Arrays and lw/sw

Consider the following C code segment:

        /* Initialize every other element in array A to 0 */
        /* (assume numVals is number of values in A) */
        int i;
        for ( i = 0; i < numVals; i += 2 )
            A[i] = 0;

MIPS translation using array indexing:

        move $t0, $zero

LOOP:   slt $t1, $t0, $a1
        beq $t1, $zero, ENDLP

        sll $t2, $t0, 2
        add $t3, $a0, $t2
        sw  $zero, 0($t3)

        addi $t0, $t0, 2
        j LOOP
ENDLP:  ...

# $t0 = i = 0

# $t1 = 1 if i < numVals
# leave loop if i >= numVals

# $t2 = 4 * i
# t3 = A + 4i, i.e., &A[i]
# store 0 out to A[i]

# i += 2
# jump to top of loop

    Step 1: i=0, t2=0, t3=1000, store 32 0's to A[0]
    Step 2: i=2, t2=8, t3=1008, store 32 0's to A[2]
    -----------
    Step 3: i=4, t2=16, t3=1016, store 32 0's to A[4]
    Step 4: i=6, t2=24, t3=1024, store 32 0's to A[6]
    Step 5: i=8, t2=32, t3=1032, store 32 0's to A[8]

What's the deal with the lw/sw offset?

The instruction

    sw $zero, 0($t3) 

means: "store the contents of $zero in memory at address $t3 + 0". Why did we need to calculate $t3 = A + $t2? Why couldn't we just use A as the base address and $t2 as the offset?

    sw $zero, $t2($a0)

Syntax: the offset has to be a constant. When stepping through an array, we don't have a constant offset, so we have to calculate the exact address and then use a 0 offset.

Is the lw/sw offset ever useful?

Yes, but not when stepping through arrays.

Stack example

Assume that $sp is 2048, $ra holds 32772, $a0 holds 1026, and $a1 holds 85.
PROC: addi $sp, $sp, -12 sw $ra, 0($sp) sw $a0, 4($sp) sw $a1, 8($sp) ... lw $a1, 8($sp) lw $a0, 4($sp) lw $ra, 0($sp) addi $sp, $sp, 12 jr $ra	# adjust $sp to 2036 for 3 values # save $ra to 2036 # save $a0 to 2040 # save $a1 to 2044 # ... procedure contents here ... # restore $a1 from 2044 # restore $a0 from 2040 # restore $ra from 2036 # adjust $sp back to 2048 # jump back to calling routine	Addresses Memory: Addressable Bytes 2020  -------- -------- -------- -------- 2024  -------- -------- -------- -------- 2028  -------- -------- -------- -------- 2032  -------- -------- -------- -------- 2036  00000000 00000001 00000000 00000100 2040  00000000 00000000 00000100 00000010 2044  00000000 00000000 00000000 01010101 2048  -------- -------- -------- -------- 2052  -------- -------- -------- -------- 2056  -------- -------- -------- --------