Contemporary operating systems still appear as huge monolithic objects. Each attempt to explain one of them's architecture typically results in the tedious enumeration of a large number of mutually depending features. To overcome that situation, a discipline on system architecture should be established aimed at the provision of understandable construction, composition, and behaviour rules. In the following we outline such a discipline as a framework for defining and handling functional units, and provide guidelines on how to fill them up with the required content resp. organize their execution.