Fetch-Execute Cycle (AQA GCSE Computer Science)

Revision Note

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Robert Hampton

Written by: Robert Hampton

Reviewed by: James Woodhouse

What is the purpose of the CPU?

  • The purpose of the Central Processing Unit (CPU) is to fetch, decode and execute instructions

  • The CPU is the brain of the computer and its job is to take an input, process data and produce an output

Fetch-Execute Cycle

What is the Fetch-Execute cycle?

  • The Fetch-Execute Cycle is the cycle that the central processing unit (CPU) runs through billions of times per second to make a computer work

  • The CPU is ‘the brain’ of a computer and is made up of components and registers

  • A computer takes an input, processes the input and then delivers an output for the user

    • Input: Clicking a button on the gamepad 

    • Process: The CPU inside the console follows a set of instructions to carry out the task

    • Output: The player moving on screen

the-cpu

The Fetch-Execute cycle stages

Fetch stage

  • During the fetch stage of the cycle, the next instruction or data must be fetched from the computer's memory (RAM)

  • The instruction or data is brought back to the CPU

Decode stage

  • During the decode stage of the cycle, the CPU needs to work out what is required from the instruction 

  • This could be a range of tasks depending on what the instruction or data included

Execute stage

  • During the execute stage of the cycle, the CPU will carry out the instruction that was fetched

  • Some examples that would take place at this stage are

    • Performing a calculation

    • Storing a result or data back in main memory (RAM)

    • Going to main memory to fetch data from a different location

The important things to remember are:

  • An instruction or data is fetched from memory

  • The instruction is decoded 

  • The instruction is executed

  • The cycle repeats billions of times per second

Worked Example

Identify two events that take place during the fetch-execute cycle. [2]

Answer

Any two of the following

  • An instruction is fetched from memory 

  • The instruction is decoded

  • The decoded instruction is executed

  • The instruction is transferred to the MDR

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Robert Hampton

Author: Robert Hampton

Expertise: Computer Science Content Creator

Rob has over 16 years' experience teaching Computer Science and ICT at KS3 & GCSE levels. Rob has demonstrated strong leadership as Head of Department since 2012 and previously supported teacher development as a Specialist Leader of Education, empowering departments to excel in Computer Science. Beyond his tech expertise, Robert embraces the virtual world as an avid gamer, conquering digital battlefields when he's not coding.

James Woodhouse

Author: James Woodhouse

Expertise: Computer Science

James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.