📚 Knowledge Library — Topic 4.3 — Software

Interrupts Explained Simply

Understand how an interrupt tells the processor that something needs attention, without diving too deeply into queues, stacks or full operating system handling.

Start LearningWorked Example
1. Invitation

Computers need a way to get attention.

Imagine typing while a video is playing. The computer needs to notice your key press without stopping everything else forever.

An interrupt is a signal that tells the processor something needs attention.

💡 Remember: an interrupt signals the processor, not the user.
Figure 1.1
Attention Signal
Keyboard press
↓
Interrupt signal
↓
Processor responds
2. Big Idea

An interrupt is a signal to the CPU.

The CPU is the Central Processing Unit. It is busy processing instructions all the time.

An interrupt is like saying, “Something needs attention now.” The processor can then decide whether to deal with it straight away.

💡 Key idea: interrupt = signal to the CPU that something needs attention.
Figure 2.1
CPU Attention
Current task
+
Interrupt arrives
↓
CPU checks it
3. FutureLogic Bridge

Think of a phone call during work.

You are writing a report. Your phone rings.

If the call is important, you pause your report, answer the call, then return to where you left off. That is the idea behind an interrupt.

💡 Bridge: an interrupt is like a phone call that may need your attention.
Figure 3.1
Phone Call Model
Writing report
↓
Phone rings
↓
Answer or ignore
4. Worked Example

A keyboard press creates an interrupt.

When a key is pressed, the keyboard can send a hardware interrupt.

The processor is told that input has arrived and can respond to it.

Simple example

Key pressed
Interrupt sent
CPU responds
Model answer: "An interrupt is a signal sent to the processor to show that something requires its attention."
Figure 4.1
Keyboard Interrupt
Keyboard
→
Interrupt
→
Processor
5. Types of Interrupt

Interrupts can come from hardware or software.

A hardware interrupt comes from a physical device, such as a keyboard, mouse or printer.

A software interrupt comes from a program, such as a division by zero error.

TypeExample
Hardware interruptKeyboard key pressed
Software interruptDivision by zero
Figure 5.1
Two Sources
Hardware
keyboard / mouse

+

Software
program error
6. ISR

The ISR handles the interrupt.

The ISR stands for Interrupt Service Routine.

It is the routine that knows how to deal with a particular interrupt. The Learning Hub will explain the full process in detail.

💡 For now, just remember: ISR = Interrupt Service Routine.
Figure 6.1
Interrupt Handler
Interrupt arrives
↓
ISR runs
↓
Problem handled
7. Exam Tip

Say processor, not user.

Many students write that an interrupt tells the user something has happened.

That is not precise enough. The interrupt is sent to the processor.

🎯 Exam Tip: write "processor" or "CPU" in your answer.
Figure 7.1
Who Gets the Signal?
Interrupt
→
Processor / CPU

Not the user
8. Common Mistake

Replace vague answers with precise ones.

Students often understand the idea, but lose marks because the answer is too vague.

⚠️ Common Mistake:

Student answer: "An interrupt tells the user there is a problem."

❌ This is wrong because the interrupt signals the processor, not the user.

Better answer: "An interrupt is a signal sent to the processor to show that something requires its attention."
Figure 8.1
Better Exam Wording
Weak answer
tells the user

↓

Better answer
signals processor
9. Summary

Interrupts in one screen.

An interrupt is a signal sent to the processor when something needs attention.

Interrupts can be caused by hardware or software. The ISR is the Interrupt Service Routine that handles the interrupt.

The detailed queue, stack and priority process will be explored in the Learning Hub.

💡 Key idea: interrupts help the computer respond to events while other work is happening.
Figure 9.1
Interrupt Summary
Event happens
↓
Interrupt sent
↓
Processor responds