Central Processing Unit (CPU) (Edexcel IGCSE Ict)

Spec point — 1.7.1

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

Worked Example Describe how the speed of the processor affects a users experience when playing a game

[4]

Answer

A description to include four from:

• Faster processors fetch/decode/execute more instructions/data / have more cycles [1] per second [1]

so:

• graphics render more quickly / at greater resolution [1]
• making the visuals smoother / graphics can be displayed at greater resolution [1]
• making the environment more realistic [1]
• more actions can be carried out [1]
• making the gameplay more immersive/exciting [1]