioned above. Overlay files are .EXEs (containing header, relocation table, and memory image). 8) When function 00h returns, all registers are changed, including the stack. You must resore SS, SP, and any other required registers. Function 4Ch Terminate a Process (EXIT) Quit with ERRORLEVEL exit code entry AH 4Ch AL exit code in AL when called, if any, is passed to next process return none note 1) Control passes to DOS or calling program 2) return code from AL can be retrieved by ERRORLEVEL or function 4Dh 3) all files opened by this process are closed, buffers are flushed, and the disk directory is updated 4) Restores Terminate vector from PSP:000Ah Ctrl-C vector from PSP:000Eh Critical Error vector from PSP:0012h Function 4Dh Get Return Code of a Subprocess (WAIT) Gets return code from functions 31h and 4Dh (ERRORLEVEL) entry AH 4Dh return AL exit code of subprogram (functions 31h or 4Ch) AH circumstance which caused termination 00h normal termination 01h control-break 02h critical device error 03h terminate and stay resident (function 31h) note The exit code is only returned once Function 4Eh Find First Matching File (FIND FIRST) entry AH 4Eh CX search attributes DS:DX pointer to ASCIIZ filename (with attributes) return CF set AX error code (2, 12h) clear data block written at current DTA format of block is: (info from BIX) documented by Micro- |00h 1 byte attribute byte of search soft as "reserved for |01h 1 byte drive letter for search DOS' use on subsquent |02h 11 bytes the search name used Find Next calls" |0Ch 2 bytes word value of last entry function 4Fh |0Fh 4 bytes dword pointer to this DTA |13h 2 bytes word directory start | PC-DOS 3.10 (from INTERRUP.ARC) |00h 1 byte drive letter |01h-0Bh bytes search template |0Ch 1 byte search attributes | DOS 2.x (and DOS 3.x except 3.1?) (from INTERRUP.ARC) |00h 1 byte search attributes |01h 1 byte drive letter |02h-0Ch bytes search template |0Dh-0Eh 2 bytes entry count within directory |0Fh-12h bytes reserved |13h-14h 2 bytes cluster number of parent directory 15h 1 byte file attribute 16h 2 bytes file time 18h 2 bytes file date 1Ah 2 bytes low word of file size 1Ch 2 bytes high word of file size 1Eh 13 bytes name and extension of file found, plus 1 byte of 0s. All blanks are removed from the name and extension, and if an extension is present it is preceded by a period. note 1) Will not find volume label 2) This function does not support network operations 3) Wildcards are allowed in the filespec 4) If the attribute is zero, only ordinary files are found. If the volume label bit is set, only volume labels will be found. Any other attribute will return that attribute and all normal files together. 5) To look for everything except the volume label, set the hidden, system, and subdirectory bits all to 1 Function 4Fh Find Next Matching File (FIND NEXT) Find next ASCIIZ file entry AH 4Fh return CF clear data block written at current DTA set AX error code (2, 12h) note 1) If file found, DTA is formatted as in call 4Eh 2) Volume label searches using 4Eh/4Fh reportedly aren't 100% reliable under DOS 2.x. The calls sometime report there's a volume label and point to a garbage DTA, and if the volume label is the only item they often won't find it 3) This function does not support network operations 4) Use of this call assumes that the original filespec contained wildcards Function 50h "Used Internally by DOS" - Set PSP * Set new Program Segment Prefix (current Process ID) entry AH 50h BX segment address of new PSP return none - swaps PSP's regarded as current by DOS note 1) By putting the PSP segment value into BX and issuing call 50h DOS stores that value into a variable and uses that value whenever a file call is made. 2) Note that in the PSP (or PDB) is a table of 20 (decimal) open file handles. The table starts at offset 18h into the PSP. If there is an 0FFh in a byte then that handle is not in use. A number in one of the bytes is an index into an internal FB table for that handle. For instance the byte at offset 18h is for handle 0, at offset 19h handle 1, etc. up to 13h. If the high bit is set then the file associated by the handle is not shared by child processes EXEC'd with call 4Bh. 3) Function 50h is dangerous in background operations prior to DOS 3.x as it uses the wrong stack for saving registers. (same as functions 0..0Ch in DOS 2.x) 4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler without setting the Critical Error flag 5) Open File information, etc. is stored in the PSP DOS views as current. If a program (eg. a resident program) creates a need for a second PSP, then the second PSP should be set as current to make sure DOS closes that as opposed to the first when the second application finishes. 6) See PC Mag Vol.5, No 9, p.314 for discussion. 7) Used by DOS 3.3 PRINT & DEBUG, DesQview 2.01, Windows 1.03, SYMDEB from MASM 4.0 Function 51h "Used Internally by DOS" - Get Program Segment Prefix * Returns the PSP address of currently executing program entry AH 51h return BX address of currently executing program offset 00h program exit point 02h memory size in paragraphs 04h unused (0) 05h CP/M style entry point (far call to DOS) 0Ah terminate address (old int 22h) 0Ch terminate segment 0Eh break address (old int 23h) 10h break segment 12h error address (old int 24h) 14h error segment 16h parent PSP segment 18h DOS 2.0+ open files, 0FFh = unused 2Ch DOS 2.0+ environment segment 2Eh far ptr to process's SS:SP 32h DOS 3.x max open files 34h DOS 3.x openfile table address 36h DOS 3.x openfile table segment 38h unused by DOS versions <= 3.3 50h DOS function dispatcher (FAR routine) 53h unused 55h FCB #1 extension 5Ch FCB #1 6Ch FCB #2 80h command tail / default DTA buffer note 1) Used in DOS 2.x, 3.x uses 62h 2) Function 51h is dangerous in background operations prior to DOS 3.x as it uses the wrong stack for saving registers. (same as functions 0..0Ch in DOS 2.x) 3) 50h and 51h might be used if you have more than one process in a PC. For instance if you have a resident program that needs to open a file you could first call 51h to save the current id and then call 50h to set the ID to your PSP. 4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler without setting the Critical Error flag 5) Used by DOS 3.3 PRINT, DEBUG Function 52h "Used Internally by DOS" - IN-VARS * Returns a pointer to a set of DOS data variables MCB chain, pointer to first device driver and a pointer to disk parameter blocks (first one) entry AH 52h return ES:BX pointer to the DOS list of lists, for disk information. Does not access the disk, so information in tables might be incorrect if disk has been changed. Returns a pointer to the following array of longword pointers: Bytes Value -2h,-1h segment of first memory control block 00h-03h pointer to first DOS disk block (see function 36h) 04h-07h Pointer to list of DOS file tables dword pointer to next file table word number of files in this table 35h bytes per file 00h-01h number of file handles referring to this file 02h-06h unknown 07h-0Ah pointer to device driver header if character device; pointer to DOS Device Control Block if block device (see fn 32h for format) 0Bh-1Fh unknown 20h-2Ah filename in FCB format (no path, no period, blank-padded) 2Bh-2Ch PSP segment of file's owner 2Dh-30h unknown - 0 always 31h-32h unknown 33h-34h unknown 8h-0Bh pointer to CLOCK$ device driver, whether installable or resident 0Ch-0Fh pointer to actual CON: device driver, whether installable or resident (DOS 2.x) 10 number of logical drives in system 11-12 maximum bytes/block of any block device 13-16 unknown 17 beginning (not a pointer. The real beginning!) of NUL device driver. This is the first device on DOS's linked list of device drivers. (DOS 3.x) 10h-11h maximum bytes/block of any block device (0200h) 12h-15h pointer to first disk buffer 16h-19h partially undefined: Pointer to array of drive info: 51h bytes per drive, starting with A: ... 00h-3Fh current path as ASCIIZ, starting with 'x:\' 40h-43h unknown zeros always 44h unknown flags? Usually 40h, except for entry after last valid entry = 00h 45h-48h pointer to DOS disk block for this drive 49h-4Ah unknown. Current track or block? -1 if never accessed 4Bh-4Eh unknown -1 always 4Fh-52h unknown 2 always 1Ah-1Dh pointer to FCB table (if CONFIG.SYS contains FCBS=) 1Eh-1Fh size of FCB table 20h number of block devices 21h value of LASTDRIVE command in CONFIG.SYS (default 5) 22h beginning (not a pointer. The real beginning!) of NUL device driver. This is the first device on DOS's linked list of device drivers. note 1) This call is not supported in OS/2 1.0's DOS Compatibility Box 2) Used by DOS 4.0 MEM.EXE, DOS 3.3 ASSIGN.COM, PRINT.COM, SUBST.EXE 3) IMPORTANT: The structure of this list changes with EVERY version of DOS. It is only partially supported by DR-DOS, and isn't supported under PC-MOS, OS/2's DOS box, or Wendin-DOS. Since the information changes so much, I feel it should be put in the "interesting, but not real useful" category. If you depend on this stuff in production code, you're going to regret it. Function 53h "Used Internally by DOS" - Translate BPB * Translates BPB (BIOS Parameter Block, see below) into a DOS Disk Block (see function call 32h). entry AH 53h DS:SI pointer to BPB ES:BP pointer to area for DOS Disk Block. Layout of Disk Block: bytes value 00h-01h bytes per sector, get from DDB bytes 02h-03h. 02h sectors per cluster, get from (DDB byte 4) + 1 03h-04h reserved sectors, get from DDB bytes 06h-07h 05h number of FATs, get from DDB byte 08h 06h-07h number of root dir entries, get from DDB bytes 09h-0Ah 08h-09h total number of sectors, get from: ((DDB bytes 0Dh-0Eh) - 1) * (sectors per cluster (BPB byte 2)) + (DDB bytes 0Bh-0Ch) 0Ah media descriptor byte, get from DDB byte 16h 0Bh-0Ch number of sectors per FAT, get from DDB byte 0Fh return unknown Function 54h Get Verify Setting Get verify flag status entry AH 54h return AL 00h if flag off 01h if flag on note Flag can be set with function 2Eh Function 55h "Used Internally by DOS" - Create "Child" PSP * Create PSP: similar to function 26h (which creates a new Program Segment Prefix at segment in DX) except creates a "child" PSP rather than copying the existing one. entry AH 55h DX segment number at which to create new PSP. return unknown note 1) This call is similar to call 26h which creates a PSP except that unlike call 26h the segment address of the parent process is obtained from the current process ID rather than from the CS value on the stack (from the INT 21h call). DX has the new PSP value and SI contains the value to be placed into PSP:2 (top of memory). 2) Function 55 is merely a substitute for function 26h. It will copy the current PSP to the segment address DX with the addition that SI is assumed to hold the new memory top segment. This means that function 26h sets SI to the segment found in the current PSP and then calls function 55h. Function 56h Rename a File entry AH 56h DS:DX pointer to ASCIIZ old pathname ES:DI pointer to ASCIIZ new pathname return CF clear successful rename set AX error code (2, 3, 5, 11h) note 1) Works with files in same drive only 2) Global characters not allowed in filename 3) The name of a file is its full pathname. The file's full pathname can be changed, while leaving the actual FILENAME.EXT unchanged. Changing the pathname allows the file to be "moved" from subdirectory to subdirectory on a logical drive without actually copying the file. 4) DOS 3.x allows renaming of directories