INT 21H: DOS Functions

                INT 21H is the portal to most DOS functions.

              See DOS Function Index for a complete list.
              See DOS Fns QuickRef for an often-used subset list.
              See DOS Fns by Version for DOS version compatibility.
              See DOS Fns by Category for functional groupings.

              See About DOS Functions for related information.

 Programs requesting a DOS service must prepare all registers and set
 AH (or AX) to the desired DOS function number, then issue an INT 21H.
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 The following are some DOS services that are NOT called via INT 21H:

          INT 25H Absolute Disk Read
          INT 26H Absolute Disk Write

          INT 28H DOS Idle
          INT 29H DOS Internal Fast Screen Write ◄undocumented►

          INT 2fH Multiplex Interrupt is the portal to:
                  ANSI.SYS installed state
                  Append Command API
                  Doskey Services
                  Dosshell Task-Switcher API
                  DoubleSpace API
                  HIMEM.SYS / XMS API
                  National Language Support services
                  Network services
                  Print Command API
                  Other Microsoft and after-market services

          INT 15H APM Advanced Power Management (APM)
          INT 31H DPMI Services
          INT 33H Mouse Services
          INT 67H EMS Services

See Also: API Index
          Interrupts and BIOS Services
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