Loop implementation in assembly in ARM Architecture

Introduction

Loops let a program repeat actions multiple times without writing the same code again and again.

When you want to repeat a task like counting numbers.

When processing items in a list one by one.

When waiting for a condition to become true.

When performing repeated calculations.

When reading data until there is no more left.

Core Concept

ARM Architecture

MOV R0, #count  ; Set loop counter
loop_label:
  ; Your repeated code here
  SUBS R0, R0, #1  ; Decrease counter by 1
  BNE loop_label   ; If counter not zero, repeat loop

SUBS subtracts and updates flags to check if zero.

BNE means 'branch if not equal to zero', so it repeats the loop.

Key Points

This loop runs 5 times, decreasing R1 each time until it reaches zero.

ARM Architecture

MOV R1, #5
loop_start:
  ; Do something
  SUBS R1, R1, #1
  BNE loop_start

Loop repeats 10 times using R2 as the counter.

ARM Architecture

MOV R2, #10
count_loop:
  ; Process data
  SUBS R2, R2, #1
  BNE count_loop

Detailed Explanation

This program runs a loop 3 times. Each time it subtracts 1 from R0. When R0 reaches zero, it exits the loop and ends the program.

ARM Architecture

AREA LoopExample, CODE, READONLY
ENTRY

    MOV R0, #3          ; Set loop count to 3
loop:
    ; Here we could do some work, for example, toggle an LED (not shown)
    SUBS R0, R0, #1     ; Decrement counter and update flags
    BNE loop            ; If not zero, repeat loop

    MOV R7, #1          ; Exit syscall number
    MOV R0, #0          ; Exit code 0
    SVC 0               ; Make syscall to exit
END

Important Notes

Loops in assembly use labels and branch instructions to repeat code.

Always update the loop counter and check it to avoid infinite loops.

ARM assembly uses condition flags set by instructions like SUBS to decide branching.

Summary

Loops repeat code by using a counter and branch instructions.

Use SUBS to decrease the counter and update flags for branching.

BNE branches back if the counter is not zero, continuing the loop.