Return value in R0 in ARM Architecture

Introduction

In ARM assembly, the first register R0 is used to hold the return value of a function. This helps the program know the result after a function finishes.

When writing a function that needs to send a result back to the caller.

When you want to check the outcome of a calculation done inside a function.

When passing a simple value like a number or address back after a process.

When debugging to see what value a function returns.

When calling system or library functions that return results in R0.

Core Concept

ARM Architecture

MOV R0, #value  ; Move the return value into R0
BX LR           ; Return from function

The return value must be placed in R0 before returning.

Use BX LR to return to the caller after setting R0.

Key Points

Returns the number 5 from the function.

ARM Architecture

MOV R0, #5
BX LR

Returns the sum of R1 and R2 in R0.

ARM Architecture

ADD R0, R1, R2
BX LR

Returns the address 0x1000 in R0.

ARM Architecture

LDR R0, =0x1000
BX LR

Detailed Explanation

This program adds two numbers 3 and 4 by calling a function. The function adds the values in R0 and R1 and returns the sum in R0. After the call, R0 holds the result 7.

ARM Architecture

; Function to add two numbers and return the result in R0
ADD_TWO_NUMBERS:
    ADD R0, R0, R1  ; Add R0 and R1, store result in R0
    BX LR           ; Return to caller

; Main program
    MOV R0, #3      ; First number
    MOV R1, #4      ; Second number
    BL ADD_TWO_NUMBERS  ; Call function
    ; Now R0 contains 7

Important Notes

Only simple return values fit in R0, like integers or pointers.

For larger data, other registers or memory are used.

Always set R0 before returning to ensure correct results.

Summary

R0 holds the return value of a function in ARM assembly.

Set R0 before returning with BX LR.

This allows the caller to get the function's result easily.