The MS-DOS API is an API which originated with 86-DOS and 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.
Read more about MS-DOS API: History of The DOS API, The DOS API and Windows, Interrupt Vectors Used By DOS, DOS INT 21h Services, Operating Systems With Support For The MS-DOS API, Programs With Support For The MS-DOS API, See Also