2. The Control Unit

Everything that happens to data inside a general purpose computer has to be  co-ordinated or it just won't work properly.

The Control Unit co-ordinates the work of the whole computer system.  It has three main tasks:

1. It follows a set of instructions called the computer 'program'.

2. It controls the hardware attached to the system.  The Control Unit monitors the hardware to make sure that the commands given to it by the current program are activated.

3. It controls the input and output of data, so all the signals go to the right place at the right time.

It is part of the Central Processing Unit (CPU)

The Control Units reads an single instruction from the computer program and carries it out (executes). Once that instruction is carried out, it moves on to the next one and so on. This often means that it has to fetch some data from memory (RAM) and then do some arithmetic on it with the help of the ALU.