Memory Access with $sp and $fp

Memory
Data memory “grows” from highest to lowest.

MIPS has a stack pointer $sp that points to the last memory allocated (used).

To store local memory (in the memory stack)

(1) Copy stack pointer to frame pointer

move $fp, $sp

$fp is the frame pointer which we will use to reference all stack memory. That means all memory will be reference with reference to the frame pointer ($fp).

(2) Allocate memory for local variables

subu $sp, $sp, 12     # Assuming 3 ints (4-byte each)

(3) Referce variables by substracting from frame pointer

# a = -4($fp)
# b = -8($fp)
# c = -12($fp)

First variable is -4($fp) and second is -8($fp) ... -4-4n($fp) n = 0, 1, 2 ...

Here is a short program from CSE1303’s Slide

C Code

#include <stdio.h>
#include <stdlib.h>

/* A global variable. */
int g = 123;

int main()
{
    /* Three local variables. */
    int a = -5;
    int b;
    int c = 0x12345678;

    /* Do some arithmetic. */
    b = g + a;

    /* Do some more arithmetic. */
    printf("%d", c - a);

    exit(0);
}

The MIPS assembly

# MIPSMemory.asm
# Test the memory access

# a = -12($fp)
# b = -8($fp)
# c = -4($fp)

 .data
g: .word 123

 .text
main:
 # Copy $sp to $fp
 move $fp, $sp

 # Allocate 4 words (12 bytes)
 subu $sp, $sp, 12

 # int a = -5
 li $t0, -5
 sw $t0, -12($fp)

 # int c = 0x12345678;
 li $t0, 0x12345678
 sw $t0, -4($fp)

 # c - a
 lw $t0, -12($fp)  # $t0 = a
 lw $t1, -4($fp)   # $t1 = c
 sub $t0, $t1, $t0

 # display result
 move $a0, $t0
 li $v0, 1
 syscall

exit:
 li $v0, 10
 syscall

...