Return value in R0 in ARM Architecture - Mini Project: Build & Apply

Return Value in R0

📖 Scenario: You are learning how ARM processors return values from functions. In ARM assembly, the return value of a function is placed in the R0 register. This is important because when a function finishes, the calling code looks in R0 to get the result.Imagine you have a simple function that adds two numbers and returns the sum. You want to see how the sum is stored in R0 before the function ends.

🎯 Goal: Build a simple ARM assembly function that adds two numbers and returns the result in R0. You will create the data setup, configure input values, write the addition logic, and complete the function so it returns the sum correctly.

📋 What You'll Learn

Create two registers with exact values 5 and 7

Use a register to hold the sum

Add the two numbers and store the result in R0

Complete the function with a return instruction

💡 Why This Matters

🌍 Real World

Understanding how return values are passed in ARM assembly is essential for low-level programming, debugging, and working with embedded systems.

💼 Career

This knowledge is important for roles in embedded software development, firmware engineering, and systems programming where ARM processors are common.

1

DATA SETUP: Load values into registers

Write ARM assembly instructions to load the value 5 into register R1 and the value 7 into register R2.

ARM Architecture

; Load 5 into R1 and 7 into R2
; Your code here

Need a hint?

Use the MOV instruction to put immediate values into registers.

2

CONFIGURATION: Prepare R0 for the result

Add a line to clear register R0 by moving 0 into it before the addition.

ARM Architecture

MOV R1, #5
MOV R2, #7
; Clear R0 before addition
; Your code here

Need a hint?

Use MOV R0, #0 to clear R0.

3

CORE LOGIC: Add the two numbers

Write an instruction to add the values in R1 and R2, and store the result in R0.

ARM Architecture

MOV R1, #5
MOV R2, #7
MOV R0, #0
; Add R1 and R2, store in R0
; Your code here

Need a hint?

Use the ADD instruction with destination R0.

4

COMPLETION: Return from the function

Add the instruction to return from the function using the BX LR instruction.

ARM Architecture

MOV R1, #5
MOV R2, #7
MOV R0, #0
ADD R0, R1, R2
; Return from function
; Your code here

Need a hint?

Use BX LR to return from the function.