O was asked this question today and I can’t help but feel that the pupil wasn’t happy with the answer. So here I go again.

The Control Unit is a sub part of the processor (along with ALU & Registers). It is responsible for the sequencing, decoding and it also synchronizes the execution of the program instructions.

I am not sure that you will be happy with this description so I went to find some support notes and an animation ( I even found it in scratch) to help you to visualise it.

I did find this video though that covers a range of topics.

[kml_flashembed movie="http://www.youtube.com/v/HEjPop-aK_w" width="425" height="344" allowfullscreen="true" fvars="fs=1" /]

So to sum up, the Control Unit, controls all operations of the CPU, It also controls the flow of data along the buses.