What is the CPU?

The CPU performs almost all the calculations that the computer needs. The CPU also controls the running of the operating system (Windows for example) and the application programs, such as spreadsheets, databases and word processing. The CPU is the most important item in a computer since it controls the overall speed and performance of your computer. The speed of a CPU is related to the frequency of its clock and is measured in Hz. However these speeds are so high that GHz are used to indicate speeds of CPU’s. (1 GHz = 1 GigaHertz = 109 Hertz). Typical speeds are now in the range of 1 to 3 GHz. The higher the speed of the CPU the faster your computer will run however the overall performance of your computer depends also on other hardware such as the RAM of your computer.

Below is shown the diagram of the CPU where the main parts are:

Diagram of a CPU.

Fig. 1 - What makes a CPU?

Arithmetic and Logic Unit (ALU)

It carries out all arithmetic calculations such as addition, multiplication, ... and also logic operations involving comparison of numbers. (greater than, less than, ...)


Data and results of calculations in the ALU are temporarily stored in registers. Data is accessed very quickly in the registers.

Control Unit

The control unit directs the flow of data between CPU, memory and the peripherals linked to the computer system. It is the most important item in a CPU.