← IGCSE Computer Science Paper 1

Why computers use interrupts

A signal that tells the CPU an event needs attention, so it responds only when needed instead of constantly checking devices (polling). This frees the CPU and gives a quick response.

Part of: IGCSE Computer Science 0478 — 4.1 Types of Software and Interrupts — Module 46

Understanding Why computers use interrupts in IGCSE Computer Science Paper 1

What it does: The CPU carries on with its program and does not waste cycles checking devices, which is what an interrupt-driven system allows. When the interrupt signal arrives, the CPU deals with the event straight away, so urgent events get a quick response. Polling means the CPU keeps asking devices if they need attention, which wastes processing time. Interrupts avoid this. An interrupt is a signal sent to the CPU by a device or a program to say that something needs attention.

Common mistakes: Interrupts mean the CPU reacts only WHEN the key is pressed. Checking the keyboard every cycle is polling, which wastes CPU time. Line 1 should be WHEN key is pressed. With interrupts the device sends a signal to the CPU when it needs attention. It does not wait to be polled. Line 3 should be Printer sends an interrupt.

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This module's key terms are also in a printable crossword (Modules 390–408) and word search (Modules 390–418).

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Learning
Duo
Why do computers use interrupts instead of the CPU constantly checking every device?
The CPU only responds when a device needs attention, so it is free to run programsThe CPU must check every device after every instruction to stay safe
Matches
Interrupts make the CPU check the mouse every millisecond to see if it moved
↓
This describes polling, not interrupts. With interrupts the mouse sends a signal only when it moves, so the CPU does not keep checking.
Applying
Meanings
Term
interrupt
Meaning
A signal that tells the CPU an event needs attention, so it responds only when needed instead of constantly checking devices (polling). This frees the CPU and gives a quick response.
Practice
// Polling: CPU keeps checking the keyboard
Testing
Gaps
TRUE
Check Answer
Spots
1EVERY cycle check the keyboard
2 Keyboard sends interrupt

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Quick Questions

1. Why do computers use interrupts instead of the CPU constantly checking every device?
A. The CPU only responds when a device needs attention, so it is free to run programs ✓
B. The CPU must check every device after every instruction to stay safe
Interrupts let the CPU get on with running programs and respond only when an event actually happens.
2. Which statement describes what happens WITHOUT interrupts (constant checking, called polling)?
A. The CPU wastes time repeatedly checking whether a device needs attention ✓
B. The CPU is told straight away when a device needs attention
Polling means the CPU keeps asking devices if they need attention, which wastes processing time. Interrupts avoid this.
3. A user presses a key while a program is running. Why is an interrupt useful here?
A. The CPU has to stop the program and keep checking the keyboard until a key is pressed
B. The keyboard signals the CPU, which gives a quick response to the key press ✓
The keyboard sends an interrupt signal only when a key is pressed, so the CPU does not need to keep checking it.
4. Which is a benefit of using interrupts?
A. Urgent events have to wait until the CPU's next scheduled check
B. Urgent events get a quick response from the CPU ✓
An interrupt signals the CPU straight away, so urgent events are dealt with quickly instead of waiting to be noticed.
5. True or false: an interrupt is a signal that tells the CPU an event needs attention.
A. True ✓
B. False
An interrupt is a signal sent to the CPU by a device or a program to say that something needs attention.

Match the Pairs

while no_key_pressed: check_keyboard() → Polling: the CPU keeps checking the keyboard again and again, wasting time that could be used to run programs
CPU runs program; key pressed -> interrupt signal sent → An interrupt: the CPU keeps running programs and only responds when the keyboard signals that it needs attention
Interrupts free the CPU to run programs, because it does not have to keep checking each device → RUN program_A // no checking of devices needed
Interrupts give a quick response to an urgent event, such as a device needing attention immediately → IF interrupt_received THEN handle_event()
Interrupts make the CPU check the mouse every millisecond to see if it moved → This describes polling, not interrupts. With interrupts the mouse sends a signal only when it moves, so the CPU does not keep checking.
A printer has to wait until the CPU happens to check it, so the CPU is always busy with it → This is wrong because with interrupts the printer signals the CPU when it needs attention, so the CPU is not stuck checking and can run other programs.

Key Term

interrupt
A signal that tells the CPU an event needs attention, so it responds only when needed instead of constantly checking devices (polling). This frees the CPU and gives a quick response.

Fill in the Blanks

1. WHILE TRUE CHECK keyboard ENDWHILE
Hint: Type the loop keyword that makes the CPU keep checking a device over and over, which wastes CPU time.
2. // An interrupt alerts the CPU to an event
Hint: Type the name of the signal that tells the CPU a device or program needs attention.
3. // Constant checking of devices = polling
Hint: Type the term for the CPU repeatedly checking devices to see if they need attention.
4. // Interrupts give a quick response
Hint: Type the word describing how fast the CPU can react to an urgent event.
5. // The CPU is free to run other programs
Hint: Type the word that describes the CPU's state because it does not need to keep checking devices.

Spot the Error

Question 1
A EVERY cycle check the keyboard
B Keyboard sends interrupt
C CPU runs the keyboard ISR
Line A — Interrupts mean the CPU reacts only WHEN the key is pressed. Checking the keyboard every cycle is polling, which wastes CPU time. Line 1 should be WHEN key is pressed.
Question 2
A CPU runs a spreadsheet
B Printer is out of paper
C Printer waits to be polled
Line C — With interrupts the device sends a signal to the CPU when it needs attention. It does not wait to be polled. Line 3 should be Printer sends an interrupt.
Question 3
A Polling saves CPU time
B Interrupts only act on events
C So CPU can run other programs
Line A — Polling repeatedly checks devices, so it wastes CPU time. Interrupts avoid this. Line 1 should say polling wastes CPU time.
Question 4
A CPU is running a video editor
B Battery level becomes critical
C An interrupt waits its turn
Line C — A key benefit of interrupts is a quick response to urgent events. The CPU does not leave them waiting. Line 3 should say an interrupt gets a quick response.
Question 5
A No interrupt has happened
B CPU must wait for the device
C Interrupt signal arrives
D CPU then deals with the event
Line B — Until an interrupt arrives the CPU is free to run programs. It does not have to wait for the device. Line 2 should say the CPU is free to run programs.

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