The OSDI (device integration) pack includes a template device driver C module, which you modify for your device. The template code provides empty functions for each of the actions to be taken, such as initialise the device, start input and send output.
For example, you might write a device driver for a special video display. You might include instructions to write data to a specific address in memory, or call a device specific DLL. When the application "prints" to the display, the data is passed to your function, and is in turn output to the device.