Exception entry and exit sequence in ARM Architecture

Introduction

When something unexpected happens in a program, the processor needs to stop what it is doing and handle the problem safely. The exception entry and exit sequence is how the processor pauses normal work, deals with the issue, and then goes back to what it was doing.

When a hardware device needs attention, like a timer or keyboard input.

When the program tries to do something wrong, like dividing by zero.

When the system needs to switch from one task to another safely.

When an error or interrupt occurs and the processor must respond immediately.

Core Concept

ARM Architecture

Exception Entry:
1. Save current processor state (registers, program counter).
2. Switch to exception mode.
3. Disable or mask further interrupts if needed.
4. Jump to the exception handler address.

Exception Exit:
1. Restore saved processor state.
2. Switch back to the previous mode.
3. Resume normal program execution from where it stopped.

The processor automatically saves some information during exception entry to remember where it left off.

The exact steps can vary slightly depending on the ARM processor version and exception type.

Key Points

This shows how the processor handles an interrupt request (IRQ) by saving state and jumping to the handler.

ARM Architecture

Exception Entry Sequence:
- Save PC and CPSR to LR and SPSR.
- Switch to IRQ mode.
- Disable IRQ interrupts.
- Branch to IRQ handler address.

This sequence returns the processor to normal operation after handling the exception.

ARM Architecture

Exception Exit Sequence:
- Restore CPSR from SPSR.
- Restore PC from LR.
- Return to previous mode and resume execution.

Detailed Explanation

This pseudocode shows the basic steps the ARM processor follows when an exception occurs and how it returns to normal work after handling it.

ARM Architecture

/* Pseudocode for Exception Entry and Exit in ARM */

// Exception Entry
save_registers();          // Save important registers
switch_to_exception_mode(); // Change to exception mode
disable_interrupts();       // Prevent nested interrupts
jump_to_handler();          // Go to exception handler

// Exception Handler
handle_exception();         // Process the exception

// Exception Exit
restore_registers();        // Restore saved registers
switch_to_previous_mode();  // Return to original mode
resume_execution();         // Continue normal program

Important Notes

ARM processors use special registers like LR (Link Register) and SPSR (Saved Program Status Register) to save state during exceptions.

Some exceptions disable further interrupts to avoid conflicts while handling the current one.

Understanding this sequence helps in debugging and writing low-level system code.

Summary

Exception entry saves the current state and switches the processor to a special mode to handle the event.

Exception exit restores the saved state and returns the processor to normal operation.

This process ensures the system can respond to events safely without losing track of what it was doing.