#
# simple example demonstrating a procedure call
# (adapted from example by former colleague M. Eggen)
#
# this program is an approximate translation of the following C code:
# 
# int main(void) {
#         int a, b, c, x;
#         a = 5; b = 6; c = 7;
#         x = addproc(a, b, c);
#         return 0;
# }
# int addproc(int a, int b, int c) {
#         return a + b + c;
# }
        .globl  main
main:
# opening linkage (save return address)
        addi    $sp, $sp, -4    
        sw      $ra, 0($sp)
#
# register usage:
#       a       $s0
#       b       $s1
#       c       $s2
#       x       $s3
#
# initialize variables
        addi    $s0, $zero, 5
        addi    $s1, $zero, 6
        addi    $s2, $zero, 7
# set up parameters for call
        add     $a0, $zero, $s0
        add     $a1, $zero, $s1
        add     $a2, $zero, $s2
# call addproc
        jal     addproc                 
# use returned value
        add     $s3, $v0, $zero         
# set return value of 0 from main
        add     $v0, $zero, $zero
# closing linkage (get return address and restore stack pointer)
        lw      $ra, 0($sp)     
        addi    $sp, $sp, 4
        jr      $ra
#
# addproc procedure
#
addproc:
# opening linkage (save return address and registers)
# we save all registers even though we aren't using them -- "good practice"
        addi    $sp, $sp, -4
        sw      $ra, 0($sp)
# here we could continue pushing things onto the stack by repeatedly
# decrementing the stack pointer, but it's quicker to do one decrement ....
        addi    $sp, $sp, -32
        sw      $s0, 0($sp)
        sw      $s1, 4($sp)
        sw      $s2, 8($sp)
        sw      $s3, 12($sp)
        sw      $s4, 16($sp)
        sw      $s5, 20($sp)
        sw      $s6, 24($sp)
        sw      $s7, 28($sp)
# do computation and set return value
        add     $v0, $a0, $a1
        add     $v0, $v0, $a2           
# closing linkage 
# "unwind the stack"
# (restore registers, get return address, restore stack pointer)
        lw      $s7, 28($sp)
        lw      $s6, 24($sp)
        lw      $s5, 20($sp)
        lw      $s4, 16($sp)
        lw      $s3, 12($sp)
        lw      $s2, 8($sp)
        lw      $s1, 4($sp)
        lw      $s0, 0($sp)
        addi    $sp, $sp, 32
        lw      $ra, 0($sp)
        addi    $sp, $sp, 4
# return to caller
        jr      $ra
        .end    main