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The '''MS-DOS API''' is an [[API]] used originally in [[MS-DOS]]/[[PC-DOS]], and later by other [[DOS]] systems. Most calls to the DOS API invoke [[software interrupt]] 21h (int 21h). By calling int 21h with the subfunction in AH and the 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. The subfunctions provided in the first release of MS-DOS in 1981 were strongly oriented to compatibility with [[CP/M]]. A major revamp of the DOS API (partially influenced by certain Unix concepts) was undertaken for the release of MS-DOS 2.0 in 1983, adding numerous new subfunctions to deal with file I/O using abstract "handles" and support for subdirectories. In these new subfunctions, strings are usually terminated by a NUL byte ([[ASCIIZ]]). In MS-DOS version 3.1 (released in 1985), several subfunctions were added for dealing with networks.
There are various implementations of the DOS API, including [[MS-DOS]], [[PC-DOS]], [[DR-DOS]], [[FreeDOS]], [[PTS-DOS]], and others. The DOS API is based on the [[BIOS]], and DOS routines often internally access [[BIOS interrupt call]]s.
The [[DOS Protected Mode Interface]] extends the DOS API to 32-bits. The MS-DOS API is partially dependent on x86 code, hence DOS cannot be ported to other chip architectures.
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