MIPS: Selective copying from one string to another error - mips

Question

I have tried to write a program in MIPS that takes a string and prints a result with only characters (assuming that the string is alphanumerical). It works perfectly for strings of size < 4, but when a 4th character occurs, it goes into infinite loop.
.data
string: .word 10
.data
result: .word 10
.data
message1: .asciiz "number\n"
.data
message2: .asciiz "letter "
.data
message3: .asciiz "finished loop\n"
.text
main:
li $v0 8 # take input as string
la $a0 string # store it in "string"
la $a1 10 # size of "string" is at most 10
syscall
la $s0 string # save address of "string" to s0
la $s1 result # save address of "result" to s1
Loop:
li $t0 10 # set t0 as '/n'
lb $t1 ($s0) # load character that we are currently checking
beq $t0 $t1 Exit # check if we are at the end of the string
li $t0 64 # set t0 to 64 to check if it is a letter (character that we are now checking must be greater than 64)
slt $t2 $t0 $t1 # t2 will store the result of comparison ($t1 - character)
li $t0 0 # set t0 to 0 to check the result of comparison
beq $t2 $t0 Increment # if 64 > ch, then we must just proceed
li $v0 4
la $a0 message2 # print message that it is a character
syscall
sb $t1 ($s1) # copy this character into our "result"
addi $s1 $s1 1 # increment the address of "result"
Increment:
li $v0 4
la $a0 message1 # print message that it is a number
syscall
addi $s0 $s0 1 # increment the address of "string" to proceed in loop
j Loop
Exit:
li $t0 10
sb $t0 ($s1) # add endline character to "result"
addi $s1 $s1 1
li $t0 0
sb $t0 ($s1) # add null character to "result"
li $v0 4
la $a0 message3 # print message that the loop has finished
syscall
li $v0 4
la $a0 result # print result
syscall
jr $ra
Thanks in advance.

Answer 1

Hard to tell without stepping through myself, but here's an idea: use the power of "i", that is, Immediate mode. That way, you don't have to worry about whether a register value isn't quite what you expected. Also, use the special $zero register instead of loading a register with zero. So change this:
li $t0 64 # set t0 to 64 to check if it is a letter (character that we are now checking must be greater than 64)
slt $t2 $t0 $t1 # t2 will store the result of comparison ($t1 - character)
li $t0 0 # set t0 to 0 to check the result of comparison
beq $t2 $t0 Increment # if 64 > ch, then we must just proceed
to this:
slti $t2, $t1, 'A' # A little different: set $t2 if $t1 < 65 ('A')
bne $t2, $zero Increment # branch if $t1 < 'A'
Maybe by eliminating registers in favor of immediates, you can find the glitch that's causing you to go into a loop.
Also, a couple of other possible glitch sources:
You're looking for '\n' to terminate the string, but I'm not sure you can guarantee that the syscall will add \n if the input is a full 10 characters. Perhaps you should also/instead check for the zero-terminator byte value.
You've defined string and result as 10 words, but you're actually storing bytes in them. That shouldn't be a problem - you've allocated 40 bytes instead of 10. But if you ever go back and change those to byte, be sure to allow for that trailing \0 (and \n).
Sorry I can't be more specific, but hope that points you in the right direction.