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In computing and especially in computer hardware, a 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 (for example a memory controller that manages access to memory for the computer) or a controller on an external device that manages the operation of (and connection with) that device.
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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.
Controller boards
Early desktop computers such as the IMSAI 8080 used expansion boards (or expansion cards) for all controllers, each handling a specific type of device. Examples of expansion board controllers are:
Controller chips
As chip densities improved controllers were implemented as single chips and often located on the motherboard. Examples are:
Further integration enabled development of Super I/O chips — single chips that could control a variety of devices such as floppy disks, parallel ports, serial ports, keyboard, and mouse.
Mainframe controllers
In IBM terminology a controller is "a device that decodes the [channel] command and effects the operation of the device."
In most mainframe systems a device-independent channel usually attaches to the CPU and to a controller or control unit which implements device-dependent logic for attaching specific devices. The functions performed by the control unit are similar to the functions performed by a device driver program on smaller systems. Some devices have integrated control units, which are logically discrete but are included with the device rather than requiring a separate box. Often a control unit can attach to multiple channels connected to a single or multiple systems. Some mainframe control units are: