Device Controllers

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I/O units often consists of a mechanical component and an electronic component.

The electronic component is called device controller (adapter).

The mechanical component is the device itself.

The interface between the controller and the device is often a very low-level one.

Each controller has few registers that are used for communicating with the CPU. By writing into theses registers, the operating system can command the device to deliver data, switch itself on or off, or otherwise perform some action. By reading from theses registers, the operating system can learn what the device's state is, whether it is prepared to accept a new command, and so on.

In addition to control registers, many devices have a data buffer that the operating system can read and write.