Fetch-Execute Cycle (AQA GCSE Computer Science)

Revision Note

Test yourself

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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