MIPS Reverse a String

MIPS example to reverse a string.

First it will be better to show how MIPS stores strings.

e.g.
the phrase "computerblog"
will be saved (address start form down to up):

??	??	??	00(zero byte)	0x...b
g = 67	o = 6f	l = 6c	b = 62	0x...8
r = 72	e = 65	t = 74	u = 75	0x...4
p = 70	m = 6d	o = 6f	c = 63	0x...0

0x??????00 0x....b
0x676f6c62 0x....8
0x72657475 0x....4
0x706d6f63 0x....0

and reversed should be:
0x20736c00 0x....b
0x636f6d70 0x....8
0x75746572 0x....4
0x626c6f67 0x....0

when making syscall with 4 (li $v0,4) and $a = x00
will start printing bytes (characters) till the ending 0 (a zero byte)

Now I think it will be easier to understand the following MIPS code.
At this point it would be good to mention that it works only if the string length is even (2,4,6...2*k).
If you want it for odd length strings too you can solve it by changing the strreverse (after exiting loop if odd do another one loop) and please leave your comments.
Anw, this is the code (lines with comment *change means that it affects the table size or the strreverse function):

.data
.align 1
String: .space 14 #*change
msg1: .asciiz "Pls give a character: "
msg2: .asciiz "\n"
msg3: .asciiz "String is: "
msg4: .asciiz "\nString Reversed is: "
.text
.globl main
main:

addi $s0,$zero,13 #*change
addi $t0,$zero,0

in:
la $a0,msg2
li $v0,4
syscall

li $v0,4
la $a0,msg1
syscall
li $v0,12
syscall

add $t1,$v0,$zero
sb $t1,String($t0)
addi $t0,$t0,1
slt $t1,$s0,$t0
beq $t1,$zero,in

sb $zero,String($t0) #ending zero

la $a0,msg2
li $v0,4
syscall
la $a0,msg2
li $v0,4
syscall
la $a0,msg3
li $v0,4
syscall

la $a0,String
li $v0,4
syscall

addi $a1,$zero,14 #pass length-*change
jal stringreverse #reverse

la $a0,msg2
li $v0,4
syscall

la $a0,msg4
li $v0,4
syscall

la $a0,String
li $v0,4
syscall

li $v0,10
syscall


stringreverse:

add $t0,$a0,$zero #beginning address

add $t1,$zero,$zero  #i=0
addi $t2,$a1,-1   #j=length-1

loop:

add $t3,$t0,$t1
lb $t4,0($t3) #lb String[i]

add $t5,$t0,$t2
lb $t6,0($t5) #lb String[j]

sb $t4,0($t5) #String[j]=String[i]
sb $t6,0($t3) #String[i]=String[j]

addi $t1,$t1,1 #i++
addi $t2,$t2,-1 #j--
#if i>=j break - $t1<$t2
slt $t6,$t2,$t1
beqz $t6,loop

jr $ra

#i-0;j=length-1;
# do {
# x = str[i]
# str[i]=str[j]
# str[j] = x
# i++;j--;
# } while(!(j<i))

Example of output:

Pls give a character: c
Pls give a character: o
Pls give a character: m
Pls give a character: p
Pls give a character: u
Pls give a character: t
Pls give a character: e
Pls give a character: r
Pls give a character: s
Pls give a character:
Pls give a character: b
Pls give a character: l
Pls give a character: o
Pls give a character: g

String is : computers blog

String Reversed is : golb sretupmoc

You may also want to see my post about how to reverse a file in C / C++.

I hope you find this post helpful and please leave your comments. Thank you.