assemblydosx86-16instructions

What does it mean by "MOV AH, 4CH" in assembly language?

Most of the assembly code is terminate by the following instructions

MOV AH, 4CH
INT 21H

What does it mean by "MOV AH, 4CH" ?

Solution

  • DOS interrupt int 21/4Ch is EXIT - TERMINATE WITH RETURN CODE, the content of al is used as the return code and the process is terminated. The documentation comes with the following note:

    Unless the process is its own parent (see #01378 [offset 16h] at AH=26h), all open files are closed and all memory belonging to the process is freed. All network file locks should be removed before calling this function