Loop implementation in assembly in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the loop counter register.

ARM Architecture

MOV R0, [1]

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A#10

BR1

C#0

DR2

Attempts:

3 left

💡 Hint

Common Mistakes

Using another register instead of an immediate value to initialize the counter.

Forgetting the '#' symbol before the number.

The instruction MOV R0, #10 sets the register R0 to the immediate value 10, which is commonly used to initialize a loop counter.

2fill in blank

medium

Complete the code to decrement the loop counter register.

ARM Architecture

SUB [1], [1], #1

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AR0

BR3

CR2

DR1

Attempts:

3 left

💡 Hint

Common Mistakes

Using different registers for destination and operand.

Using the wrong register that was not initialized as the counter.

The instruction SUB R0, R0, #1 subtracts 1 from the value in register R0, effectively decrementing the loop counter.

3fill in blank

hard

Fix the error in the loop condition check instruction.

ARM Architecture

CMP [1], #0

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AR1

BR0

C#0

DR2

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing an immediate value to another immediate value.

Using a register that is not the loop counter.

The CMP R0, #0 instruction compares the loop counter in R0 with zero to check if the loop should continue.

4fill in blank

hard

Fill both blanks to create a loop that repeats until the counter reaches zero.

ARM Architecture

BNE [1]
MOV [2], #0

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Aloop_start

BR0

CR1

Dend_loop

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong label name in the branch instruction.

Resetting the wrong register after the loop.

The BNE loop_start instruction branches back to the label loop_start if the zero flag is not set, meaning the counter is not zero yet. The MOV R0, #0 resets the counter register after the loop ends.

5fill in blank

hard

Fill all three blanks to implement a complete loop that counts down from 5 to 1.

ARM Architecture

loop_start:
MOV [1], [2]
SUB [3], [3], #1
CMP [3], #0
BNE loop_start

Drag options to blanks, or click blank then click option'

AR0

B#5

DR1

Attempts:

3 left

💡 Hint

Common Mistakes

Using different registers inconsistently for the counter.

Not initializing the counter before the loop.

This code sets register R0 to 5, then uses R0 as the loop counter. It decrements R0 by 1 each time, compares it to zero, and loops back if not zero.

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