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: pushdword 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; }