How can this encoding be done? To answer that, consider that there are a
total of K different internal and external control signals to be driven by the control
unit. In Wilkes’s scheme, K bits of the microinstruction would be dedicated
to this purpose. This allows all of the 2K possible combinations of control signals
to be generated during any instruction cycle. But we can do better than this if we
observe that not all of the possible combinations will be used. Examples include
the following: