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.