Central Processing Unit (CPU) (Edexcel IGCSE ICT)
Revision Note
Written by: Robert Hampton
Reviewed by: James Woodhouse
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]
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