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.
 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