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).