If is observed that most signals are not needed simultaneously and many signals are mutually exclusive.

As for example, only one function of the ALU can be activated at a time. In out case we are considering 16 ALU operations. Instead of using 16 different signal for ALU operation, we can group them together and reduce the number of control signal. From digital logic circuit, it is obvious that instead of 16 different signal, we can use only 4 control signal for ALU operation and them use a 4 X 16 decoder to generate 16 different ALU signals. Due to the use of a decoder, there is a reduction in the size of control word.

Another possibilities of grouping control signal is: A sources for data transfer must be unique, which means that it is not possible to gate the contents of two different registers onto the bus at the same time. Similarly Read Write signals to the memory cannot be activated simultaneously.

This observation suggests the possibilities of grouping the signals so that all signals that are mutually exclusive are placed in the same group. Thus a group can specify one micro operation at a time.

At that point we have to use a binary coding scheme to represent a given signal within a group. As for example, for 16 ALU function, four bits are enough to decode the appropriate function.

A possible grouping of the 46 control signals that are required for the above mention CPU is given in the Table 5.1.