Interrupts: how the CPU notices something happened

By the end of this lesson

By the end you can explain why interrupts exist, list typical causes and applications, say when an interrupt is detected, and describe how one is handled using an Interrupt Service Routine (ISR).

Nobody wants to wait

A printer finishes a job. A key is pressed. The clock ticks.

Without interrupts, the CPU would have to keep asking every device "are you ready?" in a loop, wasting cycles — or worse, ignore them until the program chose to look.

An interrupt is a signal, sent to the CPU along the control bus, saying "something needs your attention now." The CPU pauses what it is doing, deals with the event, then picks up exactly where it left off.

Definition

Interrupt

A signal raised by a device or by the processor itself that causes the CPU to stop executing the current program temporarily, deal with the event, and then resume.

What causes one, and why they matter

Common causes:

a key press or mouse click;
a timer reaching zero (the basis of multitasking);
a device finishing its work (printer done, download complete);
new data arriving (network packet, sensor reading);
an error, such as a division by zero or an invalid instruction.

Applications the syllabus wants you to recognise:

instant response to the user, even while a long calculation runs;
sharing the processor between programs, using timer interrupts;
real-time control, where a sensor event must be handled within a fixed time.

Pause, save, serve, restore, resume.

Worked example: A key press mid-calculation

The CPU is halfway through a long loop when a key is pressed. Follow the sequence.

Have a go first?

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Apply

DefinitionQuestion 3 · Recall

Answer the 3 questions above to continue.