Branch and link (BL) for subroutines in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What does the BL instruction do in ARM assembly?

In ARM assembly language, what is the primary function of the BL (Branch and Link) instruction?

A It jumps to a subroutine and saves the return address in the link register.

B It saves the current register values to memory before branching.

C It jumps to a subroutine without saving the return address.

D It performs a conditional branch based on the zero flag.

💡 Hint

Think about how the processor remembers where to return after a subroutine call.

The BL instruction branches to a subroutine and stores the return address in the link register (LR). This allows the program to return to the point after the BL instruction when the subroutine finishes.

📋 Factual

intermediate

Which register holds the return address after a BL instruction?

After executing a BL instruction in ARM, which register contains the return address to continue execution after the subroutine?

A Program Counter (PC)

B Link Register (LR)

C Stack Pointer (SP)

D Status Register (CPSR)

💡 Hint

It is a special register used to store return addresses.

The Link Register (LR) holds the return address after a BL instruction. This allows the program to return to the instruction following the BL call.

🔍 Analysis

advanced

What happens if the LR is overwritten before returning from a subroutine?

Consider a subroutine called using the BL instruction. What is the likely outcome if the Link Register (LR) is overwritten before the subroutine returns?

A The processor ignores the LR and uses the stack pointer to return.

B The program will return to the correct instruction after the subroutine.

C The subroutine will automatically save and restore LR, so no issue occurs.

D The program may jump to an incorrect address, causing unexpected behavior or crash.

💡 Hint

Think about what the LR stores and what happens if it loses that information.

If LR is overwritten before returning, the return address is lost or corrupted. This causes the program to jump to an incorrect location, often leading to crashes or unpredictable behavior.

❓ Comparison

advanced

Difference between BL and B instructions in ARM

Which statement correctly describes the difference between the BL and B instructions in ARM assembly?

A Both BL and B save the return address but BL is conditional.

B B branches and saves return address; BL branches without saving return address.

C BL branches and saves return address; B branches without saving return address.

D BL is used for loops; B is used for subroutines.

💡 Hint

One instruction is used for subroutine calls, the other for simple jumps.

The BL instruction branches to a subroutine and saves the return address in LR. The B instruction simply jumps to a target address without saving the return address.

❓ Reasoning

expert

Why must the LR be saved on the stack in nested subroutine calls?

In ARM assembly, when a subroutine calls another subroutine (nested calls), why is it necessary to save the Link Register (LR) on the stack?

A Because the LR will be overwritten by the nested BL instruction, losing the original return address.

B Because the stack pointer needs to be updated before returning.

C Because the processor automatically clears LR on nested calls.

D Because the LR cannot hold addresses for more than one subroutine.

💡 Hint

Consider what happens to LR when a subroutine calls another subroutine.

Each BL instruction overwrites LR with a new return address. To preserve the original return address during nested calls, LR must be saved on the stack before the nested call and restored afterward.

Practice

1. What does the BL instruction do in ARM architecture?

easy

A. It jumps to a subroutine and saves the return address in the link register.

B. It loads a value into a register.

C. It performs a bitwise AND operation.

D. It returns from a subroutine without saving the return address.

Solution

1. Step 1: Understand the purpose of BL

   The BL instruction is used to jump to a subroutine while saving the return address.

2. Step 2: Identify where the return address is saved

   The return address is saved in the link register (LR) to allow returning after the subroutine finishes.

3. Final Answer:

   It jumps to a subroutine and saves the return address in the link register. -> Option A

4. Quick Check:

   BL saves return address in LR [OK]

Hint: BL jumps and saves return address in LR [OK]

Common Mistakes:

• Confusing BL with simple jump instructions
• Thinking BL does not save return address
• Mixing BL with data load instructions

2. Which of the following is the correct syntax to return from a subroutine in ARM assembly?

easy

A. BX PC

B. MOV PC, LR

C. BX LR

D. BL LR

Solution

1. Step 1: Identify the instruction to return from subroutine

   Returning from a subroutine requires branching to the address stored in the link register (LR).

2. Step 2: Choose the correct instruction

   The instruction BX LR branches to the address in LR, effectively returning control.

3. Final Answer:

   BX LR -> Option C

4. Quick Check:

   Return uses BX LR [OK]

Hint: Return with BX LR to jump back [OK]

Common Mistakes:

• Using BX PC instead of BX LR
• Confusing BL with return instruction
• Using MOV PC, LR which is valid but less common

3. Consider this ARM assembly snippet:

BL subroutine
MOV R0, #0
B end
subroutine:
MOV R0, #1
BX LR
end:

What value will register R0 hold after execution?

medium

A. 1

B. 0

C. Undefined

D. The program will crash

Solution

1. Step 1: Follow the BL instruction

   The BL instruction jumps to subroutine and saves the return address (address of MOV R0, #0) in LR.

2. Step 2: Execute subroutine and return

   Inside subroutine, MOV R0, #1 sets R0 to 1, then BX LR returns to MOV R0, #0.

3. Step 3: Continue after BL

   MOV R0, #0 overwrites R0 to 0, followed by B end.

4. Final Answer:

   0 -> Option B

5. Quick Check:

   Post-return MOV R0, #0 overwrites subroutine value [OK]

Hint: BL returns to next instr MOV R0, #0 overwriting subroutine's R0=1 [OK]

Common Mistakes:

• Assuming R0 remains 1 after subroutine return
• Misidentifying the BL return address
• Thinking instructions after BL are skipped

4. Identify the error in this ARM assembly code snippet:

BL subroutine
MOV R0, #5
B end
subroutine:
MOV R0, #10
BX PC
end:

medium

A. No error; code is correct.

B. BL instruction is missing a label.

C. MOV R0, #5 should be before BL.

D. Using BX PC instead of BX LR to return causes incorrect return.

Solution

1. Step 1: Check return instruction in subroutine

   The subroutine uses BX PC to return, which branches to the current PC, not the saved return address.

2. Step 2: Understand correct return usage

   Correct return uses BX LR to jump back to the caller address saved by BL.

3. Final Answer:

   Using BX PC instead of BX LR to return causes incorrect return. -> Option D

4. Quick Check:

   Return must use BX LR, not BX PC [OK]

Hint: Return with BX LR, not BX PC [OK]

Common Mistakes:

• Using BX PC to return from subroutine
• Ignoring the link register usage
• Assuming BL needs no label

5. You want to call two subroutines in sequence using BL instructions. Which sequence correctly preserves the return addresses so both subroutines return properly?

hard

A. Use BL to first subroutine, then BL to second subroutine inside the first subroutine before returning.

B. Use BL to first subroutine, then BL to second subroutine without returning from the first.

C. Use two BL instructions one after another without returning in between.

D. Use BX LR to call subroutines instead of BL.

Solution

1. Step 1: Understand BL saves return address in LR

   Each BL saves the return address in LR, so calling BL twice in a row overwrites LR.

2. Step 2: Preserve return addresses by nesting calls

   Calling the second BL inside the first subroutine preserves the first return address until both return.

3. Step 3: Avoid overwriting LR by sequential BL calls without return

   Calling BL twice in sequence without returning overwrites LR, losing the first return address.

4. Final Answer:

   Use BL to first subroutine, then BL to second subroutine inside the first subroutine before returning. -> Option A

5. Quick Check:

   Nest BL calls to preserve return addresses [OK]

Hint: Nest BL calls to keep return addresses safe [OK]

Common Mistakes:

• Calling BL twice in a row without nesting
• Using BX LR to call subroutines
• Ignoring LR overwrite on multiple BL calls