Link register (R14/LR) in ARM Architecture - Mini Project: Build & Apply

Understanding the Link Register (R14/LR) in ARM Architecture

📖 Scenario: You are learning about ARM processors used in many smartphones and embedded devices. The Link Register (R14 or LR) is a special register that helps the processor remember where to return after a function call.Imagine you are writing a simple program that calls a function to calculate a value and then returns to continue the main program.

🎯 Goal: Build a step-by-step understanding of how the Link Register (R14/LR) is used in ARM assembly to store the return address during function calls and how it helps the processor return to the correct place after the function finishes.

📋 What You'll Learn

Create a variable representing the Link Register (R14) with an initial value

Set up a variable to hold the return address

Simulate storing the return address in the Link Register during a function call

Simulate returning to the stored address using the Link Register

💡 Why This Matters

🌍 Real World

Understanding the Link Register is important for debugging and writing efficient ARM assembly code used in mobile devices and embedded systems.

💼 Career

Knowledge of ARM architecture and the Link Register is valuable for embedded systems engineers, firmware developers, and anyone working with low-level programming on ARM processors.

1

Create the Link Register variable

Create a variable called link_register and set it to 0x00000000 to represent the initial empty state of the Link Register (R14).

ARM Architecture

# Create the Link Register variable
# Your code here

Hint

The Link Register starts empty before any function calls.

2

Set up the return address variable

Create a variable called return_address and set it to 0x1000 to represent the address where the program should return after a function call.

ARM Architecture

link_register = 0x00000000
# Create the return_address variable
# Your code here

Hint

The return address is where the program continues after the function finishes.

3

Store the return address in the Link Register

Assign the value of return_address to link_register to simulate storing the return address in the Link Register during a function call.

ARM Architecture

link_register = 0x00000000
return_address = 0x1000
# Store the return_address in link_register
# Your code here

Hint

When a function is called, the processor saves the return address in the Link Register.

4

Simulate returning using the Link Register

Create a variable called program_counter and set it to the value of link_register to simulate the processor returning to the stored address after the function call.

ARM Architecture

link_register = 0x00000000
return_address = 0x1000
link_register = return_address
# Set program_counter to link_register to simulate return
# Your code here

Hint

The program counter moves to the address stored in the Link Register to continue execution.

Practice

1. What is the primary purpose of the link register (R14/LR) in ARM architecture?

easy

A. To manage stack pointer operations

B. To hold the current instruction being executed

C. To store general-purpose data temporarily

D. To store the return address during function calls

Solution

1. Step 1: Understand the role of the link register

   The link register (R14/LR) is specifically designed to hold the return address when a function call is made.

2. Step 2: Differentiate from other registers

   Other registers like the program counter hold the current instruction, and the stack pointer manages the stack, but LR stores the return address.

3. Final Answer:

   To store the return address during function calls -> Option D

4. Quick Check:

   Link register = return address storage [OK]

Hint: LR always holds return address after a function call [OK]

Common Mistakes:

• Confusing LR with the program counter
• Thinking LR stores general data
• Mixing LR with stack pointer

2. Which ARM instruction saves the return address into the link register (LR)?

easy

A. BL

B. ADD

C. MOV

D. STR

Solution

1. Step 1: Identify the instruction for function calls

   The BL (Branch with Link) instruction branches to a function and saves the return address in LR.

2. Step 2: Understand other instructions

   MOV moves data, ADD adds values, and STR stores data to memory; none save return addresses.

3. Final Answer:

   BL -> Option A

4. Quick Check:

   BL saves return address in LR [OK]

Hint: BL means branch and link (save return) [OK]

Common Mistakes:

• Choosing MOV as it moves data
• Confusing STR with saving return address
• Thinking ADD saves addresses

3. Consider this ARM code snippet:
BL function
...
function:
MOV R0, #5
BX LR
What happens when the instruction BX LR executes?

medium

A. The program jumps back to the instruction after BL

B. The program jumps to the start of function

C. The program halts execution

D. The program jumps to an undefined address

Solution

1. Step 1: Understand what BL does

   BL saves the return address (next instruction after BL) into LR and jumps to 'function'.

2. Step 2: Analyze BX LR instruction

   BX LR branches to the address stored in LR, which is the return address after BL.

3. Final Answer:

   The program jumps back to the instruction after BL -> Option A

4. Quick Check:

   BX LR returns to caller address [OK]

Hint: BX LR returns to saved return address [OK]

Common Mistakes:

• Thinking BX LR jumps to function start
• Assuming program halts on BX LR
• Believing BX LR jumps to random address

4. In this ARM code, what is the error?
BL function
...
function:
MOV R0, #10
MOV PC, R0

medium

A. BL instruction is invalid

B. MOV PC, R0 does not return to caller correctly

C. MOV R0, #10 is not allowed

D. Function label is missing

Solution

1. Step 1: Check the return method

   To return from a function, the program must jump to the address in LR, not to a fixed value in R0.

2. Step 2: Identify the problem with MOV PC, R0

   MOV PC, R0 sets the program counter to 10, which is not a valid return address, causing incorrect behavior.

3. Final Answer:

   MOV PC, R0 does not return to caller correctly -> Option B

4. Quick Check:

   Return must use LR, not arbitrary register [OK]

Hint: Return by jumping to LR, not arbitrary register [OK]

Common Mistakes:

• Thinking BL is invalid
• Assuming MOV R0, #10 is wrong
• Ignoring the need to use LR for return

5. You want to write a function in ARM that calls another function and then returns correctly. Which sequence correctly uses the link register (LR)?

hard

A. Use B to call the function, then return with BX R14

B. Use B to call the function, then return with MOV PC, LR

C. Use BL to call the function, then return with BX LR

D. Use BL to call the function, then return with MOV PC, R0

Solution

1. Step 1: Understand function call with BL

   BL saves the return address in LR and branches to the called function.

2. Step 2: Understand correct return instruction

   Returning is done by branching to LR using BX LR, which jumps back to the saved return address.

3. Step 3: Evaluate other options

   B does not save return address in LR; MOV PC, R0 returns to wrong place; BX R14 is equivalent to BX LR but using B (not BL) means LR is not saved.

4. Final Answer:

   Use BL to call the function, then return with BX LR -> Option C

5. Quick Check:

   BL + BX LR = correct call and return [OK]

Hint: Call with BL, return with BX LR always [OK]

Common Mistakes:

• Using B instead of BL for calls
• Returning with MOV PC, R0 instead of BX LR
• Confusing BX R14 and BX LR usage