Need help implementing an array in MIPS

Question

I am trying to understand arrays in MIPS. I know that you need to move the stack pointer either forward one word or back (increments of 4). But I don't know how to implement that idea into MIPS code.

 j = 0
 x = a[j+k]
 a[j+k] = a[j]

What I know for certain is to load 0 into a register (j=0), then I think you load word the base address $a0 and make a[j], then you add k to it before sw that value into 0($a0).

li $t0, 0       
lw $t0 0($a0)
add $t0, $t0, $a2
sw $t0, ($a0)

We were assigned to convert this code to MIPS, and the previous was the first part of it.

# Register usage
# $a0 = base address of array
# $a1 = n, size of array
# $a2 = k, the shift amount

# j = 0
# x = a[j+k]
# a[j+k] = a[j]
# repeat n-1 times
#   j = (j+k) mod n
#   m = (j+k) mod n
#   y = a[m]
#   a[m] = x
#   x = y
# end repeat

My question is how do I do the n-1 times loop in MIPS? is it a condition then just

subi $a1 $a1

And finally how to find the mod? I believe it is with div and then the $HI. This is what I have so far.

add $t1, $t0, $a2    #j+k
div $t1, $a1         #divide (j+k) by n
mfhi $t2             #move rem into t2
move $t2, $t0        #j = (j+k) mod n

add $t1, $t0, $a2    #j+k
div $t1, $a1         #divide (j+k) by n
mfhi $t2             #move rem into t2
move $t2, $t0        #m = (j+k) mod n

sw $t0, 0($t0)
lw $t3, 0($t0)

I'm confusing myself I believe. Clarification would be appreciated.

Wisedom Wisedom · Accepted Answer · 2012-04-07T01:40:08

li $t3, 0       #j = 0
lw $t3, 0($a0)     #load the value of @a0 in to j
add $t3, $t3, $a2  # j = j+k
sw $t3, 4($a0)     # store the new value of j in to $a0

loop:
beq $a1, $zero, return
sub $a1, $a1, 1      #n = n-1
add $t4, $t3, $a2    #j+k
div $t4, $a1         #divide (j+k) by n
mfhi $t5        

move $t5, $t3        #j = (j+k) mod n


 add $t4, $t3, $a2    #j+k
 div $t4, $a1         #divide (j+k) by n
 mfhi $t5             #move rem into t2
 move $t5, $t3        #m = (j+k) mod n
 b loop

return:
sw $t3, 4($a0)
jr $ra
lw $t3, 0($a0)

I saw your code here and after some tinkering. I had the program running but the outputs were wrong and i saw an error i made with the condition and upon fixing it the program no longer returns any out put. I changed the registers.

Need help implementing an array in MIPS

1 Answers