The Fetch-Decode-Execute Cycle

This lesson explains the fetch-decode-execute (FDE) cycle, which is the fundamental process by which a CPU processes every instruction in a program. This topic is essential for AQA and OCR GCSE Computer Science.

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Overview

The CPU does not process an entire program in one go. Instead, it works through instructions one at a time in a continuous loop called the fetch-decode-execute cycle. This cycle repeats billions of times per second in a modern computer.

The three stages are:

1. Fetch — retrieve the next instruction from memory.
2. Decode — interpret what the instruction means.
3. Execute — carry out the instruction.

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Stage 1: Fetch

During the fetch stage, the CPU retrieves the next instruction from main memory (RAM). Here is what happens step by step: