Interrupts (Cambridge (CIE) O Level Computer Science)

Interrupts

What is an interrupt?

• An interrupt is a signal for the CPU so stop what it it currently doing and do something else as a higher priority

• The CPU is in a continuous loop of carrying out the fetch-decode-execute cycle, however there are occasions when this needs to be interrupted

How is an interrupt generated?

• An interrupt can be generated by hardware and software:

  • Hardware - this is caused by a hardware device such as a hardware failure

  • Software - this occurs when an application stops or requests services from the OS

• Interrupts are added to an area called the interrupt service routine

• The interrupt service routine holds instructions that will need to be fetched, decoded and executed to complete the commands of the interrupt

• The contents of the registers within the CPU cannot be lost by an interrupt, so contents are copied to a reserved area in RAM called a stack

• Contents are added to the top of the stack, which will save them for later retrieval when the interrupt is complete

• The interrupt will be executed instead of the original instructions

What are examples of hardware interrupts?

• Hardware

  • power button may have been pressed

  • moving the mouse

  • clicking an icon to open a new program

  • keyboard presses e.g. ctrl, alt, delete

What are examples of software interrupts?

• Software

  • a program is not responding

  • division by zero

  • two processes trying to access the same memory location