Nested subroutine calls in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the stack behavior in nested subroutine calls

In ARM architecture, when a subroutine calls another subroutine, what happens to the return address of the first subroutine?

A It is saved directly to memory without using the stack.

B It is discarded because only the second subroutine's return address is needed.

C It is stored in a general-purpose register unrelated to the link register.

D It is stored in the link register (LR) before the second subroutine call.

💡 Hint

Think about how ARM preserves return addresses during nested calls.

In ARM, the link register (LR) holds the return address. Before calling another subroutine, the current LR is usually saved on the stack to preserve the return address of the first subroutine.

📋 Factual

intermediate

ARM instruction for nested subroutine call

Which ARM instruction is used to call a subroutine and save the return address for nested calls?

A BL (Branch with Link)

B B (Branch)

C MOV LR, PC

D PUSH {PC}

💡 Hint

Look for the instruction that branches and saves the return address automatically.

The BL instruction branches to the target address and saves the return address in the link register (LR), enabling nested subroutine calls.

🔍 Analysis

advanced

Stack frame management in nested calls

Consider a nested subroutine call sequence where Subroutine A calls Subroutine B, which calls Subroutine C. Which of the following best describes the stack frame behavior?

A Return addresses are stored in registers only; the stack is not used.

B Each subroutine pushes its return address and local variables onto the stack, creating separate stack frames.

C Only the first subroutine pushes data; others reuse the same stack frame.

D All subroutines share a single global stack frame without pushing return addresses.

💡 Hint

Think about how local variables and return addresses are preserved during nested calls.

Each subroutine creates its own stack frame by pushing return addresses and local variables onto the stack. This preserves the context for each call.

❓ Reasoning

advanced

Effect of missing stack push in nested calls

What is the likely outcome if a subroutine in a nested call sequence fails to save the link register (LR) on the stack before calling another subroutine?

A The return address of the calling subroutine is overwritten, causing incorrect return behavior.

B The called subroutine will automatically save the LR, so no problem occurs.

C The program will run normally without any issues.

D The CPU will generate an exception and halt execution.

💡 Hint

Consider what happens to the LR register during nested calls.

If the LR is not saved before a nested call, it gets overwritten by the new return address, causing the original subroutine to return to the wrong place.

❓ Comparison

expert

Comparing nested subroutine call mechanisms

Which statement correctly compares the use of the link register (LR) and the stack in managing nested subroutine calls in ARM architecture?

A The stack is never used for return addresses; LR alone manages all nested returns.

B LR is used only for the first call; subsequent calls use a dedicated return address register.

C LR holds the immediate return address, but for nested calls, return addresses are saved on the stack to preserve all return points.

D Return addresses are saved in general-purpose registers instead of LR or stack.

💡 Hint

Think about how ARM handles multiple nested calls and return addresses.

The LR holds the return address for the current subroutine. When a nested call occurs, the LR is saved on the stack to preserve the return address of the caller, allowing multiple nested returns.