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DOS INT 21h - DOS Function Codes
The follow abridged list of DOS interrupts has been extracted from a large list compiled by Ralf Brown. These are available on any Simtel mirror (e.g. sunsite.anu.edu.au) under the directory ms-dos/info/interNNp.zip AH 01 05 09 0B 0D 19 2A 2C 2E 35 36 3A 3C 3E 40 42 47 4D 56 Description Read character from STDIN Write character to printer Writestring to STDOUT Get STDIN status Disk reset Get current default drive Get system date Get system time Set verify flag Get Interrupt vector Get free disk space Remove subdirectory Create file Close file Write file Seek file Get current directory Get return code Rename file 39 3B 3D 3F 41 43 4C 54 57 Create subdirectory Set working directory Open file Read file Delete file Get/Set file attributesExit program Get verify flag Get/Set file date AH 02 06 0A 0C 0E 25 2B 2D 30 Description Write character to STDOUT Console Input/Output Buffered input Flush buffer for STDIN Select default drive Set interrupt vector Set system date Set system time Get DOS version

07 Direct char read (STDIN), no echo 08 Char read from STDIN, no echo

AH = 01h - READ CHARACTER FROM STANDARD INPUT, WITH ECHOReturn: AL = character read Notes:
l l

^C/^Break are checked ^P toggles the DOS-internal echo-to-printer flag

l

^Z is not interpreted, thus not causing an EOF if input is redirected character is echoed to standard output

SeeAlso: AH=06h,AH=07h,AH=08h,AH=0Ah

AH = 02h -WRITE CHARACTER TO STANDARD OUTPUT
Entry: DL = character to write Return: AL = last character output Notes:
l l

l^C/^Break are checked the last character output will be the character in DL unless DL=09h on entry, in which case AL=20h as tabs are expanded to blanks if standard output is redirected to a file, no error checks (write- protected, full media, etc.) are performed

SeeAlso: AH=06h,AH=09h

AH = 05h - WRITE CHARACTER TO PRINTER
Entry: DL = character to print Notes:
l l l

keyboard checkedfor ^C/^Break STDPRN is usually the first parallel port, but may be redirected under DOS 2+ if the printer is busy, this function will wait

SeeAlso: INT 17/AH=00h

AH = 06h - DIRECT CONSOLE OUTPUT
Entry: DL = character (except FFh) Return: AL = character output Notes: does not check ^C/^Break SeeAlso: AH=02h,AH=09h

AH = 06h - DIRECT CONSOLE INPUT
Entry: AH = 06h DL = FFh Return:
l l

ZFset if no character available and AL = 00h ZF clear if character available AL = character read

Notes:
l l

l

^C/^Break are NOT checked if the returned character is 00h, the user pressed a key with an extended keycode, which will be returned by the next call of this function although the return of AL=00h when no characters are available is not documented, some programs rely on thisbehavior

SeeAlso: AH=0Bh

AH=07h - DIRECT CHARACTER INPUT, WITHOUT ECHO
Return: AL = character read from standard input Notes: does not check ^C/^Break SeeAlso: AH=01h,AH=06h,AH=08h,AH=0Ah

AH = 08h - CHARACTER INPUT WITHOUT ECHO
Return: AL = character read from standard input Notes: ^C/^Break are checked SeeAlso: AH=01h,AH=06h,AH=07h,AH=0Ah,AH=64h

AH = 09h - WRITE STRING TO STANDARD OUTPUTEntry: DS:DX -> '$'-terminated string Return: AL = 24h Notes: ^C/^Break are checked SeeAlso: AH=02h,AH=06h"OUTPUT"

AH = 0Ah - BUFFERED INPUT
Entry: DS:DX -> buffer (see below) Return: buffer filled with user input Notes:
l l

^C/^Break are checked reads from standard input

SeeAlso: AH=0Ch Format of DOS input buffer: Offset Size Description

00 01 02

1 1 n

maximum charactersbuffer can hold number of chars from last input which may be recalled OR number of characters actually read, excluding CR actual characters read, including the final carriage return

AH=0Bh - GET STDIN STATUS
Return:
l l

AL = 00h if no character available AL = FFh if character is available

Notes: ^C/^Break are checked SeeAlso: AH=06h"INPUT"

AH = 0Ch - FLUSH BUFFER AND READ STANDARD...