Interrupts and Exceptions

Exceptions and interrupts are unexpected events which will disrupt the normal flow of execution of instruction(that is currently executing by processor). An exception is an unexpected event from within the processor. Interrupt is an unexpected event from outside the process.

Whenever an exception or interrupt occurs, the hardware starts executing the code that performs an action in response to the exception. This action may involve killing a process, outputting an error message, communicating with an external device, or horribly crashing the entire computer system by initiating a “Blue Screen of Death” and halting the CPU. The instructions responsible for this action reside in the operating system kernel, and the code that performs this action is called the interrupt handler code. Now, We can think of handler code as an operating system subroutine. Then, After the handler code is executed, it may be possible to continue execution after the instruction where the execution or interrupt occurred.

Exception and Interrupt Handling :
Whenever an exception or interrupt occurs, execution transition from user mode to kernel mode where the exception or interrupt is handled. In detail, the following steps must be taken to handle an exception or interrupts.

While entering the kernel, the context (values of all CPU registers) of the currently executing process must first be saved to memory. The kernel is now ready to handle the exception/interrupt.

  1. Determine the cause of the exception/interrupt.
  2. Handle the exception/interrupt.

When the exception/interrupt have been handled the kernel performs the following steps:

  1. Select a process to restore and resume.
  2. Restore the context of the selected process.
  3. Resume execution of the selected process.

At any point in time, the values of all the registers in the CPU defines the context of the CPU. Another name used for CPU context is CPU state.

The exception/interrupt handler uses the same CPU as the currently executing process. When entering the exception/interrupt handler, the values in all CPU registers to be used by the exception/interrupt handler must be saved to memory. The saved register values can later restored before resuming execution of the process.

The handler may have been invoked for a number of reasons. The handler thus needs to determine the cause of the exception or interrupt. Information about what caused the exception or interrupt can be stored in dedicated registers or at predefined addresses in memory.

Next, the exception or interrupt needs to be serviced. For instance, if it was a keyboard interrupt, then the key code of the key press is obtained and stored somewhere or some other appropriate action is taken. If it was an arithmetic overflow exception, an error message may be printed or the program may be terminated.

The exception/interrupt have now been handled and the kernel. The kernel may choose to resume the same process that was executing prior to handling the exception/interrupt or resume execution of any other process currently in memory.

The context of the CPU can now be restored for the chosen process by reading and restoring all register values from memory.

The process selected to be resumed must be resumed at the same point it was stopped. The address of this instruction was saved by the machine when the interrupt occurred, so it is simply a matter of getting this address and make the CPU continue to execute at this address.

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