The following examples are for 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
MOV AL, 3H
CALL func
.
.
ADD ESP, 12 ; Cleaning up the parameters
.
.
MOV m, EAX
.
.
Note: the MOV AL,3H instruction is not present unless the /Gp+ compiler option is set (to support parmdwords).
For the callee, the code looks like this:
func 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
func ENDP
The saved register set is EBX, ESI, and EDI. The other registers (EAX, ECX, and EDX) can have their contents changed by a called routine.
Floating-point results are returned in ST(0), which is the top of the floating-point register stack. If there is no numeric coprocessor installed in the system, the OS/2 operating system emulates the coprocessor.
Floating-point parameters are pushed on the run-time stack.
Under some circumstances, the compiler will not use EBP to access automatic and parameter values, thus increasing the efficiency of the application. Whether it is used or not, EBP will not change across the call.
When passing structures _System convention as value parameters, the compiler generates code to copy the structure on to the run-time stack. If the size of the structure is larger than a run-time page size (4K), the compiler generates code to copy the structure backward. (That is, the last byte in the structure is the first to be copied.) This operation ensures that the OS/2 guard page _System linkage method of stack growth will function properly in the presence of large structures being passed by value.
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(struct test_tag test_parm)
{
test_parm.a = 42;
return test_parm;
}
int main(void)
{
test_struct = test_function(test_struct);
return test_struct.a;
}
The code generated for this program would be:
test_function PROC PUSH ESI PUSH EDI MOV DWORD PTR [ESP+0cH], 02aH ; test_parm.a MOV EAX, [ESP+08H] ; Get the target of the return value MOV EDI, EAX ; Value LEA ESI, [ESP+0cH] ; test_parm MOV ECX, 065H REP MOVSD POP EDI POP ESI RET test_function 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 MOV AL, 065H PUSH OFFSET FLAT: test_struct ; Push the address of the target CALL test_function ADD ESP, 0198H MOV EAX, DWORD PTR test_struct ; Take care of the return POP EDI ; from main POP ESI LEAVE RET main ENDP |
Note: The MOV AL, 065H is not present unless the /Gp+ compiler option is specified.
In a slightly different case, where only one field of the structure is used by the caller (as shown in the following example), the compiler allocates sufficient temporary storage in the caller's local storage area on the stack to contain a copy of the structure. The address of this temporary storage will be pushed as the target for the return value. Once the call is completed, the desired member of the structure can be accessed as an offset from EAX, as can be seen in the code generated for the example:
struct test_tag
{
int a;
int some_array[100];
} test_struct;
struct test_tag test_function(struct test_tag test_parm)
{
test_parm.a = 42;
return test_parm;
}
int main(void)
{
return test_function(test_struct).a;
}
The code generated for this example would be:
PUBLIC main main PROC PUSH EBP MOV EBP, ESP SUB ESP, 0194H ; Allocate space for compiler-generated PUSH ESI ; temporary variable PUSH EDI SUB ESP, 0194H MOV EDI, ESP MOV ESI, OFFSET FLAT: test_struct MOV ECX, 065H REP MOVSD LEA EAX, [ESP+019cH] PUSH EAX MOV AL, 065H CALL test_function ADD ESP, 0198H MOV EAX, [EAX] ; Note the convenience of having the POP EDI ; address of the returned structure POP ESI ; in EAX LEAVE RET main ENDP |
Again, the MOV AL,3H instruction is not present unless the /Gp+ compiler option is specified.
![]()
Calling Conventions
Stack Allocation
![]()
_System Calling Convention in OS/2