Recursive function in assembly in ARM Architecture - Mini Project: Build & Apply

Recursive Function in ARM Assembly

📖 Scenario: You are learning how to write a recursive function in ARM assembly language. Recursive functions call themselves to solve smaller parts of a problem. This is useful for tasks like calculating factorials.

🎯 Goal: Create a simple recursive function in ARM assembly that calculates the factorial of a number.

📋 What You'll Learn

Create a label for the factorial function

Use the stack to save and restore the return address

Implement the base case for factorial (when input is 0 or 1)

Use recursion to calculate factorial for numbers greater than 1

💡 Why This Matters

🌍 Real World

Recursive functions are used in many algorithms like sorting, searching, and mathematical computations. Understanding recursion in assembly helps in low-level programming and optimization.

💼 Career

Knowledge of recursion and stack management in assembly is valuable for embedded systems developers, firmware engineers, and anyone working close to hardware.

1

Set up the input number in register r0

Write a line of ARM assembly code to move the number 5 into register r0. This will be the input for the factorial function.

ARM Architecture

# Move the number 5 into register r0
# Your code here

Need a hint?

Use the MOV instruction to put the number 5 into r0.

2

Create the factorial function label and base case check

Write the label factorial and add code to check if r0 is less than or equal to 1. If yes, return 1 by moving 1 into r0 and then return from the function using BX LR.

ARM Architecture

    MOV r0, #5

factorial:
    # Check if r0 <= 1
    # If yes, move 1 into r0 and return
    # Your code here

Need a hint?

Use CMP to compare r0 with 1, then BLE to branch to base_case if less or equal.

3

Implement the recursive call with stack management

In the recurse section, push the link register LR onto the stack to save the return address. Then subtract 1 from r0, call factorial recursively, pop LR from the stack, multiply the original r0 value by the returned value in r0, and finally return using BX LR. Use register r1 to store the original r0 value before the recursive call.

ARM Architecture

    MOV r0, #5

factorial:
    CMP r0, #1
    BLE base_case

    B recurse

base_case:
    MOV r0, #1
    BX LR

recurse:
    # Save LR on stack
    # Store r0 in r1
    # Subtract 1 from r0
    # Call factorial recursively
    # Restore LR from stack
    # Multiply r1 by r0
    # Return
    # Your code here

Need a hint?

Use PUSH {LR} and POP {LR} to save and restore the return address. Use BL factorial to call the function recursively.

4

Add the program entry point and call the factorial function

Add a label _start as the program entry point. From there, call the factorial function using BL factorial. After the call, add an infinite loop to end the program using a label end and a branch to itself.

ARM Architecture

    MOV r0, #5

factorial:
    CMP r0, #1
    BLE base_case

    B recurse

base_case:
    MOV r0, #1
    BX LR

recurse:
    PUSH {LR}
    MOV r1, r0
    SUB r0, r0, #1
    BL factorial
    POP {LR}
    MUL r0, r1, r0
    BX LR

_start:
    # Call factorial function
    # Infinite loop to end program
    # Your code here

Need a hint?

Use BL factorial to call the function and create an infinite loop with a label and branch to itself.