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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.)

Types of control units

All types of control units generate electronic control signals that control other parts of a CPU. Control units are usually one of these types:

1. Microcoded control units. In a microcoded control unit, a program reads signals, and generates control signals. The program itself is executed by a very simple computer, a relatively simple digital circuit called a microsequencer.
   
2. Hardware control units. In a hardware control unit, a digital circuit generates the control signals directly.