Polling and interrupts
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Flip to reveal answersWhat is the difference between polling and interrupts?
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Question
What is the difference between polling and interrupts?
Answer
With polling the CPU repeatedly asks the device whether it is ready. With interrupts the device signals the CPU when it is ready, so the CPU can work uninterrupted until then.
Question
What happens when an interrupt occurs?
Answer
The CPU finishes its current instruction, saves where it was, runs a short routine that deals with the device, then restores the saved state and carries on. That saving and restoring is the overhead.
Question
When is polling the better choice?
Answer
When the device is almost always ready, so nearly every check succeeds and you avoid saving state thousands of times a second; and when a guaranteed response time is needed, since the delay is bounded by the checking interval.
Question
Why do battery-powered devices use interrupts?
Answer
Because the CPU can sleep between events, which uses very little power. Polling forces it awake to run checks that mostly find nothing, shortening battery life for no benefit.
Question
What is the security concern with interrupt handling?
Answer
Interrupts are raised by devices, so a faulty or malicious device can raise them constantly and leave the CPU no time for other work. Polling cannot be flooded, because the CPU decides when to look.
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Topic 1.3 hub
Operating and control systems
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