We present an approach to multi-formalism modeling which addresses two important issues in this field: to provide support for the specification of the semantics of a modeling formalism, and to allow the specification of the interactions between parts of a model described using different modeling formalisms. ModHel'X focuses on the execution of models, which we consider as the computation of one possible behavior of the model. This includes simulation, code generation or real-time execution. Based on the concept of Model of Computation, ModHel'X includes a UML meta-model which defines a component-oriented and hierarchical abstract syntax for describing the structural elements of a modeling language. The semantics of a modeling language, i.e. the corresponding model of computation, is expressed in an imperative style and addresses three aspects: control, data and time. The semantic adaptation between the heterogeneous levels found in the hierarchy of a model is described using the same imperative syntax. Finally, an execution engine has been developed for ModHel'X, which is able to interpret multi-formalism models for simulation purposes.