MIPS Assembly Language Programming

34 CHAPTER 2. MIPS TUTORIAL
Algorithm 2.2 To determine if the string that starts at address S is a palindrome.
This algorithm is appropriate for the strings that end with a newline followed by a
0 character, as strings read in by the read string syscall do. (See exercise 2.10.1 to
generalize this algorithm.)
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:
(a) Let B = S.
(b) Loop:
3. Loop:
• 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.
• Otherwise, set B = (B + 1).
(a) If A ≥ B, then the string is a palindrome. Halt.
(b) If ∗A �= ∗B, then the string is not a palindrome. Halt.
(c) Set A = (A + 1).
(d) Set B = (B − 1).