The embedded real-time software systems encountered in applications such as telecommunications, aerospace, and defense typically tend to be large and extremely complex. It is crucial in such systems that the software is designed with a sound architecture. A good architecture not only simplifies construction of the initial system, but even more importantly, readily accommodates changes forced by a steady stream of new requirements. In this paper, we describe a set of constructs that facilitate the design of software architectures in this domain. The constructs, derived from field-proven concepts originally defined in the ROOM modeling language, are specified using the Unified Modeling Language (UML) standard. In particular, we show how these architectural constructs can be derived from more general UML modeling concepts by using the powerful extensibility mechanisms of UML.