This CPU contains four general purpose registers R0 , R1 , R2 and R3 . In addition there are three other register called SOURCES, DESTIN and TEMP. These are used for temporary storage within the CPU and completely transparent to the programmer. A computer programmer cannot use these three registers.

For the proper functioning of this CPU, we need all together 24 gating signals for the transfer of information between internal CPU bus and other resources like registers.

In addition to these register gating signals, we need some other control signals which include the Read, Write, Clear Y, set carry in, WMFC, and End signal. (Here we are restricting the control signal for the case of discussion in reality, the number of signals are more).

It is also necessary to specify the function to be performed by ALU. Depending on the power of ALU, we need several control lines, one control signal for each function. Assume that the ALU that is used in the design can perform 16 different operation such as ADD, SUBSTRACT, AND, OR, etc. So we need 16 different control lines.

The above discussion indicates that 46(24+6+16) distinct signals are required. This indicates that we need 46 bits in each micro instructions, therefore the size of control word is 46.

Consider the microprogram pattern that is shown for the Add instruction. On an average 4 to 5 bits are set to 1 in each micro instruction and rest of the bits are 0. Therefore, the bit utilization is poor, and there is a scope to improve the utilization of bit.