Loop implementation in assembly in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding loop counter initialization in ARM assembly

In ARM assembly, which register is commonly used to hold the loop counter before entering a loop?

A R0

B R7

C LR

D PC

💡 Hint

Think about the general-purpose registers used for data and counters.

R0 is a general-purpose register commonly used to hold values like loop counters. PC is the program counter, LR is the link register, and R7 is often used for system calls.

📋 Factual

intermediate

ARM assembly loop decrement instruction

Which ARM assembly instruction is used to decrement a register value by 1?

A CMP R0, #1

B ADD R0, R0, #1

C MOV R0, #1

D SUB R0, R0, #1

💡 Hint

Look for the instruction that subtracts a value.

SUB R0, R0, #1 subtracts 1 from R0, effectively decrementing it. ADD adds, MOV moves a value, CMP compares.

🔍 Analysis

advanced

Analyzing loop termination condition in ARM assembly

Given the following ARM assembly snippet, what condition causes the loop to exit?

loop:
    SUB R0, R0, #1
    CMP R0, #0
    BNE loop

A Loop exits when R0 becomes zero

B Loop exits when R0 is less than zero

C Loop exits when R0 is greater than zero

D Loop never exits

💡 Hint

Consider what BNE means and when it branches.

BNE means branch if not equal. The loop continues while R0 is not zero. When R0 reaches zero, the loop exits.

❓ Comparison

advanced

Comparing loop implementations in ARM assembly

Which of the following ARM assembly loops correctly counts from 5 down to 1?

A

MOV R0, #5
loop:
    CMP R0, #1
    SUB R0, R0, #1
    BNE loop

B

MOV R0, #1
loop:
    ADD R0, R0, #1
    CMP R0, #5
    BNE loop

C

MOV R0, #5
loop:
    SUB R0, R0, #1
    CMP R0, #0
    BNE loop

D

MOV R0, #5
loop:
    CMP R0, #0
    SUB R0, R0, #1
    BNE loop

💡 Hint

Check the order of instructions and the comparison value.

Option C initializes R0 to 5, decrements before comparing to zero, and loops while not zero, correctly counting down from 5 to 1.

Option C counts up, not down. Option C compares before decrementing, causing off-by-one. Option C compares before decrementing, causing incorrect behavior.

❓ Reasoning

expert

Determining the number of loop iterations in ARM assembly

Consider this ARM assembly loop:

    MOV R0, #10
loop:
    SUB R0, R0, #2
    CMP R0, #0
    BGT loop

How many times will the loop execute before exiting?

A 4 times

B 5 times

C 6 times

D 10 times

💡 Hint

Count how R0 changes each iteration and when the loop stops.

R0 starts at 10 and decreases by 2 each loop: 10, 8, 6, 4, 2, 0. The loop continues while R0 > 0. When R0 reaches 0, CMP sets zero flag and BGT (branch if greater than zero) fails, so loop exits. The loop runs 5 times (for R0=10,8,6,4,2).

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