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.