KS3 · Computer Science

What the CPU does

Your computer can't think. Its 'brain', the CPU, just follows tiny instructions like 'add two numbers', one at a time. So how does it run a game? Watch.

Computing · Inside the computer

Watch a tiny program run

This is a made-up mini computer with 8 memory boxes. The grey numbers down the side are the boxes' addresses. Each box holds a pattern of 0s and 1s, and we've written what it means next to it.

Before you press Next: what number do you think ends up in box 8?

 1  0001 0110   LOAD box 6
 2  0010 0111   ADD box 7
 3  0011 1000   STORE in box 8
 4  0100 1000   OUTPUT box 8
 5  0000 0000   STOP
 6  0000 0111   the number 7
 7  0000 0101   the number 5
 8  0000 0000   empty (for the answer)

Box	What's in it	CPU is holding
1	LOAD box 6	—

Output

Step 1: The CPU starts at box 1 and reads what's there: 0001 0110. In our mini computer that means 'load the number from box 6'. It doesn't have the number yet, just the instruction.

1 / 8

Press Next and watch which box the CPU reaches into each time. Real CPUs use much longer patterns, but the idea is the same.

Step 1 of 8: The CPU starts at box 1 and reads what's there: 0001 0110. In our mini computer that means 'load the number from box 6'. It doesn't have the number yet, just the instruction..

Check your thinking

Is the 'brain' where things are kept?

People often call the CPU the 'brain' of the computer. That nickname leads a lot of people to a wrong idea about what it does.

Which is closest to what you think the CPU does with a program?

The CPU stores all my programs and files. It's the brain, so it remembers everything.The CPU copies the whole program inside itself, then runs it from there.The CPU carries out the program's instructions and processes the data. Main memory holds the instructions and data it's using.The CPU is clever. It works out for itself what the program ought to do.

How sure are you?

I'm sureI think soI'm guessing

The CPU stores all my programs and files. It's the brain, so it remembers everything.

This is a common misconception

The nickname is the trap here. The CPU doesn't keep your programs and files. Main memory holds the instructions and data being used right now, and secondary storage keeps things when the power is off. The CPU's job is to process data and carry out instructions.

The CPU copies the whole program inside itself, then runs it from there.

This is a common misconception

Think back to the mini computer. The program stayed in the memory boxes the whole time. The CPU reached into memory for one instruction at a time, using its address, and carried each one out.

The CPU carries out the program's instructions and processes the data. Main memory holds the instructions and data it's using.

This matches the evidence

Exactly right. Memory holds, the CPU does. The CPU reads instructions and data from main memory, works on them, and can write results back.

The CPU is clever. It works out for itself what the program ought to do.

This is a common misconception

It's actually the opposite. A CPU can't decide anything on its own. It follows simple instructions like 'add two numbers' or 'move data', one after another. It only seems clever because it does them so fast.

Memory holds the program and its data. The CPU carries out the instructions, one at a time.

Memory and storage

Reason it through

If the CPU works from main memory, why aren't your programs just kept there?

Link 1 of 4

First link · your turn

What happens to everything in main memory (RAM) when you switch the computer off?

Locked — reveal the link above first

The whole system

Input, process, output, storage

A computer system can be described as input, process, output and storage. Put each part into every set it belongs to. Some belong to two sets. Some belong to none.

A Input (sends data in)
B Output (presents results)
C Storage (keeps data for later)

Keyboard
Microphone
Monitor
Speakers
Touchscreen
Headset with a microphone
Hard drive
USB memory stick
CPU
Main memory (RAM)

How fast?

Guess the speed

A CPU has a clock speed of 3 GHz. How many instruction cycles can it carry out each second? Give your answer in millions.

Your estimate

2500 million

A CPU has a clock speed of 3 GHz. How many instruction cycles can it carry out each second? Give your answer in millions. Estimate between 0 million and 5000 million.

0 million5000 million

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How a computer follows a program: one simple instruction at a time, billions of times a second.

What you need to know

The CPU carries out a program's instructions and processes data. Main memory holds the instructions and data it is using.
A program is a list of simple instructions, stored as binary at numbered addresses and carried out one after another.
Programs live on secondary storage because RAM forgets when the power goes, and a CPU's speed is linked to its clock speed and cores.

The big picture

The CPU (central processing unit) processes data and carries out the instructions of programs. A program is a list of simple instructions, stored with its data as binary at numbered addresses in main memory. The CPU reads the instructions one after another, finds data by its address and can write results back. RAM is volatile, so programs live on secondary storage and are loaded into main memory when needed. The CPU and main memory are the process part of input, process, output and storage, and a CPU's speed is linked to its clock speed and cores.

Key points

1CPU stands for central processing unit. It processes data and carries out the instructions of programs.

2A program is a sequence of simple instructions, such as add two numbers, compare two values or move data.

3Instructions and data are stored in main memory as binary (0s and 1s). Every memory location has an address, so the CPU can find what it needs.

4The CPU reads instructions and data from main memory, carries out the instructions one after another, and can write results back to memory.

5RAM is volatile: it loses its contents when the power is off. Programs and files are kept on secondary storage and loaded into main memory when needed.

6Input devices send data in, the CPU processes it using main memory, output devices present the results, and storage keeps data for later.

7Clock speed is the number of instruction cycles a CPU can carry out each second, measured in hertz (1 GHz = 1 billion each second). Each core can carry out instructions independently.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

Two laptops have different CPUs. CPU A has a clock speed of 3 GHz and 2 cores. CPU B has a clock speed of 2 GHz and 4 cores. Which CPU can carry out more instruction cycles each second in total, if a program can keep every core busy?

⚠ Watch out

Thinking the CPU is where programs and files are kept, because it's called the 'brain'. The CPU carries out instructions. Main memory holds what is being used right now, and secondary storage keeps things when the power is off.

🧠

Memory hook

Storage KEEPS it, memory HOLDS it, the CPU DOES it.

✓

Check yourself

Without looking back, explain what happens when you open a program and it runs, using the words secondary storage, main memory, address and CPU. Then say why it can't just live in RAM.

Flashcards

(14)

What does the CPU do?

It processes data and carries out (runs) the instructions of programs.

What is a computer program?

A sequence of simple instructions, such as add two numbers, compare two values or move data, carried out one after another.

What does main memory (RAM) hold?

The program instructions and the data that the CPU is currently using.

Can the CPU change what's in main memory?

Yes. As well as reading instructions and data from main memory, it can write results back to it.

In what form are instructions and data stored in memory?

As binary digits: 0s and 1s.

What is a memory address?

The number of a memory location. It lets the CPU find the instruction or data it needs.

What does 'volatile' mean for RAM?

Its contents are lost when the power is switched off.

Where are programs and files kept when the computer is off?

On secondary storage, such as a hard drive, which keeps its contents without power.

What has to happen to a program before the CPU can run it?

It must be loaded from secondary storage into main memory.

Name the four parts of the input, process, output and storage picture.

Input sends data in, process works on it, output presents the results, and storage keeps data for later.

Which part of the input, process, output and storage picture do the CPU and main memory belong to?

Process: the CPU processes the data using main memory.

What does clock speed measure?

How many instruction cycles the CPU can carry out each second, measured in hertz (Hz).

How many cycles each second is 1 GHz?

1 billion (1,000,000,000) cycles every second.

What is a core, and why can more cores help?

A core can carry out instructions independently, so a CPU with more cores can carry out more instructions at the same time.

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.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

Watch a tiny program run

Is the 'brain' where things are kept?

Input, process, output, storage

Guess the speed

More KS3 Computer Science topics