MIPS Code that reads number of lower case letters

Question

I need to write a program that reads a string from the user and outputs the number of lowercase letters in the string. This is what I wrote

.data
msg1 : .word 0:24
.text
.globl main

main :
addu $s0 , $0 , $ra         #save the return address
li $v0 , 8          #syscall for read str
la $a0 , msg1           #load address of msg1 to store string
li $a1 , 100            #msg1 is 100 bytes
syscall

add $t6, $t6, $0

compare :
lb $t0 , 0($a0)         #load the character into $t0
beq $t0, $0, endloop
li $t1 , 'a'            #get value of 'a'
blt $t0 , $t1 , nomodify    #do nothing if letter is less than 'a'
li $t1 , 'z'            #get value of 'z'
bgt $t0 , $t1 , nomodify    #do nothing if letter is great than 'z'
addi $t6, $t6, 1        #add one to the character count
addi $a0, $a0, 1        #move to next character
beq $0,$0,compare       #branch to compare 

nomodify :
addi $a0, $a0, 1        #next character
j compare

endloop :
addu $a0, $0, $t6       
li $v0 , 1          #syscall for print int
syscall
addu $ra , $s0 , $0     #restore return address
jr $ra  

However, when it runs, it terminates with errors and I'm not entirely sure what it is that I'm doing wrong. Any suggestions/advice is much appreciated! Thanks in advance.

Answer 1

There's an exit syscall available in MARS/SPIM to exit your application. So instead of ending your program with this:

addu $ra , $s0 , $0     #restore return address
jr $ra 

you should use:

li $v0, 10    
syscall    # syscall 10 = exit

jr $ra just jumps back to whatever code was calling your main routine. In MARS that appears to be "nowhere", i.e. when your main starts running $ra is 0.
In SPIM there's some setup code that calls your main, and then performs a syscall 10 if and when you return. So in SPIM your code would work as-is.