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.