The big idea: A device needs attention at some point. The CPU can find out two ways.
Polling — the CPU keeps asking the device "ready yet?". Interrupts — the device signals the CPU when it is ready.
The same key press under both methods. Watch what the CPU was doing before it noticed.
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Polling
- The CPU stops its own work to check, again and again
- Most checks find nothing, and that time is wasted
- Simple to write — no special hardware support needed
- Notices at a predictable moment: the next check
- Wasted checks drain a battery for no result
Interrupts
- The CPU works continuously until the device signals
- No time wasted when nothing is happening
- Needs hardware support and more complex code
- Arrival time is unpredictable — it depends on the device
- The CPU can sleep between events, saving power
What an interrupt actually does: The device raises a signal. The CPU finishes its current instruction, saves where it was, runs a short piece of code that deals with the device, then restores what it saved and carries on.
That saving and restoring is the overhead of an interrupt.
The rule of thumb: Rare events → interrupts. Polling would spend nearly every check finding nothing.
Very frequent events → polling can win. If the device is almost always ready, the check nearly always succeeds and you avoid the cost of saving and restoring each time.
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Keyboard and mouse — interrupts
A high-speed network card — polling
A battery device — interrupts
A real-time controller — polling
The security angle: Interrupts are raised by devices, so a faulty or malicious device can raise them constantly and leave the CPU with no time for anything else.
Polling cannot be flooded that way, because the CPU decides when to look.
How this is tested — you must EVALUATE — weigh both methods against the system's own demands and then decide. It comes up two ways:
Paper 1 Section A
- Evaluate or compare polling and interrupts, 4-6 marks
- Explain what happens when an interrupt occurs
- Recommend one for a stated device
Paper 1 Section B — case study
- Choose a method for a device in the case study
- Justify it from event frequency, power or timing
The classic trap: Writing "interrupts are better". They usually are for rare events — and they are the wrong answer for a constantly busy device, a system needing guaranteed timing, or one exposed to an untrusted device.
A battery-powered sensor wakes rarely to report a reading. A separate industrial controller must respond to a safety switch within a guaranteed time. Evaluate polling and interrupts for each device.
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