Tom Scott — The Fetch-Execute Cycle

A companion page to Workshop 1 Hardware and Networks. Inspiration video referenced in the Workshop 1 booklet as way2.cc/ca7.

When you opened the laptop in Workshop 1 and pointed at the CPU, you saw a quiet little square of silicon. That square is doing one thing, billions of times a second, and it's the same thing every CPU has done since the 1940s: fetch, decode, execute, repeat.

Tom Scott takes you inside that loop with one of the simplest computers ever made — a relay-based machine the size of a wardrobe — so you can actually see the cycle happen at human speed. After this video, the chip in your laptop won't feel mysterious anymore. It'll just feel very, very fast.

The Video

Tom Scott visits the Centre for Computing History to demonstrate the fetch-execute cycle on a giant electromechanical computer. Each step makes a clack sound, so you can literally hear instructions being processed one at a time.

Shorter alternative video: https://www.youtube.com/watch?v=pxRpDlHZ-NU

What to Watch For

• The three steps repeating forever — listen for the clacks. Fetch (grab the next instruction), decode (work out what it means), execute (do it). Every modern CPU still does exactly this, just billions of times a second instead of once a second.
• The program counter — watch the lights step through addresses. Your CPU has the same thing inside it, just shrunk down to a few transistors.
• Why it's a "cycle" — when one instruction finishes, the next one starts. There's no break. A CPU is never resting; it's either fetching, decoding, or executing.
• The size difference — the relay computer fills a wardrobe to do what your laptop CPU does in 1mm². Same logic, very different scale.

Have a Go

Question ① — In your own words, describe the three steps the CPU repeats over and over. Use the words fetch, decode, and execute.

Question ② — A modern laptop CPU runs at about 3 GHz, which means it does this cycle about 3 billion times a second. The relay computer in the video does it about once a second. How many cycles does your laptop do in the time the relay computer finishes one?

Question ③ — Why do you think Tom Scott chose to show the cycle on a relay computer instead of just opening up a modern laptop? What can you see on the relay computer that you can't see on a modern chip?

See also: Workshop 1 Hardware and Networks · GPU vs CPU

Last updated: April 2026.

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