MIPS Palindrome Check - mips

Question

I am trying to write a MIPS program that checks if an input string is a palindrome. I tested the string "HelllleH" and when stepping through the program, I saw that during the first loop of PAL_CHECK t0 = 0 but t1 = 104. Logically, t0 = 0 and t1 = 0 also in the first loop. Can someone tell what's wrong in this program?
# a0 is input
# a1 is current character we are looking at
# a2 is length of string
# t0 is character at beginning of string
# t1 is character at end of string
# v0 stores whether string is palindrome or not (0 for false, 1 for true)
ispalindrome:
addi $a2, $0, 0 # length counter
FIND_LEN:
lbu $a1, 0($a0) # load character into $a1
beq $a1, $0, PAL_CHECK # Break if string is at the end
addi $a2, $a2, 1 # increment counter
addi $a0, $a0, 1 # increment str address
j FIND_LEN
PAL_CHECK:
# Is palindrome if length is less than 2
slti $t0, $a2, 2
bne $t0, $0, RETURN_TRUE
# Check first and last chars to see if they are the same
lbu $t0, 0($a0) # first char is t0
add $t1, $a2, $a0 # last char is t1
lbu $t1, 0($t1)
bne $t0, $t1, RETURN_FALSE # if they are not equal, return false
# continue recursion
addi $a2, $a2, -2
addi $a0, $a0, 1
j PAL_CHECK
RETURN_FALSE:
addi $v0, $0, 0
jr $ra
RETURN_TRUE:
addi $v0, $0, 1
jr $ra

Answer 1

While finding the length of the string you continually increment $a0 to point at the next character, until you find the NUL terminator at the end of the string. You never reset $a0 before the palindrome check loop, so it's still pointing at the NUL terminator when you begin that loop. So you'll actually be comparing data that's past your string.
It would make more sense to implement the check this way (I'm using C to illustrate the idea; I'll leave the MIPS translation to you):
a0 = the_string;
a1 = the_string + strlen(the_string) - 1;
while (a1 > a0) {
if (*a0 != *a1) return false;
a0++;
a1--;
}
return true;
By the way, a terminology nitpick: # continue recursion. Your implementation isn't recursive, it's iterative.