Question

for (i = 0; i < 64; i++) {
A[i] = B[i] + C[i];
}

The MIPS instructions for the above C code is:

add $t4, $zero, $zero   # I1 i is initialized to 0, $t4 = 0
Loop: 
add $t5, $t4, $t1       # I2 temp reg $t5 = address of b[i]
lw $t6, 0($t5)          # I3 temp reg $t6 = b[i]
add $t5, $t4, $t2       # I4 temp reg $t5 = address of c[i]
lw $t7, 0($t5)          # I5 temp reg $t7 = c[i]
add $t6, $t6, $t7       # I6 temp reg $t6 = b[i] + c[i]
add $t5, $t4, $t0       # I7 temp reg $t5 = address of a[i]
sw $t6, 0($t5)          # I8 a[i] = b[i] + c[i]
addi $t4, $t4, 4         # I9 i = i + 1
slti $t5, $t4, 256       # I10 $t5 = 1 if $t4 < 256, i.e. i < 64
bne $t5, $zero, Loop    # I11 go to Loop if $t4 < 256

For I8, could the sw instruction not be replaced with an addi instruction? i.e addi $t5, $t6, 0

Wouldn’t it achieve the same task of copying the address of $t6 into $t5? I would like to know the difference and when to use either of them. Same could be said about the lw instruction.

Also, maybe a related question, how does MIPS handle pointers?

edit: changed addi $t6, $t5, 0.

Answer 1

For I8, could the sw instruction not be replaced with an addi instruction? i.e addi $t6, $t5, 0

No. The sw instruction stores the result to memory. The add just manipulates registers. And lw gets a word from memory. It’s the only MIPS instruction that does so. (Other processors might and do have versions of add that access memory, but not MIPS.)

It’s necessary to adjust your thinking when working in assembly language. Registers and memory are separate. In higher level languages, registers are (nearly) completely hidden. In assembly, registers are a separate resource that the programmer must manage. And they’re a scarce resource. A HLL compiler would do this for you, but by programming in assembly, you have taken the job for yourself.

how does MIPS handle pointers?

In MIPS, pointers are just integers (in registers or memory) that happen to be memory addresses. The only way they’re distinguished from data values is by your brain. The “pointer” is something invented by higher level language designers to relieve you the programmer of this burden. If you look closely, you’ll see that $t5 actually holds a pointer. It’s a memory address used by lw and sw as the address to load from or store to.