The following examples are included for purposes of illustration only. They have not been optimized. The examples assume that you are familiar with programming in assembler. Note that, in the examples, the stack grows toward the bottom of the page, and ESP always points to the top of the stack.
For the call
m = func(a,b,c);
a, b, and c are 32-bit integers and func has two local variables, x and y (both 32-bit integers).
The stack for the call to func would look like this:
The instructions used to create this activation record on the stack look like this on the calling side:
PUSH c
PUSH b
PUSH a
CALL func@12
.
.
MOV m, EAX
.
.
For the callee, the code looks like this:
_func@12 PROC
PUSH EBP
MOV EBP, ESP ; Allocating 8 bytes of storage
SUB ESP, 8 ; for two local variables.
PUSH EDI ; These would only be
PUSH ESI ; pushed if they were used
PUSH EBX ; in this function.
.
.
MOV EAX, [EBP - 8] ; Load y into EAX
MOV EBX, [EBP + 12] ; Load b into EBX
.
.
XOR EAX, EAX ; Zero the return value
POP EBX ; Restore the saved registers
POP ESI
POP EDI
LEAVE ; Equivalent to MOV ESP, EBP
; POP EBP
RET 0CH
_func@12 ENDP
The saved register set is EBX, ESI, and EDI.
Structures are not returned on the stack. The caller pushes the address where the returned structure is to be placed as a lexically first hidden parameter. A function that returns a structure must be aware that all parameters are 4 bytes farther away from EBP than they would be if no structure return were involved. The address of the returned structure is returned in EAX.
In the most common case, where the return from a function is simply assigned to a variable, the compiler merely pushes the address of the variable as the hidden parameter. For example:
struct test_tag {
int a;
int some_array[100];
} test_struct;
struct test_tag test_function@404(struct test_tag test_parm)
{
test_parm.a = 42;
return test_parm;
}
int main(void)
{
test_struct = test_function@404(test_struct);
return test_struct.a;
}
The code generated for the above example would be:
test_function@404 PROC PUSH EBP MOV EBP, ESP PUSH ESI PUSH EDI MOV DWORD PTR [ESP+0cH], 02aH ; test_parm.a MOV EAX, [EBP+08H] ; Get the target of the return value MOV EDI, EAX ; Value LEA ESI, [EBP+0cH] ; test_parm MOV ECX, 065H REP MOVSD POP EDI POP ESI LEAVE RET 198H test_function@404 ENDP PUBLIC main main PROC PUSH EBP MOV EBP, ESP PUSH ESI PUSH EDI SUB ESP, 0194H ; Adjust the stack pointer MOV EDI, ESP MOV ESI, OFFSET FLAT: test_struct MOV ECX, 065H REP MOVSD ; Copy the parameter PUSH OFFSET FLAT: test_struct ; Push the address of the target CALL TEST_FUNCTION&404 MOV EAX, DWORD PTR test_struct ; Take care of the return POP EDI ; from main POP ESI LEAVE RET main ENDP |