Question

Demonstrate your continuous progress in learning MIPS assembly by writing and executing the program below. Edit the program using your favorite code editor (e.g. Notepad++) and load the programs into QtSpim for execution.

Activity Directions:

Write a program that will read a line of text and determine whether or not the text is a palindrome. A palindrome is a word or sentence that spells exactly the same thing both forward and backward. For example, the string “anna” is a palindrome, while “ann” is not. The algorithm you will be using to determine whether or not a string is a palindrome is given below.

This algorithm is appropriate for strings that end with a newline followed by a 0 character, as strings read in by the read string syscall do. Note that in this algorithm, the operation of getting the character located at address X is written as *X.

1. Let A = S.
   
2. Let B = a pointer to the last character of S. To find the last character in S, use the following algorithm:
   1. Let B = S.
   2. Loop:
      1. If *B == 0 (i.e., the character at address B is 0), then B has gone past the end of the string. Set B = B − 2 (to move B back past the 0 and the newline), and continue with the next step.
      2. Otherwise, set B = (B + 1).
         
3. Loop:
   1. If A >= B, then the string is a palindrome. Halt.
   2. If *A <> 'B, then the string is not a palindrome. Halt.
   3. Set A = (A + 1).
   4. Set B = (B − 1).

Answer 1

.text
main:
la $a0, string_space
li $a1, 1024
li $v0, 8
syscall
la $t1, string_space
la $t2, string_space
length_loop:
lb $t3, ($t2)
beqz$t3, end_length_loop
addu$t2, $t2, 1
b length_loop
end_length_loop:
subu$t2, $t2, 2
test_loop:
bge $t1, $t2, is_palin
lb $t3, ($t1)
lb $t4, ($t2)
bne $t3, $t4, not_palin
addu$t1, $t1, 1
subu$t2, $t2, 1
b test_loop
is_palin:
la $a0, is_palin_msg
li $v0, 4
syscall
b exit
not_palin:
la $a0, not_palin_msg
li $v0, 4
syscall
b exit
exit:
li $v0, 10
syscall
.data
string_space: .space 1024
is_palin_msg: .asciiz "The string is a palindrome.\n"
not_palin_msg: .asciiz "The string is not a palindrome.\n"