Moving values around

This program places a rectangle's width and height in $t0 and $t1, then calculates its perimeter in another register. Select Build, then use Step to watch the registers change one instruction at a time.

.text
main:
    li   $t0, 30          # width
    li   $t1, 12          # height
    add  $t2, $t0, $t1   # width + height
    add  $t2, $t2, $t2   # perimeter = 2 * (width + height)

    move $t3, $t2         # copy the perimeter
    add  $t4, $t2, $zero  # copy it again with addition
    sub  $t5, $t0, $t1   # width - height

    li   $v0, 10          # exit
    syscall

Arithmetic instructions use the destination-first shape instruction destination, source, source. The first add reads $t0 and $t1 but writes only $t2, so the original width and height remain available. The second add doubles their sum.

move $t3, $t2 copies $t2 into $t3. add $t4, $t2, $zero also copies the value, because $zero always reads as 0. Both leave $t2 alone. move is a pseudo-instruction: the assembler translates it into an instruction the processor can execute. Here you can read it simply as a copy.

Subtraction makes source order visible. sub $t5, $t0, $t1 means $t5 = $t0 - $t1, so swapping the last two operands would produce 12 - 30 = -18 instead.

Once the program stops after the exit syscall, the registers panel shows hexadecimal values by default. The decimal column gives the corresponding numbers used in the calculation:

Now compute the same two results for inputs supplied by the test: leave twice the sum of $t0 and $t1 in $t2, and $t0 - $t1 in $t3. Keep $t0 and $t1 unchanged. For $t0 = 7 and $t1 = 4, what values will the three arithmetic instructions leave in $t2 and $t3? Write them in the exercise editor, then select Test to check the final values of $t0, $t1, $t2, and $t3.

After your program passes, you can try another pair. Select Open in editor on the exercise, then Testcases. In New Testcase, add the new $t0 and $t1 values under Starting registers values. Under Expected registers values, add the values you expect for $t0, $t1, $t2, $t3, and $v0 (10 for the exit service). Use Add for each register, select Add Testcase, then select Test again.

.text
main:
    # $t0 and $t1 are supplied by the test.
    # Compute the perimeter in $t2 and $t0 - $t1 in $t3.

    li   $v0, 10
    syscall

.text
main:
    add  $t2, $t0, $t1
    add  $t2, $t2, $t2
    sub  $t3, $t0, $t1

    li   $v0, 10
    syscall