← IGCSE Computer Science Paper 1

Interrupt

A signal sent to the CPU telling it to pause its current task so something more urgent can be dealt with. The CPU then returns to the original task. It is not a crash.

Part of: IGCSE Computer Science 0478 — 4.1 Types of Software and Interrupts — Module 45

Understanding Interrupt in IGCSE Computer Science Paper 1

What it does: The interrupt itself is just a signal sent to the CPU. It is not a program, and it only asks the CPU to pause its current task for a while. Interrupts are a normal part of how a computer works. Once the interrupt is dealt with, the CPU continues the original task, so nothing has crashed. Interrupts let the CPU respond quickly to urgent events, such as input from a device, instead of ignoring them. The stop is temporary. The current task is not deleted, because the CPU needs to go back to it later. Interrupts happen all the time during normal use, for example when you move the mouse. They are not a sign of failure.

Common mistakes: An interrupt is a signal sent to the CPU, not a program. Line 1 should read: interrupt = signal to CPU. An interrupt does not make the computer crash. The CPU pauses, handles the interrupt and carries on: IF interrupt THEN handle it.

Class: _____________________  ·  Name: _____________________  ·  Date: _________

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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
Which statement correctly describes an interrupt?
A program that permanently stops the CPU from workingA signal sent to the CPU asking it to pause its current task
Matches
An interrupt is a program that runs inside the CPU
↓
An interrupt is a signal, not a program. The CPU may run a program (a routine) in response, but the interrupt itself is just the signal.
Applying
Meanings
Term
interrupt
Meaning
A signal sent to the CPU telling it to pause its current task so something more urgent can be dealt with. The CPU then returns to the original task. It is not a crash.
Practice
WHILE task is running DO
Testing
Gaps
currenttask
Check Answer
Spots
1interrupt = program run by CPU
2CPU pauses current task

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Quick Questions

1. Which statement correctly describes an interrupt?
A. A signal sent to the CPU asking it to pause its current task ✓
B. A program that permanently stops the CPU from working
An interrupt is a signal, not a program. It asks the CPU to pause so something more urgent can be dealt with.
2. What does the CPU normally do once an interrupt has been dealt with?
A. It returns to the task it was doing before ✓
B. It shuts down and has to be restarted
The stop is only temporary. After the interrupt is handled, the CPU carries on with the original task.
3. A student presses a key while a program is running and the CPU briefly pauses to deal with it. What has happened?
A. The computer has crashed
B. An interrupt was sent to the CPU ✓
Interrupts are a normal part of how computers work. They do not mean the computer has crashed.
4. Why does a computer use interrupts?
A. So that the CPU can run more slowly to save power
B. So that something more urgent can be dealt with straight away ✓
Interrupts let the CPU respond quickly to urgent events, such as input from a device, instead of ignoring them.
5. Which is true about an interrupt?
A. It stops the current task temporarily ✓
B. It deletes the current task from memory
The stop is temporary. The current task is not deleted, because the CPU needs to go back to it later.

Match the Pairs

interrupt → A signal sent to the CPU telling it to pause its current task so something more urgent can be dealt with
resume original task → After the interrupt is dealt with, the CPU goes back to the task it was doing before
An interrupt is a signal, not a program, that makes the CPU stop its current task temporarily → signal -> CPU: STOP current task
An interrupt does not mean the computer has crashed, because normal work continues afterwards → interrupt handled -> continue task
An interrupt is a program that runs inside the CPU → An interrupt is a signal, not a program. The CPU may run a program (a routine) in response, but the interrupt itself is just the signal.
interrupt received -> computer has crashed → An interrupt does not mean the computer has crashed. It is a normal signal, and the CPU returns to its original task afterwards.

Key Term

interrupt
A signal sent to the CPU telling it to pause its current task so something more urgent can be dealt with. The CPU then returns to the original task. It is not a crash.

Fill in the Blanks

1. IF interrupt = TRUE THEN PAUSE current_task ENDIF
Hint: Type the keyword for what the CPU temporarily does to its current task when an interrupt arrives.
2. IF interrupt = TRUE THEN PAUSE current_task ENDIF
Hint: Type the name of the signal sent to the CPU asking it to deal with something more urgent.
3. HANDLE interrupt RESUME original_task
Hint: Type the keyword that sends the CPU back to the task it was doing before the signal.
4. interrupt ← TRUE crashed ← FALSE
Hint: Type the Boolean value that shows an interrupt does not mean the computer has crashed.
5. IF urgent = TRUE THEN PAUSE task ENDIF
Hint: Type the keyword that starts the block of code run when the condition is true.

Spot the Error

Question 1
A interrupt = program run by CPU
B CPU pauses current task
C CPU deals with urgent event
D CPU returns to original task
Line A — An interrupt is a signal sent to the CPU, not a program. Line 1 should read: interrupt = signal to CPU.
Question 2
A WHILE computer is running
B CPU runs current task
C IF interrupt THEN crash
D ENDWHILE
Line C — An interrupt does not make the computer crash. The CPU pauses, handles the interrupt and carries on: IF interrupt THEN handle it.
Question 3
A Interrupt received: PC crashed
B CPU pauses current task
C CPU handles the interrupt
D CPU resumes original task
Line A — Receiving an interrupt is normal behaviour and does not mean the computer has crashed. The PC is fine.
Question 4
A Printer needs paper: interrupt
B CPU pauses game, handles it
C CPU never returns to the game
Line C — After dealing with the interrupt, the CPU goes back to the task it was doing. It should return to the game.
Question 5
A task = 'edit document'
B CPU runs task
C interrupt = True
D IF interrupt THEN RESTART PC
E RESUME task after handling
Line D — An interrupt makes the CPU pause the current task temporarily. It does not restart the computer. It should be IF interrupt THEN PAUSE task.

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