Transcript Lecture 7 - University of Wisconsin

Computer Architecture and
Operating Systems
CS 3230 :Assembly Section
Lecture 7
Department of Computer Science and Software Engineering
University of Wisconsin-Platteville
Stack Structure
 The stack grows “downwards”
 Doubleword alignment
 Register ESP points to the top
of stack
 PUSH and POP manipulate
the top of stack
 The stack can be used as a
convenient place to:


store data temporarily
make subprogram calls
Stack Instructions
 PUSH src
 src is doubleword
 push src onto the top of the stack
 Action:
• ESP  ESP – 4
• [ESP]  src
 POP dest



dest is doubleword
pop top of stack into dst and logically remove it from the
stack
Action:
• dest  [ESP]
• ESP  ESP + 4
Example
1- push dword 1 ; 1 stored at 0FFCh, ESP = 0FFCh
2- push dword 2 ; 2 stored at 0FF8h, ESP = 0FF8h
3- push dword 3 ; 3 stored at 0FF4h, ESP = 0FF4h
4- pop eax ; EAX = 3, ESP = 0FF8h
5- pop ebx ; EBX = 2, ESP = 0FFCh
6- pop ecx ; ECX = 1, ESP = 1000h
Function Call and Return
 The x86 uses stack to handle the function
(subroutine) call
 Stack is used to



capture return address and recover it
parameter passing
local variables
CALL: call subroutine
 Syntax:
CALL dest
 Operation (absolute call):
PUSH EIP
EIP  dest
RET: return from subroutine
 Syntax:
RET
 Operation:
POP EIP
Call Site
 Caller is responsible for
 Pushing arguments on the stack from right to left
 Execute call instruction
 Pop arguments from stack after return
Example Function
 Source code
int sumOf(int x) {
int a;
a = x*x;
a = a + x;
return a;
}
Passing parameters on Stack
 Parameters are pushed onto the stack before the
CALL instruction

If the parameter’s size is less than a double word, it must be
converted to a double word before being pushed
 Parameters must be removed from the stack after the
CALL instruction
 Example:
C++ : n = sumOf(17);
Assembly:
push dword 17 ; push parameter
call sumOf
add esp,4
; remove parameter
A single parameter on stack
Callee
 Called function must do the following
 Save registers if necessary
 Allocate stack frame for local variables
 Execute function body
 Ensure result of non-void function is in EAX
 Restore any required registers if necessary
 Return to caller
Local variables on the stack
 The stack can be used as a convenient location for
local variables (subprogram data)


Data not stored on the stack is using memory from the
beginning of the program until the end of the program (C
calls these types of variables global or static)
Data stored on the stack only use memory when the
subprogram they are defined for is active
Problem
 Parameters and local variables can be access at any
place in the subprogram
 Problem: Using push and pop makes such access
very complex
 Solution: indirect addressing (e.g. [ESP+8], [ESP] )


it can be very error prone to use ESP when referencing data
Solution: x86 supplies another stack register for indirect
addressing : EBP
• But, the original value of EBP must be restored at the end of
the subprogram
General subprogram form
Example
void cal_sum( int n, int *sump )
{
int i , sum = 0;
for ( i=1; i <= n; i++ )
sum += i;
*sump = sum;
}