Run in Spim or Mars to reverse a string with assembler! (MIPS)

stringReverse.s

# Mips program to reverse a string
# mips.s

      .data
str:     .space 128 # character buffer


      .text
.globl main

main:                  # input the string
  li    $v0, 8         # service code
  la    $a0, str       # address of buffer
  li    $a1, 128       # buffer length
  syscall

  li    $t0, 0         # push a null
  subu  $sp, $sp, 4    # onto the stack
  sw    $t0, ($sp)     # to signal its bottom
  li    $t1, 0         # index of first char in str

pushl:                 # push each character on the stack
  lbu    $t0, str($t1) # get current char into a full word
  beqz  $t0, stend     # null byte: end of string

  subu  $sp, $sp, 4    # push the full word
  sw    $t0, ($sp)     # holding the char

  addu  $t1, $t1 1     # increment the index
  j      pushl         # loop

stend:                 # pop chars from stack back into the buffer
  li    $t1, 0         # index of first byte of str
popl:
  lw    $t0, ($sp)     # pop a char off the stack
  addu  $sp, $sp, 4
  beqz  $t0, done      # null means empty stack

  sb    $t0, str($t1)  # store at string[$t1]
  addu  $t1, $t1, 1    # increment the index
  j      popl
done:                  # print the reversed string
  li    $v0, 4         # service code
  la    $a1, str       # address of string
  syscall
  li    $v0, 10        # exit
  syscall


#end of program