Loop implementation in assembly in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a loop in assembly language?

A loop in assembly language is a sequence of instructions that repeats until a certain condition is met, allowing the program to perform repetitive tasks.

beginner

Which ARM instruction is commonly used to decrease a counter in a loop?

The SUBS instruction is used to subtract and update the condition flags, often used to decrease a loop counter and check if it has reached zero.

beginner

How does the BNE instruction work in ARM loops?

BNE means 'Branch if Not Equal'. It causes the program to jump back to the loop start if the zero flag is not set, meaning the loop condition is still true.

intermediate

Why is updating the condition flags important in ARM loop implementation?

Updating condition flags after arithmetic instructions lets the processor decide whether to continue looping or exit based on the flags, enabling conditional branching.

intermediate

Describe a simple ARM loop structure using a counter register.

Load a counter register with the number of iterations, then use SUBS to decrement it each loop. Use BNE to branch back if the counter is not zero, repeating the loop.

Which instruction decreases a register and updates condition flags in ARM assembly?

A ADD

B MOV

C SUBS

D LDR

SUBS subtracts and updates flags, useful for loop counters.

What does the BNE instruction do in a loop?

A Branches if zero flag is not set

B Branches if carry flag is set

C Branches unconditionally

D Branches if zero flag is set

BNE branches if the zero flag is not set, meaning the loop continues.

In ARM loops, what is the purpose of condition flags?

A To store data

B To control branching decisions

C To hold memory addresses

D To perform arithmetic

Condition flags help decide if the loop should continue or exit.

Which register typically holds the loop counter in ARM assembly?

A Program Counter (PC)

B Link Register (LR)

C Stack Pointer (SP)

D General-purpose register (e.g., R0, R1)

General-purpose registers like R0 or R1 are used to hold counters.

What happens if the loop counter reaches zero in an ARM loop using SUBS and BNE?

A The loop exits

B The program crashes

C The loop continues

D The counter resets automatically

When the counter is zero, the zero flag is set, so BNE does not branch and the loop exits.

Explain how a simple loop is implemented in ARM assembly using a counter and branch instructions.

Think about how the counter changes and how the program decides to repeat or stop.

Describe the role of condition flags in controlling loops in ARM assembly language.

Focus on how the processor uses flags to make decisions.

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