Introduction A general purpose computer consist of Input-output units, processing unit and a memory unit. Input-output permits interaction with the physical world whereas the memory unit stores user program and the binary information. The functional part of a general purpose digital computer, its functional units, the interaction between the control and the processor and the types of the control logic. Figure-1 on top left shows the function parts of a general purpose digital computer as CPU, input-output devices and the memory unit. Functional unit of a CPU is shown in figure 2 top right as ALU, set of registers and the timing and the control logic. Basic Function of the control logic : to generate timing sequences. to take input from the external sources and