osFree wiki - en:docs:dos:api:int21
https://ftp.osfree.org/doku/
2026-04-27T11:35:17+00:00osFree wiki
https://ftp.osfree.org/doku/
https://ftp.osfree.org/doku/lib/exe/fetch.php?media=wiki:dokuwiki.svgtext/html2020-12-08T05:51:18+00:00Anonymous (anonymous@undisclosed.example.com)00
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:00&rev=1607406678&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=00H
Version
1 and higher
Brief
TERMINATE PROGRAM
Family API
DosExit
Input
AH = 00h
CS = PSP segment
Return
never returns
Macro
INCLUDE DOS.INC
TERMINATE_PROGRAMtext/html2024-04-27T06:59:18+00:00Anonymous (anonymous@undisclosed.example.com)0a
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:0a&rev=1714201158&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=0AH
Version
1 and higher
Brief
BUFFERED INPUT
Family API
DosRead
Input
AH = 0Ah
DS:DX -> buffer (see #01344)
Return
buffer filled with user input
Notestext/html2024-04-27T07:00:02+00:00Anonymous (anonymous@undisclosed.example.com)0b
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:0b&rev=1714201202&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=0BH
Version
1 and higher
Brief
GET STDIN STATUS
Family API
DosRead
Input
AH = 0Bh
Return
AL = status 00h if no character available FFh if character is availabletext/html2024-06-05T13:57:36+00:00Anonymous (anonymous@undisclosed.example.com)0c
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:0c&rev=1717595856&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=0CH
Version
1 and higher
Brief
FLUSH BUFFER AND READ STANDARD INPUT
Family API
DosRead
Input
* AH = 0Ch
* AL = STDIN input function to execute after flushing buffer other registers as appropriate for the input functiontext/html2025-08-09T14:09:12+00:00Anonymous (anonymous@undisclosed.example.com)0d
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:0d&rev=1754748552&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=0DH
Version
1 and higher
Brief
DISK RESET
Family API
DosBufReset
Input
AH = 0Dh
Return
(DOS 6 only) CF clear (earlier versions preserve CF)
Notes
This function writes all modified disk buffers to disk, but does not update the directory information (that is only done when files are closed or a SYNC call is issued)text/html2024-05-11T06:17:13+00:00Anonymous (anonymous@undisclosed.example.com)0e
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:0e&rev=1715408233&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=0EH
Version
1 and higher
Brief
SELECT DEFAULT DRIVE
Family API
DosSelectDisk
Input
* AH = 0Eh
* DL = new default drive (00h = A:, 01h = B:, etc)
Returntext/html2024-04-27T07:43:05+00:00Anonymous (anonymous@undisclosed.example.com)0f
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:0f&rev=1714203785&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=0FH
Version
1 and higher
Brief
OPEN FILE USING FCB
Family API
DosOpen
Input
AH = 0Fh
DS:DX -> unopened File Control Block (see #01345,#01346)text/html2024-04-27T06:50:35+00:00Anonymous (anonymous@undisclosed.example.com)01
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:01&rev=1714200635&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=01H
Version
1 and higher
Brief
READ CHARACTER FROM STANDARD INPUT, WITH ECHO
Family API
DosRead
Input
AH = 01h
Return
AL = character read
Macro
INCLUDE DOS.INC
READ_KBD_AND_ECHOtext/html2024-05-11T06:18:26+00:00Anonymous (anonymous@undisclosed.example.com)1a
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:1a&rev=1715408306&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=1AH
Version
1 and higher
Brief
SET DISK TRANSFER AREA ADDRESS
Family API
none
Input
* AH = 1Ah
* DS:DX -> Disk Transfer Area (DTA)
Return
*text/html2024-05-02T02:39:41+00:00Anonymous (anonymous@undisclosed.example.com)1b
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:1b&rev=1714617581&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=1BH
Version
1 and higher
Brief
GET ALLOCATION INFORMATION FOR DEFAULT DRIVE
Family API
none
Input
AH = 1Bh
Return
AL = sectors per cluster (allocation unit)
CX = bytes per sector
DX = total number of clusters
DS:BXtext/html2024-05-02T02:41:18+00:00Anonymous (anonymous@undisclosed.example.com)1c
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:1c&rev=1714617678&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=1CH
Version
1 and higher
Brief
GET ALLOCATION INFORMATION FOR SPECIFIC DRIVE
Family API
none
Input
AH = 1Ch
DL = drive (00h = default, 01h = A:, etc)text/html2024-05-02T02:54:11+00:00Anonymous (anonymous@undisclosed.example.com)1f
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:1f&rev=1714618451&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=1CH
Version
1 and higher
Brief
GET DRIVE PARAMETER BLOCK FOR DEFAULT DRIVE
Family API
Input
AH = 1Fh
Return
AL = sectors per cluster (allocation unit), or FFh if invalid drivetext/html2024-04-27T06:51:17+00:00Anonymous (anonymous@undisclosed.example.com)02
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:02&rev=1714200677&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=02H
Version
1 and higher
Brief
WRITE CHARACTER TO STANDARD OUTPUT
Family API
DosWrite
Input
AH = 02h
DL = character to write
Return
AL = last character output (despite the official docs which state nothing is returned) (at least DOS 2.1-7.0)text/html2024-05-02T04:16:17+00:00Anonymous (anonymous@undisclosed.example.com)2a
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:2a&rev=1714623377&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=2AH
Version
1 and higher
Brief
GET SYSTEM DATE
Family API
Input
AH = 2Ah
Return
CX = year (1980-2099)
DH = month
DL = day
---DOS 1.10+text/html2024-05-11T06:19:33+00:00Anonymous (anonymous@undisclosed.example.com)2b
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:2b&rev=1715408373&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=2BH
Version
1 and higher
Brief
SET SYSTEM DATE
Family API
Input
* AH = 2Bh
* CX = year (1980-2099)
* DH = month (1-12)
* DL = day (1-31)
Returntext/html2024-05-02T04:18:42+00:00Anonymous (anonymous@undisclosed.example.com)2c
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:2c&rev=1714623522&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=2CH
Version
1 and higher
Brief
GET SYSTEM TIME
Family API
Input
AH = 2Ch
Return
CH = hour
CL = minute
DH = second
DL = 1/100 secondstext/html2024-05-02T04:22:33+00:00Anonymous (anonymous@undisclosed.example.com)2d
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:2d&rev=1714623753&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=2DH
Version
1 and higher
Brief
SET SYSTEM TIME
Family API
Input
AH = 2Dh
CH = hour
CL = minute
DH = second
DL = 1/100 secondstext/html2024-05-02T04:23:24+00:00Anonymous (anonymous@undisclosed.example.com)2e
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:2e&rev=1714623804&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=2EH
Version
1 and higher
Brief
SET VERIFY FLAG
Family API
Input
AH = 2Eh
DL = 00h (DOS 1.x/2.x only)
AL = new state of verify flag
00h off
01h ontext/html2024-04-22T05:15:46+00:00Anonymous (anonymous@undisclosed.example.com)2f
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:2f&rev=1713762946&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=2EH
Version
2 and higher
Brief
GET DISK TRANSFER AREA ADDRESS
Family API
none
Input
AH = 2Fh
Return
ES:BX -> current DTA
Notes
* under the FlashTek X-32 DOS extender, the pointer is in ES:EBXtext/html2024-04-27T06:52:08+00:00Anonymous (anonymous@undisclosed.example.com)03
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:03&rev=1714200728&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=03H
Version
1 and higher
Brief
READ CHARACTER FROM STDAUX
Family API
DosRead
Input
AH = 03h
Return
AL = character read
Notes
keyboard checked for ^C/^Break, and INT 23 executed if detectedtext/html2024-05-02T07:19:38+00:00Anonymous (anonymous@undisclosed.example.com)3a
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:3a&rev=1714634378&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=3AH
Version
2 and higher
Brief
“RMDIR” - REMOVE SUBDIRECTORY
Family API
Input
AH = 3Ah
DS:DX -> ASCIZ pathname of directory to be removedtext/html2025-07-20T05:43:27+00:00Anonymous (anonymous@undisclosed.example.com)3b
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:3b&rev=1752990207&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=3BH
Version
2 and higher
Brief
“CHDIR” - SET CURRENT DIRECTORY
Family API
DosChDir
Input
* AH = 3Bh
* DS:DX -> ASCIZ pathname to become current directory (max 64 bytes)text/html2024-05-02T07:26:35+00:00Anonymous (anonymous@undisclosed.example.com)3c
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:3c&rev=1714634795&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=3CH
Version
2 and higher
Brief
“CREAT” - CREATE OR TRUNCATE FILE
Family API
DosOpen
Input
AH = 3Ch
CX = file attributes (see #01401)
DS:DX -> ASCIZ filenametext/html2024-05-02T07:55:41+00:00Anonymous (anonymous@undisclosed.example.com)3d
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:3d&rev=1714636541&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=3DH
Version
2 and higher
Brief
“OPEN” - OPEN EXISTING FILE
Family API
DosOpen
Input
AH = 3Dh
AL = access and sharing modes (see #01402)
DS:DX -> ASCIZ filename
CL = attribute mask of files to look for (server call only)text/html2024-05-18T09:04:44+00:00Anonymous (anonymous@undisclosed.example.com)3e
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:3e&rev=1716023084&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=3EH
Version
2 and higher
Brief
“CLOSE” - CLOSE FILE
Family API
Input
* AH = 3Eh
* BX = file handle
Return
* CF clear if successful
*text/html2024-05-02T07:59:40+00:00Anonymous (anonymous@undisclosed.example.com)3f
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:3f&rev=1714636780&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=3FH
Version
2 and higher
Brief
“READ” - READ FROM FILE OR DEVICE
Family API
Input
AH = 3Fh
BX = file handle
CX = number of bytes to read
DS:DX -> buffer for datatext/html2024-04-27T06:52:36+00:00Anonymous (anonymous@undisclosed.example.com)04
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:04&rev=1714200756&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=04H
Version
1 and higher
Brief
WRITE CHARACTER TO STDAUX
Family API
DosWrite
Input
AH = 04h
DL = character to write
Return
nothing
Notes
keyboard checked for ^C/^Break, and INT 23 executed if detectedtext/html2024-05-03T02:10:54+00:00Anonymous (anonymous@undisclosed.example.com)4a
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:4a&rev=1714702254&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=4AH
Version
2 and higher
Brief
RESIZE MEMORY BLOCK
Family API
Input
AH = 4Ah
BX = new size in paragraphs
ES = segment of block to resize
Return
CF clear if successful
CF set on error
AX = error code (07h,08h,09h) (see #01680 at AH=59h/BX=0000h)
BX = maximum paragraphs available for specified memory blocktext/html2024-05-03T02:15:14+00:00Anonymous (anonymous@undisclosed.example.com)4b
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:4b&rev=1714702514&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=4BH
Version
2 and higher
Brief
“EXEC” - LOAD AND/OR EXECUTE PROGRAM
Family API
Input
AH = 4Bh
AL = type of load
00h load and execute
01h load but do not execute
03h load overlay (see #01591)
04h load and execute in background (European MS-DOS 4.0 only)
"Exec & Go" (see also AH=80h)
DS:DX -> ASCIZ program name (must include extension)
ES:BX -> parameter block (see #01590,…text/html2020-12-07T02:39:22+00:00Anonymous (anonymous@undisclosed.example.com)4c
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:4c&rev=1607308762&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=4CH
Version
2 and higher
Brief
“EXIT” - TERMINATE WITH RETURN CODE
Family API
DosExit
Input
AH = 4Ch
AL = return code
Return
never returns
Notes
*text/html2024-05-03T02:20:28+00:00Anonymous (anonymous@undisclosed.example.com)4d
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:4d&rev=1714702828&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=4DH
Version
2 and higher
Brief
GET RETURN CODE (ERRORLEVEL)
Family API
Input
AH = 4Dh
Return
AH = termination type
00h normal (INT 20,INT 21/AH=00h, or INT 21/AH=4Ch)
01h control-C abort
02h critical error abort
03h terminate and stay resident (INT 21/AH=31h or INT 27)
AL = return code
CF cleartext/html2024-04-22T07:57:25+00:00Anonymous (anonymous@undisclosed.example.com)4e
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:4e&rev=1713772645&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=4EH
Version
2 and higher
Brief
“FINDFIRST” - FIND FIRST MATCHING FILE
Family API
DosFindFirst
Input
AH = 4Eh
AL = special flag for use by APPEND (refer to note below)
CX = file attribute mask (see #01420 at AX=4301h) (bits 0 and 5 ignored)
0088h (Novell DOS 7) find first deleted file
DS:DX -> ASCIZ file specification (may include path and wildcards)text/html2024-05-03T02:23:55+00:00Anonymous (anonymous@undisclosed.example.com)4f
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:4f&rev=1714703035&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=4FH
Version
2 and higher
Brief
“FINDNEXT” - FIND NEXT MATCHING FILE
Family API
Input
AH = 4Fh
Disk Transfer Area contains data block from previous FindFirst or
FindNext calltext/html2020-12-07T04:21:41+00:00Anonymous (anonymous@undisclosed.example.com)05
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:05&rev=1607314901&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=05H
Version
1 and higher
Brief
WRITE CHARACTER TO PRINTER
Family API
DosWrite
Input
AH = 05h
DL = character to print
Return
nothing
Notes
keyboard checked for ^C/^Break, and INT 23 executed if detectedtext/html2024-05-03T06:48:57+00:00Anonymous (anonymous@undisclosed.example.com)5a
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:5a&rev=1714718937&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=5AH
Version
3 and higher
Brief
CREATE TEMPORARY FILE
Family API
Input
AH = 5Ah
CX = file attribute (see #01420 at AX=4301h)
DS:DX -> ASCIZ path ending with a '\' + 13 zero bytes to receive the
generated filenametext/html2024-05-03T06:49:37+00:00Anonymous (anonymous@undisclosed.example.com)5b
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:5b&rev=1714718977&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=5BH
Version
3 and higher
Brief
CREATE NEW FILE
Family API
Input
AH = 5Bh
CX = file attribute (see #01420 at AX=4301h)
DS:DX -> ASCIZ filename
Returntext/html2021-08-01T07:40:40+00:00Anonymous (anonymous@undisclosed.example.com)5c
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:5c&rev=1627803640&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=5cH
Input
AH=5cH, AL=Subfunction
Subfunction Description Version 00 lock region of filetext/html2021-08-01T08:50:42+00:00Anonymous (anonymous@undisclosed.example.com)5d
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:5d&rev=1627807842&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=5dH
Input
AH=5dH, AL=Subfunction
Subfunction Description Version 07 GET REDIRECTED PRINTER MODEtext/html2024-05-03T07:25:57+00:00Anonymous (anonymous@undisclosed.example.com)5e
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:5e&rev=1714721157&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=5EH
Input
AH=5EH, AL=Subfunction
Subfunction Description Version 00 GET MACHINE NAMEtext/html2021-08-06T02:30:15+00:00Anonymous (anonymous@undisclosed.example.com)5f
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:5f&rev=1628217015&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=5FH
Input
AH=5FH, AL=Subfunction
Subfunction Description Version 00 GET REDIRECTION MODEtext/html2025-01-03T21:09:03+00:00Anonymous (anonymous@undisclosed.example.com)06
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:06&rev=1735938543&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=06H
Version
1 and higher
Brief
DIRECT CONSOLE OUTPUT/INPUT
Family API
DosWrite DosRead
Input
AH = 06h
DL = output character (except FFh)
DL = FFh - input charactertext/html2021-08-06T06:27:33+00:00Anonymous (anonymous@undisclosed.example.com)6a
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:6a&rev=1628231253&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=6AH
Version
4 and higher
Brief
FFLUSH - COMMIT FILE
Family API
Input
AH = 6Ah
BX = file handle
Return
CF clear if successful
all data still in DOS disk buffers is written to disk immediately,
and the file's directory entry is updated
CF set on error
AX = error code (see #01680 at AH=59h/BX=0000h)text/html2024-05-08T01:49:19+00:00Anonymous (anonymous@undisclosed.example.com)6c
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:6c&rev=1715132959&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=6cH
Version
4 and higher
Brief
EXTENDED OPEN/CREATE
Family API
Input
AX = 6C00h
BL = open mode as in AL for normal open (see also AH=3Dh)
bit 7:
Inheritance
bits 4-6:
Sharing mode
bit 3 reservedtext/html2024-04-27T06:55:12+00:00Anonymous (anonymous@undisclosed.example.com)07
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:07&rev=1714200912&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=07H
Version
1 and higher
Brief
DIRECT CHARACTER INPUT, WITHOUT ECHO
Family API
DosRead
Input
AH = 07h
Return
AL = character read from standard input
Notestext/html2024-04-27T06:56:26+00:00Anonymous (anonymous@undisclosed.example.com)08
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:08&rev=1714200986&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=08H
Version
1 and higher
Brief
CHARACTER INPUT WITHOUT ECHO
Family API
DosRead
Input
AH = 08h
Return
AL = character read from standard input
Notes
^C/^Break are checked, and INT 23 executed if detected standard input is always the keyboard under DOS 1.x, but may be redirected under DOS 2+text/html2024-04-27T06:58:07+00:00Anonymous (anonymous@undisclosed.example.com)09
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:09&rev=1714201087&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=09H
Version
1 and higher
Brief
WRITE STRING TO STANDARD OUTPUT
Family API
DosWrite
Input
AH = 09h
DS:DX -> '$'-terminated string
Return
AL = 24h (the '$' terminating the string, despite official docs which state that nothing is returned) (at least DOS 2.1-7.0 and NWDOS)text/html2024-04-27T07:44:13+00:00Anonymous (anonymous@undisclosed.example.com)10
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:10&rev=1714203853&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=10H
Version
1 and higher
Brief
CLOSE FILE USING FCB
Family API
DosClose
Input
AH = 10h
DS:DX -> File Control Block (see #01345)
Return
AL = status 00h successful FFh failedtext/html2024-04-27T07:45:12+00:00Anonymous (anonymous@undisclosed.example.com)11
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:11&rev=1714203912&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=11H
Version
1 and higher
Brief
FIND FIRST MATCHING FILE USING FCB
Family API
DosFindFirst
Input
AH = 11h
DS:DX -> unopened FCB (see #01345), may contain '?' wildcardstext/html2024-04-27T07:45:47+00:00Anonymous (anonymous@undisclosed.example.com)12
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:12&rev=1714203947&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=12H
Version
1 and higher
Brief
FIND NEXT MATCHING FILE USING FCB
Family API
DosFindNext
Input
AH = 12h
DS:DX -> unopened FCB (see #01345)
Returntext/html2024-04-27T07:46:30+00:00Anonymous (anonymous@undisclosed.example.com)13
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:13&rev=1714203990&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=13H
Version
1 and higher
Brief
DELETE FILE USING FCB
Family API
DosDelete
Input
AH = 13h
DS:DX -> unopened FCB (see #01345), filename filled with template for
deletion ('?' wildcards allowed)text/html2024-04-27T08:12:25+00:00Anonymous (anonymous@undisclosed.example.com)14
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:14&rev=1714205545&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=14H
Version
1 and higher
Brief
SEQUENTIAL READ FROM FCB FILE
Family API
DosRead
Input
AH = 14h
DS:DX -> opened FCB (see #01345)
Return
Return: AL = statustext/html2024-05-02T02:25:28+00:00Anonymous (anonymous@undisclosed.example.com)15
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:15&rev=1714616728&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=15H
Version
1 and higher
Brief
SEQUENTIAL WRITE TO FCB FILE
Family API
DosRead
Input
AH = 15h
DS:DX -> opened FCB (see #01345)
Disk Tranfer Area contains record to be writtentext/html2024-05-02T02:27:02+00:00Anonymous (anonymous@undisclosed.example.com)16
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:16&rev=1714616822&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=16H
Version
1 and higher
Brief
CREATE OR TRUNCATE FILE USING FCB
Family API
DosRead
Input
AH = 16h
DS:DX -> unopened FCB (see #01345), wildcards not allowedtext/html2024-05-11T05:10:38+00:00Anonymous (anonymous@undisclosed.example.com)17
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:17&rev=1715404238&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=17H
Version
1 and higher
Brief
RENAME FILE USING FCB
Family API
DosMove
Input
* AH = 17h
* DS:DX -> modified FCB (see also #01345) the old filename ('?' wildcards OK) is in the standard location while the new filename ('?' wildcards OK, no drive) is stored in the 11 bytes beginning at offset 11htext/html2024-05-02T02:34:50+00:00Anonymous (anonymous@undisclosed.example.com)19
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:19&rev=1714617290&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=19H
Version
1 and higher
Brief
GET CURRENT DEFAULT DRIVE
Family API
DosQCurDisk
Input
AH = 19h
Return
AL = drive (00h = A:, 01h = B:, etc)
Notes
Novell NetWare uses the fact that DOS 2.x COMMAND.COM issues this call from a particular location every time it starts a command to determine when to issue an automatic EOJtext/html2024-05-02T02:56:08+00:00Anonymous (anonymous@undisclosed.example.com)21
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:21&rev=1714618568&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=21H
Version
1 and higher
Brief
READ RANDOM RECORD FROM FCB FILE
Family API
Input
AH = 21h
DS:DX -> opened FCB (see #01345)
Return
AL = statustext/html2024-05-02T03:09:30+00:00Anonymous (anonymous@undisclosed.example.com)22
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:22&rev=1714619370&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=22H
Version
1 and higher
Brief
WRITE RANDOM RECORD TO FCB FILE
Family API
Input
AH = 22h
DS:DX -> opened FCB (see #01345)
Disk Transfer Area contains record to be writtentext/html2024-05-02T03:18:47+00:00Anonymous (anonymous@undisclosed.example.com)23
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:23&rev=1714619927&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=23H
Version
1 and higher
Brief
GET FILE SIZE FOR FCB
Family API
Input
AH = 23h
DS:DX -> unopened FCB (see #01345), wildcards not allowedtext/html2024-05-02T03:20:35+00:00Anonymous (anonymous@undisclosed.example.com)24
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:24&rev=1714620035&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=24H
Version
1 and higher
Brief
SET RANDOM RECORD NUMBER FOR FCB
Family API
Input
AH = 24h
DS:DX -> opened FCB (see #01345)
Return
nonetext/html2025-11-04T08:18:08+00:00Anonymous (anonymous@undisclosed.example.com)25
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:25&rev=1762244288&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=25H
Version
1 and higher
Brief
SET INTERRUPT VECTOR
Family API
Input
AH = 25h
AL = interrupt number
DS:DX -> new interrupt handlertext/html2024-05-02T03:52:38+00:00Anonymous (anonymous@undisclosed.example.com)26
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:26&rev=1714621958&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=26H
Version
1 and higher
Brief
CREATE NEW PROGRAM SEGMENT PREFIX
Family API
Input
AH = 26h
DX = segment at which to create PSP (see #01378)text/html2024-05-02T03:55:21+00:00Anonymous (anonymous@undisclosed.example.com)27
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:27&rev=1714622121&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=27H
Version
1 and higher
Brief
RANDOM BLOCK READ FROM FCB FILE
Family API
Input
AH = 27h
CX = number of records to read
DS:DX -> opened FCB (see #01345)text/html2024-05-02T04:02:25+00:00Anonymous (anonymous@undisclosed.example.com)28
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:28&rev=1714622545&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=28H
Version
1 and higher
Brief
RANDOM BLOCK WRITE TO FCB FILE
Family API
Input
AH = 28h
CX = number of records to write
DS:DX -> opened FCB (see #01345)
Disk Transfer Area contains records to be writtentext/html2024-05-02T04:06:29+00:00Anonymous (anonymous@undisclosed.example.com)29
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:29&rev=1714622789&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=29H
Version
1 and higher
Brief
PARSE FILENAME INTO FCB
Family API
Input
AH = 29h
AL = parsing options (see #01380)
DS:SI -> filename string (both '*' and '?' wildcards OK)
ES:DI -> buffer for unopened FCBtext/html2024-05-02T04:46:47+00:00Anonymous (anonymous@undisclosed.example.com)30
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:30&rev=1714625207&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=30H
Version
2 and higher
Brief
GET DOS VERSION
Family API
Input
AH = 30h
---DOS 5+ ---
AL = what to return in BH
00h OEM number (see #01394)
01h version flagtext/html2024-05-02T04:49:12+00:00Anonymous (anonymous@undisclosed.example.com)31
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:31&rev=1714625352&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=31H
Version
2 and higher
Brief
TERMINATE AND STAY RESIDENT
Family API
Input
AH = 31h
AL = return code
DX = number of paragraphs to keep residenttext/html2024-05-02T04:55:54+00:00Anonymous (anonymous@undisclosed.example.com)32
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:32&rev=1714625754&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=32H
Version
2 and higher
Brief
GET DOS DRIVE PARAMETER BLOCK FOR SPECIFIC DRIVE
Family API
Input
AH = 32h
DL = drive number (00h = default, 01h = A:, etc)text/html2020-12-09T05:53:28+00:00Anonymous (anonymous@undisclosed.example.com)33
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:33&rev=1607493208&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=33H
Input
AH=33H, AL=Subfunction
Subfunction Description Version 00 get current extended break statetext/html2024-05-02T05:50:47+00:00Anonymous (anonymous@undisclosed.example.com)34
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:34&rev=1714629047&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=34H
Version
2 and higher
Brief
GET ADDRESS OF INDOS FLAG
Family API
Input
AH = 34h
Return
ES:BX -> one-byte InDOS flag
Notes
this function executes on the DOS stack, and thus cannot be called while another DOS function is already executing; you should use this function once at the beginning of the program and store the returned pointer rather than calling it when requiring DOS …text/html2024-05-02T05:52:19+00:00Anonymous (anonymous@undisclosed.example.com)35
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:35&rev=1714629139&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=34H
Version
2 and higher
Brief
GET INTERRUPT VECTOR
Family API
Input
AH = 35h
AL = interrupt number
Return
ES:BX -> current interrupt handlertext/html2024-05-02T06:00:21+00:00Anonymous (anonymous@undisclosed.example.com)36
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:36&rev=1714629621&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=36H
Version
2 and higher
Brief
GET FREE DISK SPACE
Family API
DosQFSInfo
Input
AH = 36h
DL = drive number (00h = default, 01h = A:, etc)
Returntext/html2020-12-11T06:03:14+00:00Anonymous (anonymous@undisclosed.example.com)37
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:37&rev=1607666594&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=37H
Subfunction Desctiption Version 00 GET SWITCH CHARACTER 2.0+ 01 SET SWITCH CHARACTER 2.0+text/html2021-03-23T03:19:51+00:00Anonymous (anonymous@undisclosed.example.com)38
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:38&rev=1616469591&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=38H
Subfunction Desctiption Version 00 Get current country info 2.0+ 01 Get/set specified country info 3.0+text/html2025-07-12T16:20:53+00:00Anonymous (anonymous@undisclosed.example.com)39
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:39&rev=1752337253&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=39H
Version
2 and higher
Brief
“MKDIR” - CREATE SUBDIRECTORY
Family API
DosMkDir
Input
* AH = 39h
* DS:DX -> ASCIZ pathname (maximum length 67 or 260 characters)text/html2024-05-02T08:12:08+00:00Anonymous (anonymous@undisclosed.example.com)40
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:40&rev=1714637528&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=40H
Version
2 and higher
Brief
“WRITE” - WRITE TO FILE OR DEVICE
Family API
Input
AH = 40h
BX = file handle
CX = number of bytes to write
DS:DX -> data to writetext/html2024-05-02T08:15:34+00:00Anonymous (anonymous@undisclosed.example.com)41
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:41&rev=1714637734&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=41H
Version
2 and higher
Brief
“UNLINK” - DELETE FILE
Family API
Input
AH = 41h
DS:DX -> ASCIZ filename (no wildcards, but see notes)
CL = attribute mask for deletion (server call only, see notes)text/html2024-05-02T08:17:36+00:00Anonymous (anonymous@undisclosed.example.com)42
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:42&rev=1714637856&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=42H
Version
2 and higher
Brief
“LSEEK” - SET CURRENT FILE POSITION
Family API
Input
AH = 42h
AL = origin of move
00h start of file
01h current file position
02h end of file
BX = file handle
CX:DX = (signed) offset from origin of new file positiontext/html2021-04-30T04:45:25+00:00Anonymous (anonymous@undisclosed.example.com)43
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:43&rev=1619757925&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=43H
Input
AH=43H, AL=Subfunction
Subfunction Description Version 00 GET FILE ATTRIBUTEStext/html2021-04-30T05:54:30+00:00Anonymous (anonymous@undisclosed.example.com)44
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:44&rev=1619762070&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=44H
Input
AH=44H, AL=Subfunction
Subfunction Description Version 00 GET DEVICE INFORMATIONtext/html2026-03-05T02:17:01+00:00Anonymous (anonymous@undisclosed.example.com)45
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:45&rev=1772677021&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=45H
Version
2 and higher
Brief
“DUP” - DUPLICATE FILE HANDLE
Family API
DosDupHandle
Input
* AH = 45h
* BX = file handle
Return
* CF clear if successfultext/html2024-05-03T02:07:05+00:00Anonymous (anonymous@undisclosed.example.com)46
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:46&rev=1714702025&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=46H
Version
2 and higher
Brief
“DUP2”, “FORCEDUP” - FORCE DUPLICATE FILE HANDLE
Family API
Input
AH = 46h
BX = file handle
CX = file handle to become duplicate of first handletext/html2024-05-11T09:11:46+00:00Anonymous (anonymous@undisclosed.example.com)47
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:47&rev=1715418706&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=47H
Version
2 and higher
Brief
“CWD” - GET CURRENT DIRECTORY
Family API
Input
* AH = 47h
* DL = drive number (00h = default, 01h = A:, etc)text/html2021-08-14T08:46:29+00:00Anonymous (anonymous@undisclosed.example.com)48
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:48&rev=1628930789&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=48H
Version
2 and higher
Brief
ALLOCATE MEMORYY
Family API
DosAllocHuge, DosAllocSeg
Input
AH = 48h
BX = number of paragraphs to allocate
Return
CF clear if successful
AX = segment of allocated block
CF set on error
AX = error code (07h,08h) (see #01680 at AH=59h/BX=0000h)
BX = size of largest available blocktext/html2024-05-03T02:09:54+00:00Anonymous (anonymous@undisclosed.example.com)49
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:49&rev=1714702194&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=49H
Version
2 and higher
Brief
FREE MEMORY
Family API
Input
AH = 49h
ES = segment of block to free
Return
CF clear if successful
CF set on error
AX = error code (07h,09h) (see #01680 at AH=59h/BX=0000h)text/html2024-05-03T02:27:05+00:00Anonymous (anonymous@undisclosed.example.com)50
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:50&rev=1714703225&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=50H
Version
2 and higher
Brief
SET CURRENT PROCESS ID (SET PSP ADDRESS)
Family API
Input
AH = 50h
BX = segment of PSP for new process
Return
Notestext/html2024-05-03T02:28:06+00:00Anonymous (anonymous@undisclosed.example.com)51
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:51&rev=1714703286&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=51H
Version
2 and higher
Brief
GET CURRENT PROCESS ID (GET PSP ADDRESS)
Family API
Input
AH = 51h
Return
BX = segment of PSP for current process
Notestext/html2024-05-03T02:29:51+00:00Anonymous (anonymous@undisclosed.example.com)52
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:52&rev=1714703391&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=52H
Version
2 and higher
Brief
“SYSVARS” - GET LIST OF LISTS
Family API
Input
AH = 52h
Return
ES:BX -> DOS list of lists (see #01627)
Notes
partially supported bytext/html2024-05-03T02:30:50+00:00Anonymous (anonymous@undisclosed.example.com)53
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:53&rev=1714703450&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=53H
Version
2 and higher
Brief
TRANSLATE BIOS PARAMETER BLOCK TO DRIVE PARAM BLOCK
Family API
Input
AH = 53h
DS:SI -> BIOS Parameter Block (see #01663)
ES:BP -> buffer for Drive Parameter Block (see #01395 at AH=32h)
DBP drive byte must be set to valid drive (Windows95-OSR2)
---Windows95---
CX = signature 4558h ('EX') for FAT32 extended BPB/DPB (see #01664)
DX = signature 4152h ('AR'…text/html2024-05-03T02:49:26+00:00Anonymous (anonymous@undisclosed.example.com)54
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:54&rev=1714704566&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=54H
Version
2 and higher
Brief
GET VERIFY FLAG
Family API
Input
AH = 54h
Return
AL = verify flag
00h off
01h on (all disk writes verified after writing)text/html2024-05-03T03:43:04+00:00Anonymous (anonymous@undisclosed.example.com)55
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:55&rev=1714707784&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=55H
Version
2 and higher
Brief
CREATE CHILD PSP
Family API
Input
AH = 55h
DX = segment at which to create new PSP
SI = (DOS 3.0+) value to place in memory size field at DX:[0002h]text/html2024-05-03T03:46:30+00:00Anonymous (anonymous@undisclosed.example.com)56
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:56&rev=1714707990&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=56H
Version
2 and higher
Brief
“RENAME” - RENAME FILE
Family API
Input
AH = 56h
DS:DX -> ASCIZ filename of existing file (no wildcards, but see below)
ES:DI -> ASCIZ new filename (no wildcards)
CL = attribute mask (server call only, see below)text/html2021-08-01T05:51:03+00:00Anonymous (anonymous@undisclosed.example.com)57
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:57&rev=1627797063&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=57H
Input
AH=57H, AL=Subfunction
Subfunction Description Version 00 GET FILE'S LAST-WRITTEN DATE AND TIMEtext/html2021-08-01T06:19:50+00:00Anonymous (anonymous@undisclosed.example.com)58
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:58&rev=1627798790&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=58H
Input
AH=58H, AL=Subfunction
Subfunction Description Version 00 GET MEMORY ALLOCATION STRATEGYtext/html2025-07-13T12:56:51+00:00Anonymous (anonymous@undisclosed.example.com)59
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:59&rev=1752411411&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=59H
Version
3 and higher
Brief
GET EXTENDED ERROR INFORMATION
Family API
DosErrClass
Input
* AH = 59h
* BX = 0000h
Return
* AX = extended error code (see #01680)text/html2024-05-06T06:00:28+00:00Anonymous (anonymous@undisclosed.example.com)60
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:60&rev=1714975228&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=60H
Version
3 and higher
Brief
TRUENAME - CANONICALIZE FILENAME OR PATH
Family API
Input
AH = 60h
DS:SI -> ASCIZ filename or path
ES:DI -> 128-byte buffer for canonicalized nametext/html2024-05-06T06:01:29+00:00Anonymous (anonymous@undisclosed.example.com)62
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:62&rev=1714975289&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=62H
Version
3 and higher
Brief
GET CURRENT PSP ADDRESS
Family API
Input
AH = 62h
Return
BX = segment of PSP for current process
Notes
This function does not use any of the DOS-internal stacks and may thus be called at any time, even during another INT 21h call. The current PSP is not necessarily the caller's PSP. Identical to the undocumented AH=51htext/html2021-08-06T03:10:25+00:00Anonymous (anonymous@undisclosed.example.com)63
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:63&rev=1628219425&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=63H
Input
AH=63H, AL=Subfunction
Subfunction Description Version 00 GET LEAD BYTE TABLE ADDRESStext/html2024-05-08T01:22:16+00:00Anonymous (anonymous@undisclosed.example.com)64
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:64&rev=1715131336&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=64H
Version
3 and higher
Brief
SET DEVICE DRIVER LOOKAHEAD FLAG
Family API
Input
AH = 64h
AL = flag
00h (default) call device driver function 5 (non-dest read) before INT 21/AH=01h,08h,0Ah
nonzero don't call driver function 5text/html2021-08-06T04:21:45+00:00Anonymous (anonymous@undisclosed.example.com)65
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:65&rev=1628223705&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=65H
Input
AH=65H, AL=Subfunction
Subfunction Description Version 00 SET GENERAL INTERNATIONALIZATION INFOtext/html2021-08-06T05:53:53+00:00Anonymous (anonymous@undisclosed.example.com)66
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:66&rev=1628229233&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=66H
Input
AH=66H, AL=Subfunction
Subfunction Description Version 01 GET GLOBAL CODE PAGE TABLEtext/html2024-05-08T01:44:15+00:00Anonymous (anonymous@undisclosed.example.com)67
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:67&rev=1715132655&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=67H
Version
3.3 and higher
Brief
SET HANDLE COUNT
Family API
Input
AH = 67h
BX = size of new file handle table for process
Return
CF clear if successful
CF set on error
AX = error code (see #01680 at AH=59h/BX=0000h)text/html2024-05-08T01:44:52+00:00Anonymous (anonymous@undisclosed.example.com)68
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:68&rev=1715132692&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=68H
Version
3.3 and higher
Brief
FFLUSH - COMMIT FILE
Family API
Input
AH = 68h
BX = file handle
Return
CF clear if successful
all data still in DOS disk buffers is written to disk immediately,
and the file's directory entry is updated
CF set on error
AX = error code (see #01680 at AH=59h/BX=0000h)text/html2021-08-06T06:35:55+00:00Anonymous (anonymous@undisclosed.example.com)69
https://ftp.osfree.org/doku/doku.php?id=en:docs:dos:api:int21:69&rev=1628231755&do=diff
Note: This API call is for DOS and Win16 personality only. Use Family API for portability.
Int 21H, AH=69H
Input
AH=69H, AL=Subfunction
Subfunction Description Version 00 get serial number