Branch and link (BL) for subroutines in ARM Architecture - Mini Project: Build & Apply

Branch and link (BL) for subroutines

📖 Scenario: You are learning how ARM processors use the BL instruction to call subroutines (functions) in assembly language. This helps the processor jump to a different part of the program and then return back after finishing the subroutine.

🎯 Goal: Build a simple ARM assembly program that uses the BL instruction to call a subroutine which adds two numbers and returns the result.

📋 What You'll Learn

Create two registers with initial values

Create a label for the subroutine

Use the BL instruction to call the subroutine

Return from the subroutine using BX LR

💡 Why This Matters

🌍 Real World

ARM processors use the BL instruction to call functions in embedded systems, mobile devices, and many electronics.

💼 Career

Understanding BL and subroutines is essential for ARM assembly programming, firmware development, and low-level debugging.

1

DATA SETUP: Initialize registers with values

Write ARM assembly code to load the value 5 into register R0 and the value 3 into register R1.

ARM Architecture

# Load 5 into R0 and 3 into R1
# Your code here

Need a hint?

Use the MOV instruction to put numbers into registers.

2

CONFIGURATION: Define the subroutine label

Add a label called add_numbers below the register initialization to mark the start of the subroutine.

ARM Architecture

MOV R0, #5
MOV R1, #3

add_numbers:
# Your code here

Need a hint?

Labels end with a colon : and mark places to jump to.

3

CORE LOGIC: Use BL to call the subroutine

Write a BL add_numbers instruction before the label add_numbers to call the subroutine.

ARM Architecture

MOV R0, #5
MOV R1, #3
BL add_numbers

add_numbers:
# Your code here

Need a hint?

The BL instruction jumps to the label and saves the return address.

4

COMPLETION: Add subroutine instructions and return

Inside the add_numbers subroutine, add ADD R0, R0, R1 to add the two numbers, then add BX LR to return to the caller.

ARM Architecture

MOV R0, #5
MOV R1, #3
BL add_numbers

add_numbers:
# Add R0 and R1, then return
# Your code here

Need a hint?

ADD adds registers, BX LR returns from subroutine.