the portion of a computer that coordinates the operation of all the computer’s devices, prescribing specific actions according to a given program. The control unit generates control signals that ensure the required sequence of execution of operations, monitors the operation of the computer in various regimes, and provides for the interface of the human operator with the computer.

The design of a control unit depends on the type of computer and the method used to control the computing process. In synchronous computers, a previously determined time is allocated for the execution of any operation, and a single, central control unit is usually used to synchronize the overall operation of the computer. In asynchronous computers, the start of each operation is determined by the completion of the previous operation, and each component, such as the arithmetic or memory unit, often has its own local control unit. In such computers the central control unit generates only the basic control signals specifying the regimes of operation for the local control units, which organize the operation of their devices according to the control signals they receive.

Control units may operate according to hard-wired or flexible control programs. In the former, all possible combinations of control signals and time relations between signals are unchangeable and are determined by the computer’s design and construction. A change in the sequence of computations requires circuit changes in the control unit, and hard-wired programs are therefore most often used in special-purpose computers. A control unit with a flexible program is suitable for a variety of applications and allows a program to be established for the execution of a task immediately prior to such execution. These control programs are used in general-purpose computers.

The most effective control units feature multiprogramming, which allows the simultaneous solution of several tasks and independent communication between the computer and several users. Multiprogramming may be provided by several control units, each of which serves one of the programs being executed by the computer. Time sharing in the execution of several programs may also be used. The time sharing is conducted by one control unit, which is transferred from one program to another as the result of the consecutive polling of the users or an involuntary interrupt by a user in accordance with a given priority. Microprogram control units, in which a set of signals constituting a microinstruction corresponds to each machine operation, are also widely used; the microinstructions are stored in the computer’s permanent memory (seeMICROPROGRAM CONTROL). In this case, the optimum sets of control signals are chosen for each operation, and the working microprograms are constructed according to the control signals selected.

The future development of control units will center on increasing the units’ efficiency and logical capacities. The latter would make it possible to specify the structure of commands or the length of words with little or no restriction. Changes in the structure of computers, the joint operation of several computers, and other types of flexibility are also envisioned.