Preserving callee-saved registers in ARM Architecture - Mini Project: Build & Apply

Preserving Callee-Saved Registers in ARM Assembly

📖 Scenario: You are writing a simple ARM assembly function that calls another function. To keep your program stable, you need to save and restore certain registers that the called function might change.This is important because some registers must keep their values across function calls, so the caller function can continue correctly after the call.

🎯 Goal: Build a small ARM assembly code snippet that saves callee-saved registers at the start of a function and restores them before returning.This will help you understand how to preserve important data during function calls.

📋 What You'll Learn

Create a function named my_function.

At the start of my_function, push registers r4, r5, and lr onto the stack.

Add a configuration variable saved_registers that lists the registers to save.

Inside my_function, simulate some operations (can be a comment).

Before returning, pop the registers r4, r5, and lr from the stack.

End the function with a bx lr instruction to return.

💡 Why This Matters

🌍 Real World

In embedded systems and low-level programming, preserving callee-saved registers ensures that functions do not accidentally overwrite important data, preventing bugs and crashes.

💼 Career

Understanding register preservation is essential for ARM assembly programmers, firmware developers, and anyone working close to hardware or optimizing performance-critical code.

1

Create the function and push registers

Write ARM assembly code to create a function named my_function. At the start of this function, push the registers r4, r5, and lr onto the stack using the push instruction.

ARM Architecture

; Define function my_function
; Your code here

Need a hint?

Use the push instruction with the registers inside curly braces.

2

Add a configuration variable for saved registers

Add a variable named saved_registers that contains the list of registers r4, r5, and lr as a comment or label to indicate which registers are saved.

ARM Architecture

my_function:
    push {r4, r5, lr}

; Define saved_registers list
; Your code here

Need a hint?

Use a comment to define saved_registers as {r4, r5, lr}.

3

Add core logic placeholder

Inside my_function, add a comment line that says Perform operations here to simulate the function's main work.

ARM Architecture

my_function:
    push {r4, r5, lr}

; saved_registers = {r4, r5, lr}

    ; Your code here

Need a hint?

Just add a comment line inside the function body.

4

Pop registers and return

Before the function ends, pop the registers r4, r5, and lr from the stack using the pop instruction. Then add bx lr to return from the function.

ARM Architecture

my_function:
    push {r4, r5, lr}

; saved_registers = {r4, r5, lr}

    ; Perform operations here

    ; Your code here

Need a hint?

Use pop with the same registers as push, then bx lr to return.