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



MS-DOS | OS/2 | WINDOWS


OPERATING SYSTEM API
====================

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 (Command.com) 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.