Converting C to mips assembly, Unaligned address error - c

Question

i'm doing an assignment now, it's almost done but i keep getting error saying
Unaligned address in inst/data fetch: 0x10010016
at line:
lw $t3,0($a1) # get the value of b[k] and save it into t3
I search online and i find an answer saying i have to use .align 2 to fix this, but that doesn't work for my problem.
can someone please give me a hint on this, I literally spend 6 hours on this..
thank you very much
Here is my code:
# a -> $a0
# b -> $a1
# n -> $a2
# j -> $a3
# k -> $s0
# i -> $t0
.data
.align 2
arra: .word 1,2,7,4,5
arrb: .word 3,4,7,2,9
.text
la $a0, arra # we have array a[] = { 1,2,7,4,5}
la $a1, arrb # we have array b[] = {3,4,7,2,9}
addi $a2,$zero,0 # n = 0
addi $a2,$zero,3 # n = 3
addi $a3,$zero,0 # j = 0
addi $a3,$zero,3 # j = 3
addi $s0,$zero,0 # k = 0
addi $s0,$zero,2 # k = 2
g:
addi $sp, $sp, -24
sw $ra, 20($sp) # save $ra on stack
sw $s0, 16($sp) # save $s0 (k) on stack
sw $a0, 12($sp) # save a0(a) on stack
sw $a1, 8($sp) # save a1(b) on stack
sw $a2, 4($sp) # save a2(n) on stack
sw $a3, 0($sp) # save a3(j) on stack
move $a3,$s0 # set j = k
jal f # f(a,b,n,k,k)
add $a1,$a1,$s0 # return the address of b[k]
lw $t3,0($a1) # get the value of b[k] and save it into t3
add $v0, $t3,$zero # return the value
lw $a3,0($sp)
lw $a2,4($sp)
lw $a1,8($sp)
lw $a0,12($sp)
lw $s0,16($sp)
lw $ra,20($sp)
addi $sp,$sp,24
jr $ra
f:
bne $a2, $zero, ELSE # if (n != 0) go to ELSE
addi $t0, $zero, 1 # set $t0 = 1
sw $t0, 0($a1) # then set b[0] = 1
addi $t0, $zero, 1 # set $t0 = 1
sw $t0, 0($a1) # then set b[0] = 1
addi $t0, $zero, 1 # set i = 1 for the loop
forLoop:
slt $t1,$s0, $t0 # if k < i, end the loop, use $t1 to store the boolean value
bne $t1, 1, forLoopDone
add $a1, $a1, $t0 # b[i] address
add $t2,$zero,$zero
sw $t2, 0($a1) # b[i] = 0
addi $t0, $t0, 1 # i = i + 1
j forLoop
ELSE:
bne $a3, $zero, updateJ # test if (j == 0), if not, j = j -1
j iteratef
updateJ:
addi $a3, $a3, -1 # j = j -1
j iteratef
iteratef:
addi $a2, $a2, -1 # iterate, n = n - 1
j f # f(b, a, n-1, j_update, k)
bne $a3, $zero, forLoop1Ini # if (j != 0), go to for_loop1_ini
lw $a0, 0($a0) # there might be sth wrong here
sw $a1, 0($a1) # set b[0] = a[0]
addi $a3, $a3, 1 # j++
forLoop1Ini:
addi $t0, $a3, 0 # set i = j
forLoop1Start:
slt $t1,$s0, $t0
bne $t1, 1, forLoop1Done
add $a0, $a0, $t0 # get a[i] address
lw $t1, 0($a0) # t1 = a[i]
lw $t2, -4($a0) # get b[i]
add $a1, $a1, $t0 # get b[i] address
add $t1, $t1, $t2 # t1 = a[i-1] + a[i]
sw $t1, 0($t1) # b[i] = a[i-1] + a[i]
addi $t0, $t0, 1 # i++
j forLoop1Start
forLoop1Done:
nop
forLoopDone:
nop
jr $ra

Answer 1

Your problem is that you are not taking into account the size of each element of the array. Each element occupies 4 bytes.
Therefore, instead of issuing
add $a1,$a1,$s0 # return the address of b[k]
you should multiply the index by 4 (which is the size of element) to get the effective offset:
sll $s1, $s0, 2 # Compute effective offset (i.e. multiply index by 4)
add $a1,$a1,$s1 # return the address of b[k]
lw $t3,0($a1) # get the value of b[k] and save it into t3