YEAR 1 CO1 WEEK 24 INTERUPTS

More efficient way of finding out whether device needs CPU time using an interrupt. Interrupt is a signal indicating process of device needs attention.

Polling and interrupts

The purpose of the CPU is to process data and follow instructions. Also must be able to respond to events as they happen. Polling checks device if it needs to do anything which is inefficient but is simple and predictable.

Predictable and simple

Inefficient use of cpu

If Interrupt level lower/ equal to current process current process continues. If higher priority CPUs finishes current FDE cycle. The contents of CPUs registers copied to stack in memory. Location of appropriate interrupt service routine loaded into PC. When ISR complete previous contents popped from stack and loaded back into registers.

If an interrupt arrives which is higher priority it will move the lower priority to a stack but if after that a lower priority appears it is told to wait then after that if the initial task is more important then it will take it out of stank and do that first or the other way round

Stack over flow

Interrupt service routine

A key press eg CTRL/ALT /DEL

Run time error

Printer out of paper

Out of battery waring

Impending data loss

Delay between input and desired output