These programs are called software emulators . Although software emulators allow the CPU to run incompatible programs they severely slow down the performance. The control unit directs and controls the activities of the internal and external devices. It interprets the instructions fetched into the computer, determines what data, if any, are needed, where it is stored, where to store the results of the operation, and sends the control signals to the devices involved in the execution of the instructions. While performing these operations the ALU takes data from the temporary storage area inside the CPU named registers .
Registers are a group of cells used for memory addressing, data manipulation and processing.
Some of the registers are general purpose and some are reserved for certain functions. It is a high-speed memory which holds only data for immediate processing and results of this processing. If these results are not needed for the next instruction, they are sent back to the main memory and registers are occupied by the new data used in the next instruction. Top of Page All activities in the computer system are composed of thousands of individual steps. These steps should follow in some order in fixed intervals of time.