Semantic Foundation. Quoting from [Mod14, Section 1.2]: The semantics of the Modelica language is specified by means of a set of rules for translating any class described in the Modelica language to aflat Modelica structure. A class must have additional properties in order that its flat Modelica structure can be further transformed into a set of differential, algebraic and discrete equations (=flat hybrid DAE). Such classes are called simulation models. Figure 1 illustrates the basic idea. It suggests itself to discern the depicted two stages when giving semantics to Modelica models. In practical Modelica compiler implementations, the translation to aflat Modelica struc- ture is typically mapped to a “front-end” phase and the translation to an executable simulation to a “back-end” phase of the translation process. Figure 2 depicts a such a typical translation process that is classified into several stages. Sometimes a dedicated “middle-end” phase is defined; it is responsible for the symbolic equation transformations performed during sorting and optimising the hybrid differential algebraic equation (hybrid DAE) into a representation that can be efficiently solved by a numerical solver. In that case the “back-end” phase would only be responsible for code generation (typically C code) from the optimized sorted equations. The following characterisation of the different stages is taken from [Thi15, p. 39]: 1There has been research in that direction,e.g., [ Sal06, LZZ08], but so far no common agreement exists whether the Modelica language should be extended to support PDEs and how a such an extension should be done. Modelica Model Modelica Specification Flat Modelica (Hybrid DAE) Mathematical denotation for hybrid DAE system Simulation Result
