CPU Components & Their Function (Edexcel GCSE Computer Science)

Fetch-Decode-Execute Cycle

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-Decode-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

• 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-Decode-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

CPU Components & Their Function

What are the components of the CPU?

• The CPU is made up of 6 key components

  • Arithmetic Logic Unit (ALU)

  • Control Unit (CU)

  • Cache

  • Registers

  • Clock

  • Buses

• Each of the components sits within the CPU

What is the function of each component?

Arithmetic logic unit (ALU)

• Performs arithmetic operations

• Performs logical decisions
  

  • IF X > 5 THEN
          DO ……….

Control unit (CU)

• Coordinates how data moves around the CPU by sending a signal to control the movement of the data 

• Decodes the instructions fetched from memory

Registers

• Extremely small, extremely fast memory located in the CPU

• Hold small amounts of data needed as part of the fetch-execute cycle

• Each register has its own specific purpose

Cache

• Very small, very fast memory located in the CPU which is used to provide quick access to frequently used instructions and data

Clock

• Synchronises the actions of the CPU, with each tick of the clock triggering an operation

• The clock speed is measured in Hertz (Hz)

• The clock speed measures the number of fetch-decode-execute cycles that can take place in 1 second

• The faster the clock speed, the more instructions can be fetched and executed per second

• Modern computers have a clock speed in Gigahertz (GHz), meaning billion

• A clock speed of 3.5GHz can perform up to 3.5 billion instructions per second

Buses

• A bus is a set of parallel wires through which data/signals are transmitted from one component to another

• The wider a bus, the more parallel wires it has, which determines how many addressable memory locations there are

• There are 3 types of bus:

  • Address - unidirectional, carries addresses data is written to/read from

  • Data - bidirectional, carries data or instructions

  • Control - bidirectional, carries commands and control signals to tell components when they should be receiving reads or writes etc.