Interrupts

Polling repeatedly checks a status bit. Interrupts let a device notify the program when something happened.

Mechanism	Strength	Cost
Polling	simple, predictable control flow	wastes work when events are rare
Interrupts	efficient for sporadic events	requires event-loop structure

The original PC path used programmable interrupt controllers and IRQ lines. Modern controllers differ, but the idea is recognizable: subscribe, receive an event, do minimal handler work, return to main logic.

Polling is not wrong. It can be simplest when the program has nothing else to do or expects an event immediately, but it scales poorly when the CPU could work while the device is quiet.

Handler discipline

A handler should acknowledge or record the event, update a small counter or flag, and avoid rendering, parsing, allocation, or blocking calls. The handler learns that an event happened; the application loop reacts to it.

This separation keeps the program understandable once several devices are involved. In Lab 3, keyboard and timer events arrive through the same loop, and each handler must leave enough information for the main logic.

IMPORTANT

A timer interrupt is not a license to run the whole program inside the handler. Keep the handler short.