Problems parsing and printing integers with MIPS - formatting

Question

I'm doing a homework assignment where I need to calculate a parenthesized math problem using the runtime stack in MIPS and I've hit a bit of a snag:
I've gotten to the point where I'm trying to parse the integers out of the user supplied input. It worked really well when it only dealt with single digits, but when I got to two digit numbers it gave me problems (I was using Syscall 4 or the print string function). For example, I'd punch in 77 and it'd give me "H". So I switched the syscall to 1, the print integer command and now I get insanely large numbers. Is there anyway I can accomplish what I need to do?
My code so far. Ignore the add and subtract methods, they haven't been implemented yet. I feel that after I solve this problem those should be pretty easy to introduce.
.data
Welcome: .asciiz "\nCalculate a Fully Parenthesized Expression.\n"
promptExpr: .asciiz "Enter the expression: "
bufExpr: .space 200
.text
.globl main
main:
la $a0, Welcome
li $v0, 4
syscall
la $a0, promptExpr
li $v0, 4
syscall
li $v0, 8
la $a0, bufExpr
li $a1, 200
syscall
li $t0, 0
subu $sp, $sp, 4
sw $t0, ($sp)
li $t1, 0
Loop: lb $t0, bufExpr($t1)
beq $t0, 10, endProg
beq $t0, 45, negCheck
bgt $t0, 47, num
beq $t0, 41, calc
bne $t0, 32, push
addi $t1, $t1, 1
j Loop
endProg:
li $t1, 0
la $a0, ($sp)
li $v0, 1
syscall
li $v0, 10
syscall
num:
move $t2, $t0
addi $t1, $t1, 1
lb $t0, bufExpr($t1)
bgt $t0, 47, collect
subu $sp, $sp, 4
sw $t2, ($sp)
addu $t1, $t1, 1
j Loop
collect:
# collects the entire integer by multiplying the current amount by ten
# and adding the next digit.
li $t7, 10
mul $t2, $t2, $t7
addu $t2, $t2, $t0
addi $t1, $t1, 1
lb $t0, bufExpr($t1)
bgt $t0, 47, collect
subu $sp, $sp, 4
sw $t2, ($sp)
j Loop
push:
subu $sp, $sp, 4
sw $t0, ($sp)
addu $t1, $t1, 1
j Loop
negCheck:
calc:
lw $t4, ($sp)
addu $sp, $sp, 4
lw $t5, ($sp)
addu $sp, $sp, 4
move $t0, $t4
beq $t5, 40, push
lw $t6, ($sp)
addu $sp, $sp, 4
lw $t7, ($sp)
addu $sp, $sp, 4
beq $t5, 43, addMath
beq $t5, 45, subMath
addMath:
subMath:
Sorry if my code is kind of messy, MIPS gives me a headache.
Thank you in advance!

Answer 1

You need to subtract '0' (decimal 48) in your num/collect routine to convert the characters from your input string into values in the range 0..9.
Otherwise if you enter the string 12 at the prompt you'll get '1' * 10 + '2', i.e. 49 * 10 + 50 (= 540).