Question

I have this code:

.data

    array: .word 13, 11, 5, 9, 0, -3
    size: .word 6

.text

Main:
    la $a0, array
    lw $a1, size
    jal PrintIntArray
    j Exit

# $a0 - array, $a1 - size
PrintIntArray:
addi $sp, $sp, -12
li $t0, 0
sw $t0, 0($sp) # i
sw $a0, 4($sp) # array
sw $a1, 8($sp) # size

li $a0, '['
li $v0, 11
syscall

lw $t1, 8($sp) # size
ble $t1, $0, EmptyArray
    PrintLoop:
        lw $t1, 8($sp) # size
        lw $t0, 0($sp) # i
        bge $t0, $t1, PrintLoopEnd
            lw $t0, 0($sp) # i
            lw $t2, 4($sp) # array
            add $t2, $t2, $t0

            lw $a0, 0($t2) # <====== RUNTIME EXCEPTION AT THIS LINE !!!
            li $v0, 1
            syscall

            li $a0, ','
            li $v0, 11
            syscall

            lw $t0, 0($sp) # i
            add $t0, $t0, 1
            sw $t0, 0($sp)
            j PrintLoop
    PrintLoopEnd:
EmptyArray:

li $a0, ']'
li $v0, 11
syscall

jr $ra

Exit:

The line marked by me produces the following run-time exception:

Error in util.asm line 37: Runtime
exception at 0x00400060: fetch address
not aligned on word boundary
0x10010001

What did I do wrong? I suppose I made some mistake in loading/storing the address.

Answer 1

You need to multiply i by the size of the array element, then add it to the base address of the array in order to compute the address of the ith element. Note that, if the element size is 4 bytes, this multiplication can be performed easily by a left shift of two bits.