Fetch-Execute Cycle (AQA GCSE Computer Science)

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

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 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