Modern embedded systems are often heterogeneous in that their design requires several description paradigms, based on different models of computation and concurrency (MoCCs). In this paper we present Rialto, a formal language intended at expressing computations in several MoCCs. The distinguishing features of Rialto and its implementation are 1) A formal semantics: the language is formalized using SOS (structured operational semantics) rules; 2) Encapsulation of models of computation into policies: we thus distinguish between the syntactic elements of the language (parallelism, interrupts) and its semantics; 3) efficient implementation algorithms. Policies are expressed in the language itself, which allows for more expressive power and a sounder semantics.