## palindrome.asm reads a line of text and tests if it is a palindrome.
## Register usage:
## $t1 A.
## $t2 B.
## $t3 the character at address A.
## $t4 the character at address B.
## $v0 syscall parameter / return values.
## $a0 syscall parameters.
## $a1 syscall parameters.
.text
.globl __start
__start:
## read the string S:
la $a0, string_space
li $a1, 1024
li $v0, 8 # load "read_string" code into $v0.
syscall
la $t1, string_space # A = S.
la $t2, string_space ## we need to move B to the end
length_loop: # of the string:
lb $t3, ($t2) # load the byte at B into $t3.
beqz $t3, end_length_loop # if $t3 == 0, branch out of loop.
addu $t2, $t2, 1 # otherwise, increment B,
b length_loop # and repeat
end_length_loop:
subu $t2, $t2, 2 ## subtract 2 to move B back past
# the '\0' and '\n'.
test_loop:
bge $t1, $t2, is_palin # if A >= B, it's a palindrome.
lb $t3, ($t1) # load the byte at address A into $t3,
lb $t4, ($t2) # load the byte at address B into $t4.
bne $t3, $t4, not_palin # if $t3 != $t4, not a palindrome.
# Otherwise,
addiu $t1, $t1, 1 # increment A,
addiu $t2, $t2, -1 # decrement B,
b test_loop # and repeat the loop.
is_palin:
la $a0, yes
b continue
not_palin:
la $a0, no
continue:
li $v0, 4
syscall
li $v0, 10
syscall
.data
string_space: .space 1024 # set aside 1024 bytes for the string.
yes: .asciiz "Your string is a pallindrome"
no: .asciiz "Your string is not a pallindrome"