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:
Network services
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
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INT 21H: DOS Functions