If you are new to device integration in DP4, read the Overview.
If you wish to set up special devices for your system, read the explanation on how devices are defined in Device Definition Hierarchy.
If you are an application developer, and you wish to access devices in your program, read the instructions in Accessing devices from DP4 applications.
If you wish to write your own OSDI (device interface), read Writing an OSDI. You are recommended to familiarise yourself with the other areas covered in this manual first.
An apology, and explanation is perhaps required for the somewhat confusing terminology used throughout this manual. You can read it here.