---
title: "Inside the CPU: Fetch, Decode, Execute"
description: "The cycle a processor repeats: fetch the next instruction from memory, work out what it means, carry it out. Clock speed, cores and cache each change how fast that goes."
canonical: https://lightmysky.com/learn/computing/inside-the-cpu-fetch-decode-execute-mt_YT-FSWnUKR
source: https://lightmysky.com/learn/computing/inside-the-cpu-fetch-decode-execute-mt_YT-FSWnUKR.md
retrieved: 2026-09-12
---

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# Inside the CPU: Fetch, Decode, Execute

The cycle a processor repeats: fetch the next instruction from memory, work out what it means, carry it out. Clock speed, cores and cache each change how fast that goes.

Subject: Computing · Area: Computer Systems · Ages 15 to 16
Page: https://lightmysky.com/learn/computing/inside-the-cpu-fetch-decode-execute-mt_YT-FSWnUKR

## Ready when they can

- Put the three stages of the cycle in order and say what each does
- Explain what a higher clock speed buys, and what extra cores buy
- Explain why both instructions and data sit in memory while a program runs

## Lesson: Fetch it, read it, run it

Your processor repeats one cycle: fetch the next instruction from memory, decode what it means, then execute it. Fetch, decode, execute, and round again. That loop runs every app you open, billions of times a second.

**Example.** The ALU does the maths and comparisons during execute, once decode has spoken, using chains of AND, OR and NOT gates. Clock speed counts cycles per second, so 3 GHz means about 3 billion rounds each second.

A faster clock squeezes more rounds into each second, while extra cores run whole cycles side by side. Two cores are not always twice as fast, since some work refuses to split. Cache helps too: it shelves recent instructions nearby, and programs reuse the same bits so often that fetching flies.

Both instructions and data wait in memory during a run, since each fetch grabs the next order from there. Pushing clocks past stock speeds cooks extra heat, so speed always trades against cooling.

**Recap.** Fetch from memory, decode the meaning, execute the deed, and speed that loop with clocks, cores and cache.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Logic Gates and Truth Tables](https://lightmysky.com/learn/computing/logic-gates-and-truth-tables-mt_o2N0-q3A4Q)
- [Inside the Box: Input, Processing, Storage and Output](https://lightmysky.com/learn/computing/inside-the-box-input-processing-storage-and-output-mt_Oms8067Pnm)

## Opens up

- [Memory and Storage: RAM, ROM and the Backing Store](https://lightmysky.com/learn/computing/memory-and-storage-ram-rom-and-the-backing-store-mt_AiXGJioO_S)
- [Assembly Language and the Instruction Set](https://lightmysky.com/learn/computing/assembly-language-and-the-instruction-set-mt_bzmsfvdzwp)