Computer Controller is a chip, an expansion card, or a stand-alone device that interfaces with a peripheral device. This may be a link between two parts of a computer or a controller on an external device that manages the operation of that device. The term is sometimes used in the opposite sense to refer to a device by which the user controls the operation of the computer, as in game controller.
In desktop computers the controller may be a plug in board, a single integrated circuit on the motherboard, or an external device. In mainframes the controller is usually either a separate device attached to a channel or integrated into the peripheral.