 A control unit is the part of a CPU or other device that directs its operation. The outputs of the unit control the activity of the rest of the device. A control unit can be thought of as a finite state machine.
The control unit is the circuitry that controls the flow of data through the processor, and coordinates the activities of the other units within it. In a way, it is the 'brain within the brain', as it controls what happens inside the processor, which in turn controls the rest of the PC.
At one time control units for CPUs were ad-hoc logic, and they were difficult to design. Now they are often implemented as a microprogram that is stored in a control store. Words of the microprogram are selected by a microsequencer and the bits from those words directly control the different parts of the device, including the registers, arithmetic and logic units, instruction registers, buses, and off-chip input/output. In modern computers, each of these subsystems may have its own subsidiary controller, with the control unit acting as a supervisor.
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