Skip to the page
Year 8 · Autumn 1 · Computer Systems

The CPU and the fetch–decode–execute cycle

TheoryPresent ▶

What this lesson covers

Topic 8 Inside the computer

KS3 Computing · The CPU does the work, RAM holds what it's using now, and storage keeps everything long-term. The CPU repeats one loop: fetch, decode, execute.

Computer SystemsCambridge Lower Secondary Computing 0860

You will learn to

  • Identify the control unit, the ALU, the registers and the clock, and say what each one does.
  • Explain the stored-program idea: instructions and data sit in the same memory.
  • Describe the fetch–decode–execute cycle in order, using the PC, MAR and MDR.
  • Explain what the address bus and the data bus carry.

Success criteria

  • I can act out one cycle as the memory, the control unit or the ALU.
  • I can step the tiny CPU toy through fetch, decode and execute, and say what happens at each step.
  • I can say what the PC holds before and after a fetch.
  • I can work out that a 2 GHz clock ticks 2 billion times a second.

Key words

Say each word, then read what it means. Every one is in the glossary too.

Control Unit
The part of the CPU that fetches, decodes and directs.
ALU
Arithmetic Logic Unit: the CPU's calculator for maths and logic.
Registers
Tiny storage inside the CPU, like the PC or ACC.
Cache
Small, very fast memory on the CPU chip.
Clock speed
How many cycles a CPU does per second, in GHz.
Cores
One complete processing unit inside a CPU.
CPU
Central Processing Unit: the processor that runs instructions.
Register
Tiny storage inside the CPU, like the PC or ACC.
Bus
A set of wires that carries addresses, data or control signals between parts of the computer.
RAM
Random Access Memory: fast, volatile working memory.
Volatile
Loses its contents when the power goes off.
Embedded system
A small computer built into a device for one job.

The explanation

Read it here, or open the study page below to go one step at a time with things to try.

The parts of the CPU

  • Control Unit (CU): the manager. Fetches each instruction, works out what it means, tells the other parts what to do.
  • ALU: the calculator. Does maths (+, −) and logic (comparisons, AND, OR, NOT).
  • Registers: a few tiny, super-fast storage spaces inside the CPU.
  • Cache: small, very fast memory on the chip for things used often.
  • Clock: a steady pulse. Each tick lets the CPU take a step.

Fetch, decode, execute

Fetch

  1. The PC holds the next address, e.g. 20.
  2. Copy 20 into the MAR.
  3. Send it to RAM along the address bus.
  4. RAM sends back what's at address 20, along the data bus, into the MDR.
  5. The PC goes up by 1 (to 21).

Decode: the Control Unit works out what the instruction means.

Execute: do it. The ALU may calculate, data may move, the result goes to the ACC or memory. Then back to fetch.

★Remember

This loop runs billions of times a second.

What makes a CPU faster

  • Clock speed: cycles per second. 3.5 GHz = 3.5 billion ticks a second.
  • Cores: each core is a full processor. 4 cores can do 4 things at once, if the program can be split up.
  • Cache size: more cache = fewer slow trips to RAM.
!Watch out

Doubling the cores never quite doubles the speed. Not every job can be split.

Study it

Open the study page and go one step at a time.

Topic 8 · Year 8

Inside the computer

The CPU does the work, RAM holds what it's using now, and storage keeps everything long-term. The CPU repeats one loop: fetch, decode, execute.

The steps for this lesson

Try it

Check yourself

One question at a time. The reason shows after each answer.

Saved on this device.