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Interrupt 21h, the DOS Interrupt, is the most used Interrupt in DOS and Windows programming. It contains functionality for text-interface IO, exiting, and more.

Int 21h Interrupt Table:









AH/ax = 00hnonenoneExit Programmov AH,00h
int 21h
Use 4Ch instead, this is an obsolete function for DOS 1.x, and does not always clear the memory properly.
AH/ax = 01hnoneAL = key pressedEcho Keymov AH,01h
int 21h
Reads one character from the standard input (keyboard) and echoes it to the standard output (screen). DOS 1.x function; might not work the way you expect. (for example, it might use the printer for output)
AH/ax = 02hnoneDL = key pressedWrite Charmov AH,02h
int 21h

Writes one character to the standard output (screen). DOS 1.x function (obsolete)

AH/ax = 09h none nonePrint String

mov DX,(string address)

mov AH,09h
int 21h

Prints a string stored in the address in the DX Register.