DSM systems provide an easy-to-use programming model for parallel and distributed systems, but it is sometimes difficult to reach the performance characteristics of low-level message-passing programs, in particular if these have been optimized towards a specific architecture. In this article, we propose a multi-layered realization of a DSM system which provides different programming abstractions, including a level which allows an explicit control of the data placement. The programmer can select an appropriate level of abstraction for his application and it is even possible to mix program parts realized at different abstraction levels. The article gives a description of the multi-layered model, describes a prototype realization of the system and presents some preliminary experimental results on a heterogeneous system.