Fetch-Execute Cycle (AQA GCSE Computer Science)
Revision Note
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 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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