How the CPU Works: The Fetch-Execute Cycle (KS3 Computing)

The CPU (Central Processing Unit) is the component that executes every instruction a program contains. It works by repeating a three-stage loop — fetch, decode, execute — billions of times per second, turning stored instructions into everything you see, hear and do on a computer.

What is the CPU and why is it called the "brain"?

The CPU is often called the brain of the computer, though the analogy only goes so far. Unlike a biological brain, the CPU does not think or learn — it simply follows instructions with extraordinary speed and precision.

The DfE national curriculum for computing asks KS3 students to understand "the hardware and software components that make up computer systems." The CPU is the central component: without it, no instruction is ever carried out. Everything else — RAM, storage, input and output devices — exists to feed the CPU with instructions and data, or to receive its results.

A modern CPU is a chip smaller than a postage stamp, yet it contains billions of transistors working together to process information.

What are the key components inside the CPU?

The CPU has three main internal parts:

Component | Abbreviation | Role
Control Unit | CU | Fetches and decodes instructions; coordinates all other components
Arithmetic Logic Unit | ALU | Performs calculations (addition, subtraction) and logical comparisons (AND, OR, NOT)
Registers | — | Tiny, ultra-fast memory locations inside the CPU that hold data being worked on right now

Key registers include:

Program Counter (PC) — holds the memory address of the next instruction to fetch.

Current Instruction Register (CIR) — holds the instruction currently being decoded and executed.

Accumulator (ACC) — holds the result of the most recent ALU calculation.

Think of the registers as the CPU's "hands" — they are where the CPU is literally holding the thing it is working on right now.

What is the fetch-execute cycle?

The fetch-execute cycle (sometimes called the fetch-decode-execute cycle) is the repeating loop the CPU uses to carry out every program instruction. It has three stages:

Stage 1 — Fetch

The Control Unit reads the memory address stored in the Program Counter (PC).

It fetches the instruction stored at that address from RAM.

The PC is incremented so it points to the next instruction.

Stage 2 — Decode

The fetched instruction is loaded into the Current Instruction Register (CIR).

The Control Unit decodes it — works out what type of instruction it is (e.g. add two numbers, jump to a different memory location, store a value).

Stage 3 — Execute

The appropriate component carries out the instruction.

If it is an arithmetic or logical operation, the ALU performs it.

If it is a memory operation, data is read from or written to RAM.

Then the cycle immediately begins again with the next instruction.

Worked trace — a simple addition:

Cycle | Stage | What happens
1 | Fetch | PC holds address 100; instruction at address 100 is loaded into CIR; PC becomes 101
1 | Decode | Instruction decoded as "load the value 5 into the accumulator"
1 | Execute | ACC ← 5
2 | Fetch | Instruction at address 101 is loaded into CIR; PC becomes 102
2 | Decode | Instruction decoded as "add the value 3 to the accumulator"
2 | Execute | ALU computes 5 + 3 = 8; ACC ← 8

After two cycles, the accumulator holds 8 — the result of adding 5 and 3.

What is clock speed, and does it matter?

The CPU has an internal clock that pulses at a fixed rate, measured in hertz (Hz). Each pulse is one cycle. A CPU running at 3 GHz (gigahertz) performs 3 billion cycles per second.

Higher clock speed generally means faster processing — but it is not the only factor. A CPU that executes more instructions per cycle (IPC) can outperform a higher-clocked rival. Clock speed also increases heat output and power consumption, which is why phone processors often run at lower frequencies to preserve battery life.

What are CPU cores?

A core is a complete fetch-execute unit. A dual-core CPU has two cores; an octa-core has eight. Each core can carry out its own fetch-execute cycle independently.

Number of cores | Common name | Rough use case
2 | Dual-core | Basic office and school laptops
4 | Quad-core | General-purpose desktops and gaming
8 | Octa-core | High-performance workstations and modern phones
16+ | Many-core | Video editing, 3D rendering, servers

Adding cores helps when multiple programs (or multiple threads within one program) need to run simultaneously. However, a single-threaded task — one that cannot be split into parallel jobs — only benefits from a faster clock speed, not more cores.

How does cache help the CPU?

RAM is fast, but reaching it still takes precious time from the CPU's perspective. Cache is a small bank of ultra-fast memory built directly into the CPU to solve this problem.

When the CPU fetches an instruction or piece of data from RAM, it stores a copy in cache. If it needs the same data again shortly, it finds it in cache (a cache hit) rather than travelling all the way to RAM (a cache miss). Cache hits are much faster.

Cache comes in levels: L1 (smallest, fastest, closest to the core), L2, and L3 (largest, slowest relative to L1, shared between cores).

Frequently asked questions

What does CPU stand for in KS3 computing?

CPU stands for Central Processing Unit. It is the primary chip in a computer that carries out program instructions. Every calculation, comparison, and data movement a computer performs passes through the CPU.

What is the fetch-execute cycle in simple terms?

The fetch-execute cycle is the repeating three-step process the CPU uses to run programs: fetch an instruction from memory, decode what it means, then execute it. The cycle repeats billions of times per second, which is how a computer runs an entire program.

What is the difference between the CPU and the GPU?

The CPU (Central Processing Unit) is a general-purpose processor with a small number of powerful cores, optimised for sequential tasks — running your operating system, a browser, or a game's logic. The GPU (Graphics Processing Unit) has thousands of smaller cores, optimised for performing the same simple calculation on huge amounts of data in parallel — perfect for rendering graphics or training AI models. Most computers have both.

What affects CPU performance at KS3?

Three main factors affect CPU performance: clock speed (how many cycles per second), number of cores (how many instructions can be handled in parallel), and cache size (how much fast on-chip memory is available). A CPU with a high clock speed, multiple cores, and generous cache will outperform one that is weak in all three areas.

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