Example

The stack is often used to hold temporary values when most registers are already in use. An example of this is how a compiler translates an arithmetic expression into machine codes that uses a stack. Say that the arithmetic expression is ab - 12a + 18b - 7. Say that only $t0 and $t1 are available.

Before SPIM starts running a program it initializes the stack pointer $sp appropriately. On a computer with a full operating system, the stack pointer is initialized by the operating system before control is passed to a user program.

Here is the start of the program:

# Evaluate the expression ab - 12a + 18b - 7

main:   
        lw      $t0,a          # get a
        lw      $t1,bb         # get b
        mul     $t0,$t0,$t1    # a*b
        
        subu    $sp,$sp,  # push a*b onto stack
        
        sw      $t0,

        . . . . .

        .data
a:      2
bb:     3