MIPS Assembly Language Programming

36 CHAPTER 2. MIPS TUTORIAL
Note that the arithmetic done on the pointer B is done using unsigned arithmetic
(using addu and subu). Since there is no way to know where in memory a pointer
will point, the numerical value of the pointer may well be a “negative” number if it
is treated as a signed binary number .
When this step is finished, A points to the first character of the string and B points
to the last. The next step determines whether or not the string is a palindrome:
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,
addu $t1, $t1, 1 # increment A,
subu $t2, $t2, 1 # decrement B,
b test_loop # and repeat the loop.
The complete code for this program is listed in section 5.4 (on page 74).
2.9 The atoi Program
The next program that we’ll write will read a line of text from the terminal, interpret
it as an integer, and then print it out. In effect, we’ll be reimplementing the read_int
system call (which is similar to the GetInteger function in the Roberts libraries).
2.9.1 atoi-1
We already know how to read a string, and how to print out a number, so all we need
is an algorithm to convert a string into a number. We’ll start with the algorithm
given in 2.3 (on page 37).
Let’s assume that we can use register $t0 as S, register $t2 as D, and register
$t1 is available as scratch space. The code for this algorithm then is simply:
li $t2, 0 # Initialize sum = 0.
sum_loop:
lb $t1, ($t0) # load the byte *S into $t1,
addu $t0, $t0, 1 # and increment S.