Paper 1 - Computer Systems
Paper 2 - Computational Thinking, Algorithms, and Programming

Common CPU Components & CPU Registers

Whilst most of us can identify the CPU, it is also important to understand how the common CPU components work together to run instructions in the Fetch Decode Execute cycle.


Whilst it may appear that the CPU is just made up of tiny circuit boards, it actually has four main areas:

  • Arithmetic Logic Unit (ALU)
  • Control Unit (CU)
  • The Registers
  • Immediate Access Store

The CPU itself interacts with main memory. In the diagram below, you will see that the main memory (Cache & RAM) are smaller in capacity, but physically closer to the CPU than the secondary storage. This is one aspect that makes them faster.

Components of the CPU

Arithmetic Logic Unit

The ALU is responsible for the procesing of arithmetic (maths) and logical comparrisons within the CPU. When an instruction requires a logic gate to be used, this is the component that will produce the outcome.

Control Unit

The control unit acts as the manager of the CPU. It sends signals to tell the other components how to respond to the instructions that are recieved. The control unit is able to do this as it has it’s own bus for the signals called the control bus.


Registers are small pieces of memory inside the CPU that hold specific data. There are five main registers that allow the Fetch Decode Execute cycle to run alongside a number of general purpose cpu registers.

Immediate Access Store

The IAS is another name for main memory, or the umbrella term for the Cache & RAM together. This is where the instructions and currently running program and data are stored in order to run the computer system. The IAS is much faster than secondary storage allowing the CPU to keep up with the demands of the programs.

Activity – Notes

The Common CPU Components Notes provide a structured way for students to revise the areas covered in class in a visual way. Students should be encouraged to colour and add to the notes page being as creative as possible.