KS3 · Computer Science

The input-process-output model

Scan a tin of beans at a self-checkout and the price pops up in under a second. The scanner didn't work it out. Neither did the screen. So who did?

Follow one tin of beans through a self-checkout

Stage 1 of 4: Input — the barcode is scanned. paused

Input — the barcode is scanned · 1/4drag the dot →

The scanner captures the barcode as a number and sends it into the till. That's its whole job — it has no idea it's looking at beans.

Drag the dot along the path, or press Play, and watch where the data goes next.

Where does the work happen?

You press G. A G appears. Who worked that out?

You're typing a message. You press the G key and a G appears on the screen straight away.

Which is closest to what you think right now?

The keyboard does it — it knows which key I pressed, so it works out the G.The CPU does it — it follows a program's instructions to work on the data from the keyboard.The screen does it — it decides what picture to show.The keyboard and the screen share it — one half each, with nothing in between.

How sure are you?

I'm sureI think soI'm guessing

The keyboard does it — it knows which key I pressed, so it works out the G.

This is a common misconception

The keyboard is an input device: it captures your key press and sends that data into the computer. Think of it as a messenger, not a thinker — it hands the data over and decides nothing about what happens next.

The CPU does it — it follows a program's instructions to work on the data from the keyboard.

This matches the evidence

Exactly. The CPU (the processor) follows the instructions of a program to work on the input data. Here it works out that a G should appear; in other programs the same kind of work means calculating, comparing, sorting or changing data.

The screen does it — it decides what picture to show.

This is a common misconception

The screen is an output device: it presents a result that has already been worked out somewhere else. It shows what it is sent — the deciding happened before the data ever reached it.

The keyboard and the screen share it — one half each, with nothing in between.

This is a common misconception

That leaves out the middle of the journey. The keyboard's data goes in, the CPU processes it, and only then does the screen have a result to show. Input and output sit either side of the processing; they don't do it between them.

Input devices capture the data. Output devices present the result. The processing happens in between, in the CPU, by following a program.

Sort the devices

Input, output or storage?

Tick every group each device belongs to. Watch out: one device belongs in two groups, and one belongs in none of them.

A Input device
B Output device
C Storage device

Keyboard
Sensor
Camera
Monitor
Speakers
Printer
Motor
Touchscreen
Hard disk drive
Solid state drive
USB flash drive
CPU (processor)

Two kinds of "memory" — don't mix them up

Secondary storagevsMain memory

People say "memory" for both. They do very different jobs.

Focus

When the power goes off

Secondary storage

Keeps its data and programs

Main memory

Isn't where things are kept for later

The insight

This is the big one. Work only reaches secondary storage when it's saved — anything you haven't saved isn't kept once the computer is switched off.

What it holds

Secondary storage

Data and programs you might want again another day

Main memory

Only the data and instructions currently in use

Its part in the model

Secondary storage

Supplies data to processing and saves results to use again

Main memory

Holds what the CPU is using while it processes

A picture to remember it by

Secondary storage

Your school bag — things stay in it until you need them

Main memory

Your desk — only what you're working on right now

Your turn

Describe a system you haven't met in this lesson

You press a button to make your character jump. On the screen the character jumps, and the speaker plays a "boing". You switch the console off, and the next day your progress is still there.

Use the input, process, output and storage model to describe what happens in this handheld games console. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Input: the button captures the press and sends it into the console as data. (1 mark)
Process: the CPU follows the game's program to work on that data, e.g. working out the character's new position. (1 mark)
Output: the screen shows the jump and the speaker plays the sound. (1 mark)
Storage: the progress is saved to secondary storage, which keeps it while the power is off. (1 mark)
Storage supplying data: next time, the saved progress is supplied from storage to the process stage so the game can carry on. (1 mark)

Model answer

The input is the button press, which the button captures and sends into the console. The CPU processes it by following the game's program — it works out that the character should jump and where it moves to. The output is the screen showing the jump and the speaker playing the "boing". The progress is saved to secondary storage, which keeps it while the console is switched off, and next time the saved progress is supplied from storage so the game can carry on.

Four questions — what goes in, what's done to it, what comes out, what's kept — describe a system you've never seen before.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Four questions that describe any computer system: what goes in, what's done to it, what comes out, and what's kept.

What you need to know

Any computer system can be described by four stages: input, process, output and storage.
Input devices capture data and send it into the system; output devices present or act on the results.
The CPU does the processing by following a program's instructions — calculating, comparing, sorting or changing data.
Secondary storage keeps data and programs when the power is off; main memory holds only the data and instructions currently in use.

The big picture

The input-process-output model describes how the parts of a computer system work together. Every computer system — a till, a phone, a games console — can be described in four stages: input, process, output and storage. Input devices capture data and send it in. The CPU processes it by following a program's instructions. Output devices present or act on the result. Storage keeps data and programs for later, and it can supply data to processing as well as save its results. Some devices, like a touchscreen, do two jobs, and secondary storage is not the same thing as main memory.

Key points

1Data moves through the stages in order: input, then process, then output.

2Storage works with the process stage both ways — it supplies data and it saves results.

3Hardware (the devices) and software (the programs) work together, and this model describes how.

4Some devices do two jobs: a touchscreen displays output and takes input from your touch.

5The CPU isn't an input, output or storage device — it is the process stage.

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Worked example

Problem

You open the calculator app on a phone, tap 7 × 6 =, and 42 appears. Describe this using input, process, output and storage.

⚠ Watch out

Thinking storage always comes last. Storage can save the results of processing, but it can also supply data to the process stage — like the price list a till looks up, or a game you saved yesterday.

🧠

Memory hook

Capture → Crunch → Show, and Save for later. Input captures, the CPU crunches, output shows, storage saves.

✓

Check yourself

You take a photo on a phone, and it's still in your gallery tomorrow. Name the input, the process and the output — then say why the photo is still there.

Flashcards

(14)

What four questions can describe any computer system?

What goes in, what's done to it, what comes out, and what's kept.

What is input?

Data entered into, or collected by, the computer system.

What does an input device actually do?

It captures data and sends it into the system — it doesn't process it.

Which part of a computer does the processing?

The CPU (processor).

What does the CPU follow when it processes data?

The instructions of a program.

Give four kinds of work processing can do to data.

Calculating, comparing, sorting or changing it.

What is output?

The result of processing, presented to the user or passed to another device.

Does output always mean something on a screen?

No — output devices present OR act on results, e.g. a motor that makes something move, or speakers that play sound.

What is storage for?

Keeping data and programs so they can be used again later.

What happens to data in secondary storage when the power is switched off?

It is kept — that's why saved files are still there next time.

What does main memory hold?

Only the data and instructions currently in use.

Name the two ways storage connects to the process stage.

It supplies data to processing, and it saves the results of processing.

Does data reach the output stage before or after it is processed?

After. There's nothing to present until the CPU has worked on the data.

Why is a touchscreen both an input and an output device?

It displays output and takes input from the user's touch.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Follow one tin of beans through a self-checkout

You press G. A G appears. Who worked that out?

Input, output or storage?

Describe a system you haven't met in this lesson

More KS3 Computer Science topics