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

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

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

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

Hint

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

Practice

1. What does a nested subroutine call mean in ARM assembly?

easy

A. A subroutine runs without calling any other subroutine

B. A subroutine calls itself repeatedly

C. One subroutine calls another subroutine inside it

D. A subroutine is called only once in the program

Solution

1. Step 1: Understand subroutine calls

   A subroutine is a small program block that can be called to perform a task.

2. Step 2: Define nested calls

   Nested calls happen when one subroutine calls another subroutine inside its code.

3. Final Answer:

   One subroutine calls another subroutine inside it -> Option C

4. Quick Check:

   Nested call = one subroutine calls another [OK]

Hint: Nested means calling a subroutine inside another [OK]

Common Mistakes:

• Confusing nested call with recursion
• Thinking subroutine runs alone without calls
• Mixing up call and return instructions

2. Which ARM instruction is used to call a subroutine?

easy

A. BX LR

B. BL

C. MOV

D. ADD

Solution

1. Step 1: Identify call instruction

   In ARM, the instruction to call a subroutine is 'BL' which means Branch with Link.

2. Step 2: Differentiate from return instruction

   'BX LR' is used to return from a subroutine, not to call it.

3. Final Answer:

   BL -> Option B

4. Quick Check:

   Call subroutine = BL instruction [OK]

Hint: BL calls subroutine; BX LR returns [OK]

Common Mistakes:

• Confusing BX LR as call instead of return
• Using MOV or ADD as call instructions
• Mixing up branch instructions without link

3. Given the following ARM assembly snippet, what is the final value in register R0 after execution?

main:
  MOV R0, #1
  BL sub1
  BX LR

sub1:
  ADD R0, R0, #2
  BL sub2
  BX LR

sub2:
  ADD R0, R0, #3
  BX LR

medium

A. 1

B. 3

C. 0

D. 6

Solution

1. Step 1: Trace main and sub1 execution

   Start with R0=1 in main. Then BL sub1 calls sub1. In sub1, R0 = 1 + 2 = 3.

2. Step 2: Trace sub1 calling sub2

   sub1 calls sub2. In sub2, R0 = 3 + 3 = 6. Then sub2 returns, sub1 returns, main returns.

3. Final Answer:

   6 -> Option D

4. Quick Check:

   1 + 2 + 3 = 6 [OK]

Hint: Add values stepwise through nested calls [OK]

Common Mistakes:

• Ignoring sub2 addition
• Not updating R0 after each call
• Confusing return with call instructions

4. Identify the error in this ARM assembly nested subroutine code:

main:
  BL sub1
  BX LR

sub1:
  ADD R0, R0, #1
  BL sub2
  BX LR

sub2:
  ADD R0, R0, #2
  BX R0

medium

A. Using BX R0 instead of BX LR to return in sub2

B. Missing MOV instruction in main

C. BL should be replaced with BX for calls

D. R0 should not be modified in subroutines

Solution

1. Step 1: Check return instructions

   Subroutines should return using 'BX LR' to jump back to caller address stored in LR.

2. Step 2: Identify incorrect return

   In sub2, 'BX R0' tries to jump to address in R0, which is incorrect and causes error.

3. Final Answer:

   Using BX R0 instead of BX LR to return in sub2 -> Option A

4. Quick Check:

   Return must use BX LR, not BX R0 [OK]

Hint: Return with BX LR, not BX R0 [OK]

Common Mistakes:

• Using BX R0 instead of BX LR
• Confusing BL and BX instructions
• Assuming R0 holds return address

5. You want to organize a complex ARM program by splitting tasks into subroutines that call each other. Which is the best practice for nested subroutine calls to avoid errors?

hard

A. Always use BL to call and BX LR to return, saving LR if calling another subroutine

B. Use BX LR to call and BL to return, no need to save LR

C. Avoid calling subroutines inside subroutines to prevent errors

D. Use MOV to jump between subroutines and ignore LR register

Solution

1. Step 1: Understand call and return instructions

   BL saves return address in LR and branches to subroutine; BX LR returns to caller.

2. Step 2: Handle nested calls safely

   If a subroutine calls another, it must save LR (e.g., on stack) before calling to avoid losing return address.

3. Final Answer:

   Always use BL to call and BX LR to return, saving LR if calling another subroutine -> Option A

4. Quick Check:

   BL call + BX LR return + save LR for nested calls [OK]

Hint: Save LR before nested BL calls, return with BX LR [OK]

Common Mistakes:

• Using BX LR to call instead of BL
• Not saving LR before nested calls
• Jumping with MOV ignoring LR