Explain DOS function call with one illustration

DOS function call

In order to use DOS function calls, always place function number into register AH and load all other relevant information into registers, as described in the entry data table. Once this is accomplished, follow with an INT 21H to execute DOS function.

Illustration:      MOV AH, 6

                        MOV DL, 'A'

                        INT 21H

Example demonstrates how to display an ASCII A on the CRT screen at the current cursor position with a DOS function call.