1. ICS51 Introductory Computer Organization
Another Function Call Example
and Logical Operations

2. The caller should • Before calling a procedure:
• pass parameters to the procedure: push to the stack in reverse order (the last parameter is pushed first)
• execute call (to perform the procedure call)
• After the procedure returns:
• pop parameters from the stack (to restore the stack to its state before call was performed)
• the return value is in EAX

3. The callee should • At the beginning of procedure:
• access passed parameters using ESP
• When the procedure is done:
• the return value should be placed in EAX
• execute ret instruction (return to the caller)

4. Another example Passing parameters using the stack:
Save registers used inside the function on the stack then restore them after
Accessing parameters: To access the a location on the stack, use an instruction like mov REG, dword ptr[ESP+OFFSET]
Passing parameters using the stack:
Pushed on the stack from right to left
The return address of the procedure is also on the stack!!
The “ret” instruction
Return value in the “eax” register

5. void main(void) {
int arr[] = {0,1,2,3,4};
int sum = firstpass(arr,5);
printf(" The sum is = %d\n",sum); }
__declspec(naked)
int firstpass (int *arr, int len){
__asm{
/* Save the registers that may be modified by the called function*/
push ecx
push edx
/* Get parameters from the stack */
mov ecx, dword ptr[esp + 12] // load the arr parameter from the stack
mov edx, dword ptr[esp + 16] // load the len parameter from the stack
/* put the parameters on the stack for the add2nums function */
push edx // edx contains length
push ecx // ecx contains array
call add2nums
/* pop the parameters of the stack after the call – to restore the stack*/
pop ecx
pop edx
/* restore the saved registers from the stack*/
ret

6. __declspec(naked)
int add2nums(int *array, int length) {
__asm {
/* Save the registers used in this function to the stack
Do not save eax ! */
push edi
xor eax,eax
xor edi,edi
/* this accesses the "array" parameter on the stack */
mov ebx, dword ptr[esp + 8]
/* load the "length" parameter from the stack */
mov edx, dword ptr[esp + 12]
loop1:
cmp edx,edi
je all_done
/* copies i-th element or an array */
mov ecx, dword ptr[ebx+4*edi]
/* the value returned by this function is put in eax */
add eax,ecx
inc edi
jmp loop1
all_done:
/* restore the saved registers from the stack */
pop edi
ret }

7. Logical Operations Get a bit Count number of ones Be creative!
Using AND/OR
Count number of ones
Using DIV
Using AND and SHR
Be creative!