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.

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{{Video|src=https://www.youtube.com/watch?v=Z5JC9Ve1sfI}}

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.

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- **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.

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**<i class="fas fa-pencil-alt"></i> Your turn!** Grab a pen (or a keyboard) and have a crack at these.

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**<i class="fas fa-film"></i> Question** ① — In your own words, describe the three steps the CPU repeats over and over. Use the words *fetch*, *decode*, and *execute*.

**<i class="fas fa-microchip"></i> 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?

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⚡ **Hint:** It's a big number. Count the zeros. (3 billion = 3,000,000,000.)

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**<i class="fas fa-question-circle"></i> 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?

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