6. CPU Components

You need to have a basic understanding of the three main parts of a CPU. These are the:

• Control Unit
• Immediate Access Store
• Arithmetic and Logic Unit (ALU)

The Control Unit

This has three main jobs:

1. It controls and monitors the hardware attached to the system to make sure that the commands given to it by the application software are used. For example, if you send something to print, the control unit will keep a check that the instructions are sent to the printer correctly.
2. It controls the input and output of data so that the signals go to the right place at the right time
3. It controls the flow of data within the CPU

The Immediate Access Store

This holds the data and programs needed at that instant by the Control Unit.

The CPU reads data and programs kept on the backing storage and stores them temporarily in the IAS's memory.

The CPU needs to do this because Backing Storage (e.g. the hard disk) is much to slow to be able to run the applications from it directly.

For example, lets pretend that a modern CPU was slowed down to carry out one instruction per second. In this scenario, the hard disk slowed down to an equivalent rate would take three months to supply the data needed by the CPU!

Arithmetic and Logic Unit

This is where the computer processes data either by manipulating it or acting upon it. It consists of two parts:

1. Arithmetic part - which does exactly what you think it should - it performs the calculations on the data e.g. 3 + 2 = 5
2. Logic part - this deals with logic and comparisons. For example, it works out if one value is greater, less than or equal to another.