Would like to know the answer to excercise 2.37 in Computer Organization and Des

Question

Would like to know the answer to excercise 2.37 in Computer Organization and Design by Patterson and Hennesy.

The question is:

Write a program in MIPS assembly language to convert an ASCII number string containing positive and negative integer decimal strings. $a0 holds the address of a null-terminated string containing digits between 0 and 9. The program should compute the integer value equivalent to the string of digits and then place it in $v0. If a non-digit character appears anywhere in the string, the program should stop with a value of -1 in $v0. For example, if $a0 points to a sequence of 3 bytes (base 10) 50, 52, 0 (the null-terminated string is "24"), then when the program stops, $v0 should contain the value in base ten, 24.

Explanation / Answer

Positive And Negative Integer Decimal strings

MAIN: addi $sp, $sp, 4
sw $ra, ($sp)
add $t6, $0, 0x30 # ‘0’
add $t7, $0, 0x39 # ‘9’
add $s0, $0, $0
add $t0, $a0, $0

LOOP: lb $t1, ($t0)
slt $t2, $t1, $t6
bne $t2, $0, DONE
slt $t2, $t7, $t1
bne $t2, $0, DONE
sub $t1, $t1, $t6
beq $s0, $0, FIRST
mul $s0, $s0, 10

FIRST: add $s0, $s0, $t1
addi $t0, $t0, 1
j LOOP

DONE: add $v0, $s0, $0
lw $ra, ($sp)
addi $sp, $sp, 4

jr $ra

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