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 -♦-