I am trying to learn how to call procedures in assembly language. Below is a simple example that shows my problem. I push 7 unto the stack, call the procedure; when the procudeure pops from the stack, the value is not the one I pushed. Can someone please help me understand what is happening and what I can do to make this work?
PUSH 7
CALL FOOBAR
MOV AH, 4CH
INT 21H
FOOBAR PROC
POP AX ; Not 7
RET
FOOBAR ENDP
The call instruction puts the return address on the stack, so when you pop ax in your procedure, it doesn't get the 7 you pushed, but the return address. The ret won't work, either (it expects to find the return address there!) try something like...
FOOBAR proc
push bp ; save caller's reg
mov bp, sp
mov ax, [bp + 4]
; do something with it
; leave - equivalent to:
mov sp, bp
pop bp
ret
There's a possible "gotcha" here. A "far" procedure has both the segment (cs) and offset on the stack, so 4 byes for the return address, and two bytes for the push bp puts the first parameter at [bp + 6]. I guess just proc defaults to proc near - you might want to say that, just for clarity. If you need a proc far it's probably time to graduate to 32-bit code (or 64-bit). 16-bit code is such a PITA - we're really glad to forget it! :)