Link register (R14/LR) in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence: The link register (R14) in ARM architecture stores the {{BLANK_1}} address during a function call.

ARM Architecture

The link register (R14) stores the [1] address during a function call.

Drag options to blanks, or click blank then click option'

Abase

Breturn

Cdata

Dstack

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the link register with the stack pointer.

Thinking it stores data instead of an address.

The link register (R14) holds the return address so the program knows where to continue after a function finishes.

2fill in blank

medium

Complete the sentence: When a function is called, the current program counter (PC) value is saved into {{BLANK_1}}.

ARM Architecture

When a function is called, the current program counter (PC) value is saved into [1].

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AR14

BR12

CR15

DR13

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing R13 (stack pointer) instead of R14.

Confusing R15 (program counter) with the link register.

The program counter (PC) value is saved into the link register (R14) to remember where to return after the function call.

3fill in blank

hard

Fix the error: The instruction 'BL {{BLANK_1}}' is used to branch to a function and save the return address in the link register.

ARM Architecture

BL [1]

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AR14

BLR

Clabel

DR13

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use a register name after BL instead of a label.

Confusing the link register name with the branch target.

The 'BL' instruction branches to a label (function address), saving the return address in the link register automatically. You do not specify registers here.

4fill in blank

hard

Fill both blanks to complete the sentence: The link register ({{BLANK_1}}) is used to store the {{BLANK_2}} address during a function call.

ARM Architecture

The link register ([1]) is used to store the [2] address during a function call.

Drag options to blanks, or click blank then click option'

AR14

BR13

Creturn

Dstack

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up R13 (stack pointer) with R14 (link register).

Thinking it stores the stack address instead of the return address.

R14 is the link register, and it stores the return address so the program knows where to continue after a function call.

5fill in blank

hard

Fill all three blanks to complete the explanation: The instruction 'MOV {{BLANK_1}}, {{BLANK_2}}' copies the value from the link register ({{BLANK_3}}) to another register.

ARM Architecture

MOV [1], [2]

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AR0

BLR

CR14

DR1

Attempts:

3 left

💡 Hint

Common Mistakes

Using R13 instead of R14 or LR as the link register.

Confusing source and destination registers.

This instruction moves the value from the link register (R14, also called LR) into register R0.

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