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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