We have added 6 new instructios. Still two codes are unused, which can be used for other purposes. We show it as NOP means No Operation.

We have seen that for ALU operation, instruction decoder generated the signal for appropriate ALU operation.

Apart from that we need many more signals for proper functioning of the computer. Therefore, we need a module, which is known as control unit, and it is a part of CPU. The control unit is responsible to generate the appropriate signal.

As for example, for LDAI instruction, control unit must generate a signal which enables the register A to store in data into register A.

One major task is to design the control unit to generate the appropriate signal at appropriate time for the proper functioning of the computer.

Consider a simple problem to add two numbers and store the result in memory, say we want to add 7 to 5.

To solve this problem in computer, we have to write a computer program. The program is machine specific, and it is related to the instruction set of the machine.