这个都是过去DOS时代的汇编源码，虽然已经过去了，但是对于学习汇编还是有帮助的，汇编语言只是程序员一门基础语言，大多人掌握即可，不一定要深入研究.......
title MSDOS 2.00 Function Library for Lattice C
subttl -
;;
;;FUNCTION: Sets and returns switch char-
;; acter and device availability.
;;
;;
;;CALL:
;;
;; ret= _charop(al,dl)
;; int ret; DL return value,
;; int al; charoper function
;; int dl; charoper data
;;
;;RETURN:
;; See the DOS docs for details.
;;_charop(0,0) returns the ASCII switch char,
;;_charop(1,'-') sets the switch to -,
;;_charop(2,0) returns device availability,
;;_charop(3,i) sets device availability.
;;
;;
;;DESCRIPTION:
;;
;;EXAMPLE:
;;
;;
;;CAUTIONS:
;;
;;
;;ASSUMPTIONS:
;;
;;LONG 32 bits (4 bytes)
;;INT 16 bits (2 bytes)
;;CHAR 8 bits (1 byte)
;;
page
pgroup group prog
prog segment byte public 'prog'
assume cs:pgroup,ds:pgroup
public _charop
_charop proc near
push bp
mov bp,sp
mov al,[bp+4]
mov dl,[bp+6]
mov ah,55
int 33
mov al,dl
mov ah,0
pop bp
ret
_charop endp
prog ends
end