Return value in R0 in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the R0 register in ARM architecture?

R0 is used to hold the return value of a function in ARM architecture. When a function finishes, it places its result in R0 for the caller to use.

beginner

Which register typically holds the return value after a function call in ARM?

The R0 register typically holds the return value after a function call in ARM processors.

beginner

If a function returns an integer value, where is this value stored in ARM assembly?

The integer return value is stored in the R0 register before returning to the caller.

intermediate

Why is R0 used for return values in ARM instead of other registers?

R0 is used because ARM calling conventions designate it as the primary register for passing return values, simplifying function calls and returns.

intermediate

Can R0 be used for purposes other than returning values in ARM functions?

Yes, R0 can be used as a general-purpose register during function execution, but before returning, it must hold the return value.

In ARM architecture, which register holds the return value of a function?

A R0

B R1

C R7

D LR

R0 is the standard register used to hold the return value after a function call in ARM.

What type of value is typically stored in R0 after a function returns?

A Program counter

B Stack pointer

C Return value

D Status flags

R0 holds the return value from a function, such as an integer or pointer.

If a function returns no value (void), what is stored in R0?

A Zero always

B Stack pointer

C Function address

D Undefined or ignored

If no value is returned, R0 may contain any value and is generally ignored.

Which ARM register is NOT used to return a function value?

A R1

B LR

C R2

D R0

LR (Link Register) holds the return address, not the return value.

Why is it important to place the return value in R0 before returning from a function?

A Because the caller expects the result in R0

B Because R0 controls the program flow

C Because R0 stores the stack pointer

D Because R0 holds the return address

The calling convention requires the return value in R0 so the caller can access it easily.

Explain the role of the R0 register in returning values from functions in ARM architecture.

Think about how functions communicate results back to the caller.

Describe what happens to the R0 register during and after a function call in ARM.

Consider the lifecycle of R0 in a function call.

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