real mode API (Int 21h etc) of the MS-DOS operating system

The MS-DOS API is an API which originated with 86-DOS and is used in MS-DOS/PC DOS and other DOS-compatible operating systems. Most calls to the DOS API are invoked using software interrupt 21h (INT 21h). By calling INT 21h with a subfunction number in the AH processor register and other parameters in other registers, one invokes various DOS services. DOS services include keyboard input, video output, disk file access, executing programs, memory allocation, and various other things. In the late 1980s, DOS extenders along with the DOS Protected Mode Interface (DPMI) allow the programs to run in either 16-bit or 32-bit protected mode and still have access to the DOS API.


History of the DOS APIEdit

The original DOS API in 86-DOS and MS-DOS 1.0 was designed to be functionally compatible with CP/M. Files were accessed using file control blocks (FCBs). The DOS API was greatly extended in MS-DOS 2.0 with several Unix concepts, including file access using file handles, hierarchical directories and device I/O control.[1] In DOS 3.1, network redirector support was added. In MS-DOS 3.31, the INT 25h/26h functions were enhanced to support hard disks greater than 32 MB. MS-DOS 5 added support for using upper memory blocks (UMBs). After MS-DOS 5, the DOS API was unchanged for the successive standalone releases of DOS.

The DOS API and WindowsEdit

In Windows 9x, DOS was generally used as a bootloader which loaded the protected-mode operating system and graphical shell. DOS was usually accessed from a virtual DOS machine (VDM) but it was also possible to boot directly to real mode MS-DOS 7.0 without loading Windows. The DOS API was extended with enhanced internationalization support and long filename support, though the long filename support was only available in a VDM. With Windows 95 OSR2, DOS was updated to 7.1, which added FAT32 support, and functions were added to the DOS API to support this. Windows 98 and Windows ME also implement the MS-DOS 7.1 API, though Windows ME reports itself as MS-DOS 8.0.

Windows NT and the systems based on it (e.g. Windows XP and Windows Vista) are not based on MS-DOS, but use a virtual machine, NTVDM, to handle the DOS API. NTVDM works by running a DOS program in virtual 8086 mode (an emulation of real mode within protected mode available on 80386 and higher processors). NTVDM supports the DOS 5.0 API. DOSEMU for Linux uses a similar approach.

Interrupt vectors used by DOSEdit

Interrupt vector Description Version Notes
20h Terminate program 1.0+ Implemented in DOS kernel
21h Main DOS API 1.0+ Implemented in DOS kernel
22h Program terminate address 1.0+ Return address in calling program
23h Control-C handler address 1.0+ Default handler is in the command shell (usually COMMAND.COM)
24h Critical error handler address 1.0+ Default handler is in the command shell (usually COMMAND.COM)
25h Absolute disk read 1.0+ Implemented in DOS kernel, enhanced in DOS 3.31 to support up to 2 GB partitions
26h Absolute disk write 1.0+ Implemented in DOS kernel, enhanced in DOS 3.31 to support up to 2 GB partitions
27h Terminate and stay resident 1.0+ Implemented in COMMAND.COM in DOS 1.0, DOS kernel in DOS 2.0+
28h Idle callout 2.0+ Called by DOS kernel when waiting for input
29h Fast console output 2.0+ Implemented by the builtin console device driver or a replacement driver like ANSI.SYS
2Ah Networking and critical section 3.0+ Called by DOS kernel to interface with networking software
2Bh Unused
2Ch Unused
2Dh Unused
2Eh Reload transient 2.0+ Implemented in COMMAND.COM
2Fh Multiplex 3.0+ Implemented in DOS kernel and various programs (PRINT, MSCDEX, DOSKEY, APPEND, etc.) depending on subfunction number

DOS INT 21h servicesEdit

AH Description Version
00h Program terminate 1.0+
01h Character input 1.0+
02h Character output 1.0+
03h Auxiliary input 1.0+
04h Auxiliary output 1.0+
05h Printer output 1.0+
06h Direct console I/O 1.0+
07h Direct console input without echo 1.0+
08h Console input without echo 1.0+
09h Display string 1.0+
0Ah Buffered keyboard input 1.0+
0Bh Get input status 1.0+
0Ch Flush input buffer and input 1.0+
0Dh Disk reset 1.0+
0Eh Set default drive 1.0+
0Fh Open file 1.0+
10h Close file 1.0+
11h Find first file 1.0+
12h Find next file 1.0+
13h Delete file 1.0+
14h Sequential read 1.0+
15h Sequential write 1.0+
16h Create or truncate file 1.0+
17h Rename file 1.0+
18h Reserved 1.0+
19h Get default drive 1.0+
1Ah Set disk transfer address 1.0+
1Bh Get allocation info for default drive 1.0+
1Ch Get allocation info for specified drive 1.0+
1Dh Reserved 1.0+
1Eh Reserved 1.0+
1Fh Get disk parameter block for default drive 1.0+
20h Reserved 1.0+
21h Random read 1.0+
22h Random write 1.0+
23h Get file size in records 1.0+
24h Set random record number 1.0+
25h Set interrupt vector 1.0+
26h Create PSP 1.0+
27h Random block read 1.0+
28h Random block write 1.0+
29h Parse filename 1.0+
2Ah Get date 1.0+
2Bh Set date 1.0+
2Ch Get time 1.0+
2Dh Set time 1.0+
2Eh Set verify flag 1.0+
2Fh Get disk transfer address 2.0+
30h Get DOS version 2.0+
31h Terminate and stay resident 2.0+
32h Get disk parameter block for specified drive 2.0+
33h Get or set Ctrl-Break 2.0+
34h Get InDOS flag pointer 2.0+
35h Get interrupt vector 2.0+
36h Get free disk space 2.0+
37h Get or set switch character 2.0+
38h Get or set country info 2.0+
39h Create subdirectory 2.0+
3Ah Remove subdirectory 2.0+
3Bh Change current directory 2.0+
3Ch Create or truncate file 2.0+
3Dh Open file 2.0+
3Eh Close file 2.0+
3Fh Read file or device 2.0+
40h Write file or device 2.0+
41h Delete file 2.0+
42h Move file pointer 2.0+
43h Get or set file attributes 2.0+
44h I/O control for devices 2.0+
45h Duplicate handle 2.0+
46h Redirect handle 2.0+
47h Get current directory 2.0+
48h Allocate memory 2.0+
49h Release memory 2.0+
4Ah Reallocate memory 2.0+
4Bh Execute program 2.0+
4Ch Terminate with return code 2.0+
4Dh Get program return code 2.0+
4Eh Find first file 2.0+
4Fh Find next file 2.0+
50h Set current PSP 2.0+
51h Get current PSP 2.0+
52h Get DOS internal pointers (SYSVARS) 2.0+
53h Create disk parameter block 2.0+
54h Get verify flag 2.0+
55h Create program PSP 2.0+
56h Rename file 2.0+
57h Get or set file date and time 2.0+
58h Get or set allocation strategy 2.11+
59h Get extended error info 3.0+
5Ah Create unique file 3.0+
5Bh Create new file 3.0+
5Ch Lock or unlock file 3.0+
5Dh File sharing functions 3.0+
5Eh Network functions 3.0+
5Fh Network redirection functions 3.0+
60h Qualify filename 3.0+
61h Reserved 3.0+
62h Get current PSP 3.0+
63h Get DBCS lead byte table pointer 3.0+
64h Set wait for external event flag 3.2+
65h Get extended country info 3.3+
66h Get or set code page 3.3+
67h Set handle count 3.3+
68h Commit file 3.3+
69h Get or set media id 4.0+
6Ah Commit file 4.0+
6Bh Reserved 4.0+
6Ch Extended open/create file 4.0+

Operating systems with support for the MS-DOS APIEdit

Programs with support for the MS-DOS APIEdit

See alsoEdit


  1. ^ Ray Duncan (1988). Advanced MS-DOS Programming: The Microsoft Guide for Assembly Language and C Programmers. Microsoft Press. ISBN 0914845772.