Loop implementation in assembly in ARM Architecture - Step-by-Step Execution

Concept Flow - Loop implementation in assembly

Initialize counter

↓

Check condition: counter > 0?

No→EXIT

Yes↓

Execute loop body

↓

Update counter: counter = counter - 1

↩Back to Check

The loop starts by setting a counter, then checks if the counter is greater than zero. If yes, it runs the loop body, decreases the counter, and repeats. If no, it exits.

Execution Sample

ARM Architecture

MOV R0, #3
LOOP:
SUBS R0, R0, #1
BNE LOOP

This code sets R0 to 3, then loops subtracting 1 until R0 reaches zero.

Analysis Table

Step	Instruction	R0 Before	Operation	R0 After	Branch Taken
1	MOV R0, #3	-	Set R0 to 3	3	No branch
2	SUBS R0, R0, #1	3	R0 = 3 - 1	2	Check Z flag (not zero)
3	BNE LOOP	-	Branch if R0 != 0	-	Branch to LOOP
4	SUBS R0, R0, #1	2	R0 = 2 - 1	1	Check Z flag (not zero)
5	BNE LOOP	-	Branch if R0 != 0	-	Branch to LOOP
6	SUBS R0, R0, #1	1	R0 = 1 - 1	0	Check Z flag (zero)
7	BNE LOOP	-	Branch if R0 != 0	-	No branch, exit loop

💡 R0 reaches 0, zero flag set, BNE does not branch, loop ends

State Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
R0	-	2	1	0	0

Key Insights - 2 Insights

Why does the loop stop when R0 reaches zero?

Because the SUBS instruction updates the zero flag when R0 becomes zero, and the BNE (Branch if Not Equal) checks this flag. When zero flag is set, BNE does not branch, so the loop exits (see execution_table steps 6 and 7).

What does the SUBS instruction do differently than SUB?

SUBS subtracts and updates the condition flags (like zero flag), which is needed for the BNE instruction to decide whether to branch. Without updating flags, the loop condition can't be checked properly.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of R0 after step 4?

A 2

B 0

C 1

D 3

💡 Hint

Check the 'R0 After' column for step 4 in the execution_table.

At which step does the loop condition become false and the loop exits?

A Step 5

B Step 7

C Step 3

D Step 1

💡 Hint

Look at the Branch Taken column where BNE does not branch.

If the initial MOV instruction set R0 to 5 instead of 3, how many times would the loop body execute?

A 5 times

B 4 times

C 3 times

D 6 times

💡 Hint

The loop runs until R0 reaches zero, starting from the initial value.

Concept Snapshot

Loop in ARM assembly uses a register as counter.
MOV sets initial count.
SUBS subtracts and updates flags.
BNE branches if counter not zero.
Loop repeats until counter is zero.
Zero flag controls loop exit.

Full Transcript

This example shows how to implement a loop in ARM assembly using a register as a counter. First, the MOV instruction sets the counter register R0 to 3. Then the loop label marks the start. The SUBS instruction subtracts 1 from R0 and updates the zero flag. The BNE instruction checks if R0 is not zero and branches back to the loop start if true. This repeats until R0 reaches zero, at which point the zero flag is set, BNE does not branch, and the loop ends. The execution table traces each step, showing R0's value changes and branch decisions. Key points include the importance of SUBS updating flags and how the zero flag controls loop exit.

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