A control unit drives the corresponding processing hardware by generating a set of signals that are in sync with the master clock. The two major operations performed by the control unit are instruction interpretation and instruction sequencing.
The control unit is a part of the Central Processing Unit (CPU). The CPU is divided into the arithmetic logic unit and the control unit. The control unit generates the appropriate timing and control signals to all the operations involved with a computer. The flow of data between the processor, memory, and other peripherals is controlled using the timing signals of the control unit.
The main function of a control unit is to fetch the data from the main memory, determine the devices and the operations involved with it, and produce control signals to execute the operations.
The functions of the control unit are as follows −
The processor contains several registers and special function registers for temporary storage purposes, in addition to the arithmetic logic unit and control unit. Program Counters (PC), Instruction Registers (IR), Memory Address Registers (MAR), and Memory Data Register (MDR) are special function registers.
The figure shows these special function registers. PC is one of the main registers in the CPU. The instructions in a program must be executed in the right order to obtain the correct results. The sequence of instructions to be executed is maintained by the PC.
The IR holds the instruction that is presently being executed. The timing signals generated by the control unit are based on the content of IR. The signals help in controlling the various processing elements that are necessary to execute the instruction.
The function of the other registers MAR and MDR is to transfer data. The address of the main memory to/from which data is transferred is stored in MAR. The data that is to be read/written from the specified address to the main memory is stored in MDR.