Interrupts

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Interrupts

The CPU needs to know when a device needs attention. It does this by sending an interrupt.

An interrupt has a certain priority.

Interrupts can only take processor time from tasks with lower priority.

When an interrupt is raised the operating system runs the relevant interrupt service routine.

At the end of the fetch-decode-execute cycle, the processor checks if there is any interrupts. If there is an interrupt with a higher priority than the current task being carried out then these tasks are carried out:

The contents of the program counter and other registers are copied to a stack

The relevant interrupt service routine is then loaded by changing the program counter to where the ISR starts in memory

When the ISR is complete the previous values are restored from the stack and put in all the registers.

If, while an interrupt is being processed, a new higher priority interrupt is raised, the interrupt currently being processed is added to the stack