MIPS - how to write a for loop while also accessing and saving arrays - mips

Question

I won't lie, I'm a total beginner at programming in general with my first time being last year when I started programming in Computer Programming I. A lot of this is still over my head so uh... go easy on me will ya?
The question poised is as follows:
"1. (5 pts) Translate the following C code to MIPS. Assume that the variables i and j are assigned to registers $s0 and $s1, respectively. Assume that the base address of the arrays A and B are in registers $s6 and $s7, respectively. Assume that the elements of the arrays A and B are 4-byte words:
for (i = 0; i< j; i++)
B[i] = A[i+1] - A[i];
For my program I wrote:
.data
arrayA: .word 5,8, 12, 13, 28
sizeA: .word 5
arrayB: .space 4
i: .word 0
_j: .word 5
.text
# for (i = 0; i< j; i++)
# B[i] = A[i+1] - A[i];
main:
la $s6, arrayA #loads the base address of arrayA into register s6
la $s7, arrayB #loads the base address of arrayB into register s7
lw $s0, i #Loads zero into register s0 for i
lw $s1, _j #loads the value of 4 into register s2 for j
Loop:
beq $s0, $s1, Exit #Does the operation i < j in the for-loop
addi $s0, $s0, 1 #Does the operation i++ in the for-loop
mul $t0, $s0, 4 #Get address of i
add $t1, $t0, $s6 #Finds and stores the base address of a[i]
add $t2, $t0, $s7 #Finds and stores the base address of b[i]
lw $t3, ($s6) #get the value of a[i]
lw $t4, 4($10) #get the value of a[i + 1]
sub $t5, $t4, $t3 #subtracts a[i + 1] and a[i]
sw $t5, ($t2) #Stores the above into b[i]
add $a0, $zero, $t5
li $v0, 1
syscall
J Loop #loops back to beginning
Exit:
li $v0 10
syscall #syscall to exit the program
When I run it, it spits out 33333, which means it isn't incrementing. How do I fix this?

Answer 1

arrayA: .word 5,8, 12, 13, 28
may be wrong. You didn't add a space between 5, and 8 while giving spaces between the other elements. My QtSpim Version 9.1.21 didn't accept that.
arrayB: .space 4
is wrong. You are allocating only 4 bytes while space for 4 words (16 bytes) is required.
_j: .word 5
is wrong. The stored value is 5 despite of the comment saying "loads the value of 4 into register s2 for j". Using value 5 is wrong also because the array a has only 5 elements.
addi $s0, $s0, 1 #Does the operation i++ in the for-loop
is in the wrong place. The increment should be after the loop body.
lw $t3, ($s6) #get the value of a[i]
lw $t4, 4($10) #get the value of a[i + 1]
is wrong. The address of a[i] is stored in $t1, so you should use that.
J Loop #loops back to beginning
may be wrong. You used an uppercase letter for instruction name only here. At least my QtSpim Version 9.1.21 didn't accept that.
li $v0 10
may be wrong. You didn't use a comma for separating the operands while you used that in previous use of li: li $v0, 1. I'm surprised by seeing my QtSpim Version 9.1.21 accepting this.
Fixed code:
.data
arrayA: .word 5, 8, 12, 13, 28
sizeA: .word 5
arrayB: .space 4 * 4
i: .word 0
_j: .word 4
.text
# for (i = 0; i< j; i++)
# B[i] = A[i+1] - A[i];
main:
la $s6, arrayA #loads the base address of arrayA into register s6
la $s7, arrayB #loads the base address of arrayB into register s7
lw $s0, i #Loads zero into register s0 for i
lw $s1, _j #loads the value of 4 into register s2 for j
Loop:
beq $s0, $s1, Exit #Does the operation i < j in the for-loop
mul $t0, $s0, 4 #Get address of i
add $t1, $t0, $s6 #Finds and stores the base address of a[i]
add $t2, $t0, $s7 #Finds and stores the base address of b[i]
lw $t3, ($t1) #get the value of a[i]
lw $t4, 4($t1) #get the value of a[i + 1]
sub $t5, $t4, $t3 #subtracts a[i + 1] and a[i]
sw $t5, ($t2) #Stores the above into b[i]
add $a0, $zero, $t5
li $v0, 1
syscall
addi $s0, $s0, 1 #Does the operation i++ in the for-loop
j Loop #loops back to beginning
Exit:
li $v0, 10
syscall #syscall to exit the program

Answer 2

Don't mix friendly register names with the raw register names — very confusing.
lw $t3, ($s6) #get the value of a[i]
lw $t4, 4($10) #get the value of a[i + 1]
Here you're mixing $t3, $t4, $s6, and $10, which is actually $t2.
Your assembly code is doing the following C code (modulo the above bug):
int *s6 = A;
int *s7 = B;
for ( int i = 0; i != j; ) {
i++;
int *t1 = s6 + i; // i is automatically scaled by C
int *t2 = s7 + i; // ditto
int t3 = *s6; // A[0]
int t4 = s6[1]; // A[1]
int t5 = t4 - t3;
*t2 = t5; // B[i]=...
}
Can you see why it keeps loading A[0] & A[1]?
You should have noticed these problems during single stepping debugging.  After each step, check that every effect you're expecting happens as you expect.  When it doesn't, look for typos or logic problems.