Loops

Loops in Assembly

ARM64 has no dedicated loop instruction. Loops are built from the same branches and comparisons you already know -- but combined in specific patterns.

"All hands, brace for impact!" -- as the crew of the Enterprise-D learned in "Cause and Effect," getting caught in a temporal causality loop means repeating the same sequence until you find the exit condition. Loops in assembly work the same way.

While Loop Pattern

The standard while loop tests the condition first, then executes the body:

// while (i < N):
loop:
    CMP X0, X1
    B.GE done        // if i >= N, exit
    // ... loop body ...
    ADD X0, X0, #1   // i++
    B loop           // repeat
done:

This is the most common loop structure. The condition check at the top ensures the body is skipped entirely if the condition is false initially.

Do-While Loop Pattern

A do-while loop executes the body first, then tests. This is useful when you know the loop runs at least once:

// do { ... } while (counter > 0):
loop:
    // ... loop body (runs at least once) ...
    SUB X0, X0, #1   // counter--
    CBNZ X0, loop    // repeat if counter != 0

CBNZ (Compare and Branch if Not Zero) is perfect here -- it combines the test and branch into one instruction.

For Loop Pattern

for (i = 1; i <= N; i++):

MOV X0, #1           // i = 1
MOV X1, #0           // sum = 0
MOV X2, #10          // N = 10
loop:
    CMP X0, X2
    B.GT done        // if i > N, exit
    ADD X1, X1, X0   // sum += i
    ADD X0, X0, #1   // i++
    B loop
done:
// X1 now contains the sum (55)

Countdown Loops

Counting down is sometimes more efficient because CBNZ saves a separate CMP:

MOV X0, #5           // counter = 5
loop:
    // ... loop body ...
    SUB X0, X0, #1   // counter--
    CBNZ X0, loop    // repeat if counter != 0

Loop Building Blocks

Every loop is made from these pieces:

Tip: When translating a high-level loop, identify these five parts first, then map each to assembly.

Your Task

Write a program that computes the sum of integers from 1 to 10 using a loop. The answer is 55. Print it followed by a newline.