Central Processing Unit (CPU) (Edexcel IGCSE ICT)

Central Processing Unit (CPU)

What is the purpose of the CPU?

• The purpose of the Central Processing Unit (CPU) is to execute instructions

• The CPU achieves this by completing processor cycles

• A modern CPU is capable of performing billions of processor cycles in one second

The processor 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

How is the speed of a processor measured?

• The speed of a processor is measured by it's clock speed

• Clock speed is measured in Hertz (Hz)

• The clock speed measures the number of processor 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

Number of cores

• A core works like it is its own CPU

• Multiple core processors mean they have multiple separate processing units that can fetch, decode and execute instructions at the same time

• Multi-core processors can run more powerful programs with greater ease

• Multiple cores increase the performance of the CPU by working with the clock speed

  • Example: A quad-core CPU (4 cores), running at a clock speed of 3Ghz

    • 4 cores x 3GHz

    • 4 x 3 billion instructions

    • 12 billion instructions per second