Consider a typical computer operation performing I/O. To start an I/O operation, the device driver loads the appropriate registers in the device controller, in turn, examines the contents of these registers to determine what action to take (such as "read a character from the keyboard"). The controller starts the transfer of data from the device to its local buffer. Once the transfer of data is complete, the device controller informs the device driver that it has finished its operation. The device driver then gives control to other parts of the operating system, possibly returning the data or a pointer to the data if the operation was a read. For other operations, the device driver returns statrus information such as "write completed successfully" or "device busy". But how does the controller inform the device dirver that it has finished its operation? This is accomplished via an interrupt.
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