Link register (R14/LR) in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Link Register (LR) in ARM architecture?

The Link Register (LR), also known as R14, is a special register in ARM processors that stores the return address when a function call is made. It helps the processor know where to continue after a function finishes.

beginner

How does the Link Register (LR) help during a function call?

When a function is called, the address of the next instruction (after the call) is saved in the LR. This allows the program to return to the correct place after the function completes.

intermediate

What happens to the Link Register (LR) during nested function calls?

In nested calls, the LR is often saved (pushed) to the stack before calling another function to avoid losing the return address. After the nested call finishes, the LR is restored to return correctly.

intermediate

Why is the Link Register (LR) important for efficient function calls?

Using the LR avoids the need to use memory to store return addresses for simple calls, making function calls faster and saving memory access time.

beginner

Which ARM register number corresponds to the Link Register?

The Link Register corresponds to register number 14, often written as R14 or LR.

What does the Link Register (LR) store in ARM architecture?

A The stack pointer address

B The current instruction being executed

C The return address after a function call

D The value of the program counter

The LR stores the return address so the program knows where to continue after a function finishes.

Which register number is the Link Register in ARM?

A R14

B R12

C R15

D R13

The Link Register is register number 14, called R14 or LR.

What must happen to the Link Register during nested function calls?

A It is ignored

B It is saved to the stack and restored later

C It is overwritten without saving

D It is cleared to zero

To keep the correct return address, LR is saved to the stack before nested calls and restored after.

Why is using the Link Register efficient for function calls?

A It avoids using memory for return addresses

B It speeds up arithmetic operations

C It stores function parameters

D It controls interrupts

LR stores return addresses in a register, avoiding slower memory access.

What happens if the Link Register is not saved during nested calls?

A Nothing, it works fine

B The program crashes immediately

C The function executes twice

D The return address may be lost, causing incorrect program flow

Not saving LR can overwrite the return address, leading to wrong program behavior.

Explain the role of the Link Register (R14/LR) in ARM function calls.

Think about how the processor remembers where to go back after a function.

Describe what must be done with the Link Register during nested function calls and why.

Consider what happens if multiple functions call each other.

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