CSC231 SumProc3.asm
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;;; sumproc3.asm
;;; D. Thiebaut CSC231
;;; third of a series of programs illustrating how to pass and receive
;;; information to and from a function in assembly.
;;;
;;; This and all associated programs simply add the contents of
;;; two variables a and b and store the resulting sum in a variable
;;; called result (all are double words)
;;;
;;; this version passes a and b by value through the stack. the
;;; sum is passed back in the stack.
SYS_EXIT equ 1
SYS_WRITE equ 4
STDOUT equ 1
;; -------------------------
;; data segment
;; -------------------------
section .data
a dd 0x1234 ; operand 1
b dd 0x5555 ; operand 2
result dd 0 ; where sum will end up
;; -------------------------
;; code area
;; -------------------------
section .text
global _start
_start:
push eax ; make room in the stack for result
push dword [a] ; pass copy of a
push dword [b] ; pass copy of b
call sum
pop dword [result] ; get sum from stack and restore stack
;; exit()
mov eax,SYS_EXIT
mov ebx,0
int 0x80 ; final system call
;;; -------------------------------------------------------
;;; sum function
;;; gets 2 dwords from stack and puts sum in stack for main
;;; program to receive.
;;; registers modified: eax
;;;
;;; stack looks like this after first 2 instructions:
;;;
;;; +----------+
;;; | "empty" | <-- ebp+16
;;; +----------+
;;; |copy of a | <-- ebp+12
;;; +----------+
;;; |copy of b | <-- ebp+8
;;; +----------+
;;; |retrn addr| <-- ebp+4 (plus 4 because double words!)
;;; +----------+
;;; |"old" ebp | <-- ebp <-- esp
;;; +----------+
;;; | |
;;; -------------------------------------------------------
sum: push ebp
mov ebp,esp
mov eax,dword [ebp+8] ; get copy of b
add eax,dword [ebp+12] ; get copy of a
mov dword[ebp+16],eax ; put sum in reserved space in stack
pop ebp
ret 8 ; we get rid of 2 dwords in stack and
; keep one for main program to get sum
