Exception entry and exit sequence in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Exception Entry Sequence

What is the first action performed by the ARM processor when an exception occurs?

A The processor disables all interrupts and continues executing the next instruction.

B The processor immediately jumps to the reset vector address without saving any state.

C The processor saves the current program counter (PC) and status register to a specific banked register.

D The processor clears all registers to zero before handling the exception.

💡 Hint

Think about how the processor preserves the current state before handling an exception.

When an exception occurs, the ARM processor first saves the current PC and status register to banked registers. This preserves the execution state so it can return after handling the exception.

📋 Factual

intermediate

Exception Vector Address

Where does the ARM processor jump to handle an exception after saving the state?

A To the address stored in the link register (LR).

B To a fixed exception vector address specific to the exception type.

C To the current program counter (PC) value plus 4 bytes.

D To the address stored in the stack pointer (SP).

💡 Hint

Exception handling starts at a known fixed location depending on the exception.

The ARM processor jumps to a fixed exception vector address that corresponds to the type of exception. This address contains the exception handler code.

🔍 Analysis

advanced

Exception Exit Sequence Behavior

What happens during the exception exit sequence in ARM architecture?

A The processor restores the saved PC and status register from banked registers and resumes normal execution.

B The processor disables interrupts permanently and halts execution.

C The processor jumps to the reset vector and restarts the program from the beginning.

D The processor clears all banked registers and resets the stack pointer before resuming execution.

💡 Hint

Consider how the processor returns to the point where the exception happened.

During exception exit, the processor restores the saved PC and status register from banked registers. This allows the program to continue from where it was interrupted.

❓ Comparison

advanced

Difference Between IRQ and FIQ Exception Entry

Which statement correctly describes a key difference in the entry sequence between IRQ and FIQ exceptions in ARM?

A FIQ uses more banked registers than IRQ, allowing faster context saving.

B IRQ disables all interrupts, while FIQ allows nested interrupts during handling.

C IRQ saves the PC to a general-purpose register, FIQ saves it to the stack.

D FIQ jumps to a different vector address than IRQ but uses the same banked registers.

💡 Hint

Think about how ARM optimizes fast interrupt handling.

FIQ mode has more banked registers than IRQ mode, which reduces the need to save and restore registers on the stack, making FIQ faster to enter and exit.

❓ Reasoning

expert

Effect of Incorrect Exception Exit Sequence

What is the most likely outcome if the ARM processor fails to restore the correct status register during exception exit?

A The processor will switch to user mode and disable all interrupts permanently.

B The processor will automatically reset to recover from the error.

C The processor will ignore the error and continue executing the exception handler indefinitely.

D The processor may continue execution with incorrect processor mode or interrupt settings, causing unpredictable behavior.

💡 Hint

Consider what the status register controls in the processor.

The status register controls processor mode and interrupt enable flags. If it is not restored correctly, the processor may run in the wrong mode or with interrupts disabled/enabled incorrectly, leading to unpredictable program behavior.