Return value in R0 in ARM Architecture - Step-by-Step Execution

Concept Flow - Return value in R0

Function starts

↓

Compute result

↓

Place result in R0

↓

Return to caller

↓

Caller reads R0 for return value

The function computes a result, stores it in register R0, then returns. The caller reads R0 to get the return value.

Execution Sample

ARM Architecture

MOV R0, #5
BX LR

This code sets the return value to 5 by placing it in R0, then returns to the caller.

Analysis Table

Step	Instruction	Action	R0 Value	Notes
1	MOV R0, #5	Load immediate value 5 into R0	5	R0 now holds the return value
2	BX LR	Branch to link register (return)	5	Function returns, R0 unchanged
3	Caller reads R0	Caller accesses R0 to get return value	5	Return value is 5

💡 Function returns after placing value in R0; caller uses R0 as return value

State Tracker

Variable	Start	After Step 1	After Step 2	Final
R0	undefined	5	5	5

Key Insights - 2 Insights

Why is the return value placed in R0?

In ARM calling conventions, R0 is the standard register to hold the return value, as shown in execution_table step 1 and 3.

What happens if the function does not place a value in R0 before returning?

The caller may read an undefined or old value from R0, leading to incorrect results. The execution_table shows the importance of step 1.

Visual Quiz - 3 Questions

Look at the execution_table, what value does R0 hold after step 1?

A 5

B 0

C undefined

D LR

💡 Hint

Check the 'R0 Value' column in execution_table row for step 1

At which step does the function return to the caller?

A Step 1

B Step 2

C Step 3

D No return

💡 Hint

Look at the 'Instruction' and 'Action' columns in execution_table

If the MOV instruction was changed to MOV R0, #10, what would be the final R0 value?

A 5

B 0

C 10

D Undefined

💡 Hint

Refer to variable_tracker and how R0 changes after MOV instruction

Concept Snapshot

In ARM, the return value from a function is placed in register R0.
The function sets R0 before returning using BX LR.
The caller reads R0 to get the result.
Always ensure R0 holds the correct value before return.

Full Transcript

In ARM architecture, when a function finishes, it places its return value in the register R0. This is a standard rule so the caller knows where to look. The function uses instructions like MOV to put the value in R0, then returns using BX LR. The caller then reads R0 to get the returned result. If the function does not set R0 properly, the caller may get wrong data. This trace showed a simple example where the function returns 5 by moving 5 into R0 and then returning.

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