5.118 · exercise

Predict: loops

Trace counting loops, a post-test loop, a loop inside a loop, and an unsigned test, and say what a register holds when each one ends.

0 of 5 correct

Predict the result

    mov w19, 0  // sum
    mov w20, 1  // i
    b test
body:
    add w19, w19, w20
    add w20, w20, 2
test:
    cmp w20, 9
    b.le body

What does w19 hold when the loop ends? Answer in decimal.

Predict the result

    mov w19, 0  // passes
    mov w20, 12 // i
again:
    add w19, w19, 1
    add w20, w20, 1
    cmp w20, 10
    b.lt again

This loop has no jump to its test before the first pass. How many passes does it make, as counted in w19? Answer in decimal.

Predict the result

// for (i = 0; i < 10; i += 4)
    mov w20, 0  // i
    b test
body:
    add w20, w20, 4
test:
    cmp w20, 10
    b.lt body

What does w20 hold when the loop ends? Answer in decimal.

Predict the result

    mov w19, 0  // count
    mov w20, 0  // row
row_top:
    mov w21, 0  // col
col_top:
    add w19, w19, 1
    add w21, w21, 1
    cmp w21, w20
    b.le col_top
    add w20, w20, 1
    cmp w20, 4
    b.lt row_top

How many times does the inner body run, as counted in w19? Answer in decimal.

Predict the result

    mov w19, 0  // passes
    mov w20, -2 // i
loop:
    add w19, w19, 1
    add w20, w20, 1
    cmp w20, 2
    b.lo loop

How many passes does the loop make, as counted in w19? Answer in decimal.