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.

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