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.

Practice

1. What is the main purpose of a loop in ARM assembly language?

easy

A. To store data in memory

B. To jump to a different program section once

C. To perform arithmetic operations

D. To repeat a set of instructions multiple times

Solution

1. Step 1: Understand the concept of loops

   Loops are used to repeat instructions multiple times to avoid writing the same code repeatedly.

2. Step 2: Identify the purpose in ARM assembly

   In ARM assembly, loops use registers and branch instructions to repeat code sections.

3. Final Answer:

   To repeat a set of instructions multiple times -> Option D

4. Quick Check:

   Loops repeat instructions [OK]

Hint: Loops repeat code blocks until a condition ends [OK]

Common Mistakes:

• Confusing loops with data storage
• Thinking loops perform only arithmetic
• Assuming loops run only once

2. Which ARM assembly instruction is used to subtract 1 from a register and update condition flags for looping?

easy

A. SUBS

B. MOV

C. ADD

D. BNE

Solution

1. Step 1: Identify the instruction that subtracts and sets flags

   SUBS subtracts a value and updates condition flags used for branching decisions.

2. Step 2: Differentiate from other instructions

   ADD adds values, MOV moves data, BNE branches if not equal; only SUBS subtracts and sets flags.

3. Final Answer:

   SUBS -> Option A

4. Quick Check:

   SUBS subtracts and sets flags [OK]

Hint: SUBS subtracts and updates flags for loop control [OK]

Common Mistakes:

• Choosing ADD instead of SUBS
• Confusing MOV with arithmetic
• Using BNE as arithmetic instruction

3. Consider this ARM assembly loop snippet:

MOV R0, #3
loop:
SUBS R0, R0, #1
BNE loop

How many times will the instructions inside the loop execute?

medium

A. 2 times

B. Infinite loop

C. 3 times

D. 4 times

Solution

1. Step 1: Analyze initial register value

   R0 starts at 3, so the loop counter is 3.

2. Step 2: Understand SUBS and BNE behavior

   SUBS subtracts 1 and updates flags; BNE branches if result is not zero, so loop runs while R0 > 0.

3. Final Answer:

   3 times -> Option C

4. Quick Check:

   Loop runs 3 times as R0 counts down from 3 [OK]

Hint: Loop runs as many times as initial counter value [OK]

Common Mistakes:

• Counting one less iteration
• Assuming infinite loop
• Confusing BNE with BEQ

4. Identify the error in this ARM assembly loop code:

MOV R1, #5
loop:
SUB R1, R1, #1
BNE loop

medium

A. SUB does not update condition flags, causing an infinite loop

B. MOV instruction is incorrect syntax

C. BNE should be replaced with BEQ

D. Loop counter should start at 0

Solution

1. Step 1: Check SUB vs SUBS instruction

   SUB subtracts but does not update condition flags; BNE depends on flags to branch.

2. Step 2: Understand effect on loop

   Since flags are not updated by SUB, they remain unchanged and BNE continues to branch back, causing an infinite loop.

3. Final Answer:

   SUB does not update condition flags, causing an infinite loop -> Option A

4. Quick Check:

   SUB vs SUBS affects flags needed for BNE [OK]

Hint: Use SUBS, not SUB, to update flags for BNE [OK]

Common Mistakes:

• Using SUB instead of SUBS
• Changing BNE to BEQ incorrectly
• Starting counter at zero

5. You want to create a loop in ARM assembly that counts down from 4 to 1 and sums these values in R2. Which code snippet correctly implements this?

hard

A. MOV R0, #1
MOV R2, #0
loop:
ADD R2, R2, R0
SUBS R0, R0, #1
BNE loop

B. MOV R0, #4
MOV R2, #0
loop:
ADD R2, R2, R0
SUBS R0, R0, #1
BNE loop

C. MOV R0, #4
MOV R2, #0
loop:
ADD R2, R2, R0
SUB R0, R0, #1
BNE loop

D. MOV R0, #4
MOV R2, #0
loop:
ADD R2, R2, R0
SUBS R0, R0, #1
BEQ loop

Solution

1. Step 1: Initialize counter and sum registers

   R0 starts at 4 (countdown), R2 starts at 0 (sum).

2. Step 2: Use ADD to accumulate and SUBS to decrement with flags

   ADD adds R0 to R2; SUBS decrements R0 and updates flags for BNE to loop while R0 != 0.

3. Step 3: Confirm correct branch instruction

   BNE loops while R0 not zero; BEQ would loop only when zero, incorrect here.

4. Final Answer:

   Code snippet correctly sums 4+3+2+1 in R2 -> Option B

5. Quick Check:

   SUBS + BNE loop counts down and sums [OK]

Hint: Use SUBS with BNE and add inside loop for sum [OK]

Common Mistakes:

• Starting counter at 1 instead of 4
• Using SUB instead of SUBS
• Using BEQ instead of BNE for loop