Heterogeneous Models of Computation

The variety of models of computation that have been developed is only partially due to immaturity in the field. It appears that differentmodels fundamentally have different strengths and weaknesses, and that attempts to find their common features result in models that are very low level, difficult to use. These low level models (such as Dijkstra’s P/V systems [52]) provide a good theoretical foundation, but not a good basis for design. Thus we are faced with two alternatives in designing complex, heterogeneous systems. We can either use a single unified approach and suffer the consequences, or we can mix approaches. To use the unified approach today we could choose between VHDL and C for a mixed hardware and software design, doing the entire design in one or the other (i.e. specifying the software in VHDL or the hardware in C). Or worse, we could further bloat the VHDL language by including a subset designed for software specification (e.g. by making Ada a subset of VHDL). In the alternative that we advocate, we mix approaches while keeping them conceptually distinct, for example by using both
VHDL and C in a mixed hardware/software design. The key problem in the mixed approach, then, is to define the semantics of the interaction of fundamentally different models of computation. It is not simply a problem of interfacing languages. It is easy, for example, to provide a mechanism for calling C procedures from VHDL. But what does it mean if two concurrent VHDL entities call C procedures that interact? The problem is exacerbated by the lack of agreed-upon semantics for C or VHDL. Studying the interaction semantics of mixed models of computation is the main objective of the Ptolemy project [14]. There, a hierarchical framework is used, where a specification in one model of computation can contain a primitive that is internally implemented using another model of computation. The object-oriented principle of information hiding is used to isolate the models from one another as much as possible.