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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.
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. (See also CPU design and computer
architecture.)
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