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Microsoft KB Archive/103891

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Overview: Operating System Application Programmer Interface PSS ID Number: Q103891 Article last modified on 11-15-1993 PSS database name: PSS



Applications request operating system services, such as I/O, by calling some sort of application programmer interface (API) that is “exposed”–or made available and accessible–by the operating system.

MS-DOS exposes its I/O API by means of interrupt routines, primarily interrupt number x21.

  • From an assembly language program, a few key CPU registers are loaded with parameter information, and the interrupt instruction (INT) is used to pass the request to MS-DOS.
  • When a function (such as “Read”) for a high level language (such as C or Pascal) is compiled, linked, and executed, it causes events to occur that are equivalent to those invoked by the assembly language program: registers are loaded with parameters and an interrupt is generated, again passing control to MS-DOS.
  • When you issue a command from the MS-DOS command line, the resident command interpreter ( causes the same events to occur again: parameters are loaded in registers, an interrupt is generated, control is passed to MS-DOS.

OS/2 operates much the same way, except it does not expose an interface directly through interrupts: you must issue a high-level API call such as “DosRead,” with the appropriate parameters.

KBCategory: KBSubCategory: msna

Copyright Microsoft Corporation 1993.