# InvertText.s
#
# Author: Andrea Micheloni (http://www.tankmiche.com)
# License: Creative Commons Attribute-NonCommercial-ShareAlike 3.0
# Tested on: xspim (spim v7.3)
# Description: Waits for an input text (32 chars max), then outputs the same text reversed
# Comments are in pseudo-code
#

.globl main

.data

  fub: .space 32
  fin: .space 4
  buf: .space 32
  intro: .asciiz "\nWelcome! Type something (max 32 chars), and I will output it in reverse: \n\n"
  out: .asciiz "\nAnd the reversed string is: \n"

.text
  
  main:

    li $v0, 4				# mode = print_string
    la $a0, intro			# str = intro
    syscall				# >> output

    li $v0, 8				# mode = read_string
    la $a0, buf				# buffer = buf
    li $a1, 33				# length = 32
    syscall				# input >> buf

    lui $s0, 0x1001			# s0 = 0x10010000
    lui $s1, 0x1001			# s1 = 0x10010000
    addi $s1, $s1, 0x0020		# s1 += 0x0020
    li $t0, 32				# t0 = 32

  loop:
  incr:

    add $s3, $s1, $t0			# s3 = s1+t0
    addi $t0, -4			# t0--

    lw $t1, 0($s3)			# t1 = mem[ s1 + 0 ]

    beq $t1, $0, incr			# if ( t1 == 0 ) -> incr

#   The following instructions reverse the loaded word
#   stored in $t1, it changes from 0xABCDEFGH to 0xGHEFCDAB

    andi $t2, $t1, 0x00ff               # t2 = 0x000000yy
    sll $t2, $t2, 24			# t2 = 0xyy000000
    andi $t3, $t1, 0xff00               # t3 = 0x0000yy00
    sll $t3, $t3, 8			# t3 = 0x00yy0000   
    srl $t1, $t1, 16			# t1 = 0x0000yyyy
    andi $t4, $t1, 0x00ff               # t4 = 0x000000yy
    sll $t4, $t4, 8			# t4 = 0x0000yy00
    andi $t5, $t1, 0xff00               # t5 = 0x0000yy00
    srl $t5, $t5, 8			# t5 = 0x000000yy

    or $t5, $t5, $t4			# t5 = t5 OR t4
    or $t3, $t3, $t2			# t3 = t3 OR t2
    or $t1, $t3, $t5			# t1 = t5 OR t3

    sw $t1, 0($s0)			# mem[ s0 + 0 ] = t1
    addi $s0, $s0, 4			# s0 += 4

    bgtz $t0, loop			# if( t0 > 0 ) -> loop

    li $v0, 4				# mode = print_string
    la $a0, out				# str = out
    syscall				# >> output

#
#   Ok, we have the text in memory (buf). But what if it starts with the null char?
#   it would print nothing at all, thinking the string is finished!
#   so we have to find where the first NULL is, and print from that location on.
#

    lui $t2, 0x00FF			# t2 = 0x00FF0000
    li $t0, 3				# t0 = 3
    lui $s0, 0x1001			# s0 = 0x10010000
    lw $t1, 0($s0)			# t1 = mem[ s0 + 0 ]

    and $t3, $t2, $t1			# t3 = t2 AND t1
    beq $t3, $0, end			# if( t3 == 0 ) -> end

    addi $t0, $t0, -1			# t0--
    srl $t2, $t2, 8			# t2 = 0x0000FF00
    and $t3, $t2, $t1			# t3 = t2 AND t1
    beq $t3, $0, end			# if( t3 == 0 ) -> end

    addi $t0, $t0, -1			# t0--
    srl $t2, $t2, 8			# t2 = 0x000000FF
    and $t3, $t2, $t1			# t3 = t2 AND t1
    beq $t3, $0, end			# if( t3 == 0 ) -> end

    addi $t0, $t0, -1

  end:

    add $a0, $s0, $t0			# str = mem[ s0 + t0 ]
    syscall				# >> output

    li $v0, 10				# mode = exit
    syscall				# >> quit