Nested subroutine calls in ARM Architecture - Mini Project: Build & Apply

Understanding Nested Subroutine Calls in ARM Assembly

📖 Scenario: You are learning how ARM assembly handles nested subroutine calls. Imagine you have a main program that calls a subroutine, which itself calls another subroutine. This is common in real programs where tasks are broken down into smaller functions.

🎯 Goal: Build a simple ARM assembly program with nested subroutine calls: a main routine calls sub1, and sub1 calls sub2. Each subroutine will return to its caller correctly.

📋 What You'll Learn

Create a main label as the program entry point

Create two subroutines named sub1 and sub2

Use bl (branch with link) instruction to call subroutines

Use bx lr to return from subroutines

Ensure main calls sub1, and sub1 calls sub2

💡 Why This Matters

🌍 Real World

Nested subroutine calls are common in software development for organizing code into reusable functions. ARM assembly programmers must understand how to manage these calls to ensure correct program flow.

💼 Career

Embedded systems developers and firmware engineers often write ARM assembly code. Mastering nested subroutine calls is essential for debugging and writing efficient low-level code.

1

Set up the main label

Write the ARM assembly code to create a label called main which will be the program's starting point.

ARM Architecture

main:
    // Your code here

Need a hint?

The main label marks where the program begins execution.

2

Create the sub1 subroutine

Add a subroutine labeled sub1 below main. This subroutine should end with bx lr to return to the caller.

ARM Architecture

main:
    // Program starts here

sub1:
    // Your code here

Need a hint?

The bx lr instruction returns from the subroutine to the address stored in the link register.

3

Make main call sub1

In the main label, add the instruction bl sub1 to call the sub1 subroutine.

ARM Architecture

main:
    // Program starts here
    // Your code here

sub1:
    bx lr

Need a hint?

The bl instruction calls a subroutine and saves the return address in the link register.

4

Add sub2 and call it from sub1

Create a new subroutine labeled sub2 that ends with bx lr. Then, inside sub1, add the instruction bl sub2 to call sub2.

ARM Architecture

main:
    bl sub1

sub1:
    // Your code here

// Add sub2 below

Need a hint?

Remember to call sub2 from sub1 using bl sub2 and return properly with bx lr.