Eduqas / WJEC
1.1 The Central Processing Unit (CPU)
The Central Processing Unit (CPU) is the most important component in any computer system.
The purpose of the CPU is to process data and instructions by constantly repeating the fetch - decode - execute cycle.
The control unit directs the flow of dataand information into the CPU.
It also controls the other parts of the CPU.
ALU stands for ‘Arithmetic and Logic Unit’.
It performs simple calculations and logical operations.
The registers are temporary storage spaces for data and instructions inside the CPU.
The registers are used during the FDE cycle.
Five essential registers are explained in 1.2.
Cache memory is used to temporarily store data that is frequently accessed.
Cache memory is split into different levels. Level 1 and level 2 (L1 & L2) are usually within the CPU and level 3 (L3) is just outside it. See 1.3 and 1.5 for more information about cache.
You should know: The control unit is also known as the controller and cache memory is sometimes called internal memory.
The way a computer is designed and laid out is known as its architecture.
The most common type of computer architecture is Von Neumann.
Von Neumann Architecture
The CPU is the most important component in Von Neumann architecture as it is constantly fetching and decoding instructions from RAM and controlling the other parts of the system.
Von Neumann architecture also stores both instructions and data in memory. Being able to store programs in memory allows computers to be re-programmed for other tasks - this enables it to multitask and run several applications at the same time.
Data input and output is another key feature of this architecture.
An alternative architecture is Harvard, which features the control unit as the most essential component.
1.1 - The Central Processing Unit (CPU):
1a. What does 'CPU' stand for? 
1b. What is the purpose of the CPU? 
2a. Draw a diagram of the CPU, use the same symbols as shown on this page. 
2b. Label the four main components of the CPU. 
3. Describe the purpose of:
a. The Control Unit 
b. The ALU 
c. The registers 
d. Cache memory 
4a. Describe the key features of Von Neumann architecture. 
4b. Explain why storing data in memory is important. 
4c. State an alternative architecture.