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

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

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

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

Hint

Use BX LR to return from the function.

Practice

1. In ARM assembly, which register is used to hold the return value of a function?

easy

A. LR

B. R1

C. R0

D. SP

Solution

1. Step 1: Understand ARM calling convention

   In ARM, the first register R0 is used to pass the return value from a function to the caller.

2. Step 2: Identify the return value register

   Since R0 holds the return value, the caller reads the result from R0 after the function returns.

3. Final Answer:

   R0 -> Option C

4. Quick Check:

   Return value register = R0 [OK]

Hint: Return values are always in R0 register in ARM [OK]

Common Mistakes:

• Confusing LR with return value
• Using SP as return value
• Thinking R1 holds return value

2. Which ARM assembly instruction is commonly used to return from a function after setting R0?

easy

A. MOV R0, #0

B. ADD R0, R0, #1

C. STR R0, [SP]

D. BX LR

Solution

1. Step 1: Identify return instruction in ARM

   The instruction BX LR branches to the address stored in the link register (LR), effectively returning from the function.

2. Step 2: Confirm usage after setting R0

   After placing the return value in R0, BX LR is used to return control to the caller.

3. Final Answer:

   BX LR -> Option D

4. Quick Check:

   Return instruction = BX LR [OK]

Hint: Return with BX LR after setting R0 [OK]

Common Mistakes:

• Using MOV instead of BX LR to return
• Storing R0 to stack before return
• Using ADD as return instruction

3. Consider this ARM assembly snippet:

MOV R0, #5
BX LR

What value will the caller receive after this function returns?

medium

A. 5

B. 0

C. LR value

D. Undefined

Solution

1. Step 1: Analyze MOV instruction

   The instruction MOV R0, #5 sets the R0 register to 5, which is the return value.

2. Step 2: Analyze return instruction

   BX LR returns control to the caller, with R0 holding the return value 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   R0 value after return = 5 [OK]

Hint: Value in R0 before BX LR is the return value [OK]

Common Mistakes:

• Thinking LR value is returned
• Assuming R0 resets to 0
• Ignoring MOV sets return value

4. Identify the error in this ARM function that intends to return 10:

ADD R0, R0, #10
BX LR

medium

A. R0 is not initialized before ADD

B. BX LR should be MOV PC, LR

C. ADD instruction syntax is incorrect

D. Function should use R1 for return value

Solution

1. Step 1: Check initial value of R0

   The instruction ADD R0, R0, #10 adds 10 to R0, but R0 is not initialized, so the result is unpredictable.

2. Step 2: Confirm correct return method

   BX LR is correct to return; the problem is the uninitialized R0.

3. Final Answer:

   R0 is not initialized before ADD -> Option A

4. Quick Check:

   Uninitialized R0 causes wrong return value [OK]

Hint: Initialize R0 before adding to it [OK]

Common Mistakes:

• Assuming R0 starts at zero
• Confusing BX LR with MOV PC, LR
• Using wrong register for return value

5. You want to write an ARM function that returns the sum of two input values passed in R0 and R1. Which code correctly sets the return value in R0 before returning?

hard

A. MOV R0, R1
BX LR

B. ADD R0, R0, R1
BX LR

C. ADD R1, R0, R1
BX LR

D. SUB R0, R0, R1
BX LR

Solution

1. Step 1: Understand input and output registers

   Inputs are in R0 and R1; the sum should be stored in R0 as the return value.

2. Step 2: Check each option

   ADD R0, R0, R1 BX LR adds R0 and R1, stores result in R0, then returns with BX LR, which is correct.

3. Final Answer:

   ADD R0, R0, R1 BX LR -> Option B

4. Quick Check:

   Sum in R0 before BX LR returns correct value [OK]

Hint: Sum inputs into R0, then BX LR to return [OK]

Common Mistakes:

• Storing sum in R1 instead of R0
• Using SUB instead of ADD
• Not returning with BX LR