Link register (R14/LR) in ARM Architecture

Introduction

The link register (R14 or LR) is used to store the return address when a function or subroutine is called. It helps the processor know where to go back after finishing the function.

When calling a function or subroutine in ARM assembly.

When you want to return to the instruction after a function call.

When saving the return address before branching to another code section.

When implementing nested or recursive function calls.

When debugging to track where the program will return after a call.

Core Concept

ARM Architecture

BL label  ; Branch with link to 'label', saves return address in LR
MOV PC, LR ; Return from function by jumping to address in LR

BL instruction saves the next instruction address in LR before jumping.

To return from a function, the program counter (PC) is set to the value in LR.

Key Points

This calls 'my_function' and saves the return address in LR.

ARM Architecture

BL my_function
; LR now holds address after BL instruction

This returns from the function by jumping back to the saved address in LR.

ARM Architecture

MOV PC, LR
; Return to the instruction after the function call

Save LR to stack to preserve return address during nested calls.

ARM Architecture

PUSH {LR}
; Save LR on stack before calling another function

Restore LR from stack and return to caller.

ARM Architecture

POP {LR}
MOV PC, LR
; Restore LR and return from function

Detailed Explanation

This program calls 'function' using BL, which saves the return address in LR. The function sets R0 to 42 and returns by moving PC to LR. After returning, the program sets R0 to 0 and ends.

ARM Architecture

start:
    BL function
    ; After function returns, continue here
    MOV R0, #0
    B end

function:
    ; Do some work here
    MOV R0, #42
    MOV PC, LR

end:
    ; Program end

Important Notes

LR is automatically updated by BL and BLX instructions.

Always save LR if you call another function inside your function to avoid losing the return address.

Using LR helps avoid using memory for return addresses, making function calls faster.

Summary

The link register (R14/LR) stores the return address during function calls.

BL instruction saves the return address in LR before jumping.

Returning from a function is done by moving PC to LR.

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