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 information 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 sellicks, each of these subsystems may have its own subsidiary controller, with the control unit acting as a supervisor.
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