Consider the part of control unit, its task is to generate the appropriate signal at right moment.
There is an instruction decoder in CPU which decodes this information in such a way that computer can perform the desired task
The simple model for the decoder may be considered that there is three input lines to the decoder and correspondingly it generates eight output lines. Depending on input combination only one of the output signals will be generated and it is used to indicate the corresponding operation of ALU.
In our simple model, we use three storage units in CPU,
Two -- for storing the operand and
one -- for storing the results.
These storage units are known as register.