Loop implementation in assembly in ARM Architecture - Mini Project: Build & Apply

Loop implementation in assembly

📖 Scenario: You are learning how to write a simple loop in ARM assembly language. Loops help repeat a set of instructions multiple times, which is useful in many real-world tasks like counting, processing lists, or blinking an LED repeatedly.

🎯 Goal: Build a basic ARM assembly program that counts down from 5 to 1 using a loop and then stops.

📋 What You'll Learn

Create a register with the initial count value 5

Set up a label to mark the start of the loop

Use a decrement instruction to reduce the count by 1 each time

Use a conditional branch to repeat the loop while the count is not zero

💡 Why This Matters

🌍 Real World

Loops in assembly are used in embedded systems to repeat tasks like reading sensors, controlling motors, or blinking lights.

💼 Career

Understanding loops in assembly helps in low-level programming jobs such as embedded software development, firmware engineering, and systems programming.

1

DATA SETUP: Initialize the count register

Write an ARM assembly instruction to load the immediate value 5 into register r0. This will be our loop counter.

ARM Architecture

    MOV r0, #5  // Your code here

Hint

Use the MOV instruction to put the number 5 into r0.

2

CONFIGURATION: Create the loop start label

Add a label called loop_start to mark the beginning of the loop.

ARM Architecture

    MOV r0, #5
loop_start:  // Your code here

Hint

Labels end with a colon : and are placed at the start of a line.

3

CORE LOGIC: Decrement the counter and check if zero

Inside the loop, write an instruction to subtract 1 from r0. Then add a conditional branch instruction BNE loop_start to jump back to loop_start if r0 is not zero.

ARM Architecture

    MOV r0, #5
loop_start:
    SUB r0, r0, #1  // Your code here
    BNE loop_start  // Your code here

Hint

Use SUB to decrease the value in r0. Use BNE to branch if not equal to zero.

4

COMPLETION: Add a stop instruction

Add the B . instruction after the loop to create an infinite stop (hang) so the program does not continue running random code.

ARM Architecture

    MOV r0, #5
loop_start:
    SUB r0, r0, #1
    BNE loop_start
    B .  // Your code here

Hint

The instruction B . means branch to the current address, creating an infinite loop to stop the program.

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