MIPS Help. Load Byte error. Address out of range - mips

Question

I am trying to add numbers that are in base 5.
Im supposed to prompt the user for 2 separate base 5 number in the form XXXXXX.XXXXX, and then add them.
My current problem is that I am getting an error when I am loading the last digit into a register, so I can start the calculation.
please help! Thanks
.data
str: .space 13 # XXXXXX.XXXXX hold 12 chars + 0x00
str2: .space 13
sum: .space 14
printstr: .space 13
printstr2: .space 13
printsum: .space 14
message1: .asciiz "Enter a base 5 number A: "
message2: .asciiz "Enter a base 5 number B: "
message3: .asciiz "Sum of A and B: "
newline: .asciiz "\n"
.text
main:
#get number A
li $v0, 4
la $a0, message1
syscall
li $v0, 8
li $a1, 13
la $a0, str
syscall
li $v0, 4
la $a0, newline
syscall
#get number B
li $v0, 4
la $a0, message2
syscall
li $v0, 8
li $a1, 13
la $a0, str2
syscall
li $v0, 4
la $a0, newline
syscall
#pointer to input string A
la $s1, str
addi $s1, $s1, 12
#pointer to input string A
la $s2, str2
addi $s2, $s2, 12
#pointer to sum
la $s3, sum
addi $s3, $s3, 13
#other variables
li $t0, 0 # inital carry is 0
li $t1, 1 # counter for looping through string
li $t2, 5 # base 5
loop:
lb $t3, ($s1) ############################# PROBLEM HERE######
lb $t4, ($s2)
beq $t3, '.', next
beq $t4, '.', next
subi $t3, $t3, 0x30
subi $t4, $t4, 0x30
add $t5, $t3, $t4
add $t5, $t5, $t0
div $t5, $t2
mflo $t0
mfhi $t5
addi $t5, $t5, 0x30
sb $t5, ($s3)
subi $s1, $s1, 1
subi $s2, $s2, 1
subi $s3, $s3, 1
addi $t1, $t1, 1
ble $t1, 13, loop
addi $t0, $t0, 0x30
sb $t0, ($s3)
next:
subi $s1, $s1, 1
subi $s2, $s2, 1
#subi $s3, $s3, 1
addi $t1, $t1, 1
j loop
PRINT: #print the sum
exit:
li $v0, 10
syscall

Answer 1

You set up $s1 to point past str and decrement it backwards. Your loop: termination is the ble $t1,13,loop. When that completes, $s1 is pointing to the start of the .data section (i.e. 0x10010000).
But, then, you "fall through" to next: [probably wrong] and decrement $s1 so it has the value 0x1000ffff. Then, you do j loop so the fetch is now coming from an address one byte below the start of the .data segment (i.e. memory that does not exist)
That's the source of the crash. You probably need a jump inst after the ble to continue on to some other code.
But, also, when you do beq $t3,'.',next you go to next: and decrement both pointers. Unless you restrict your input there is no guarantee that both $s1 and $s2 both point to '.' at the same time, so you only want to decrement one of them. (e.g. suppose you had str: 12.3 and str2: 123.41)
So, you may need some code to align the two numbers with respect to their decimal points beforehand.