Abstract: This paper proposes a methodology for modelling the timing behaviour of hard real-time systems oriented to compositionality and reusability. When a system is built according to a modular structure, the methodology provides the system designer with capacity to build the real-time model of the system as a composition of the reusable timing models of the modules that make up the system. The modularization is applied at all levels: software, hardware and middleware. The methodology relies on a reactive modelling approach, i.e. the timing behaviour of a system is modelled by identifying and describing the timing behaviour of the activities executed in the system in response to events, coming either from the environment or from the timer. The methodology is based on the complementary concepts of model descriptor and model instance. The reusable timing model of a software or hardware module is formulated as a parameterized descriptor, which contains all the information about the internal elements of the module that is required to evaluate the behaviour of any application in which the module may be used. The analysable real-time model of a system is built by composing the model instances of the modules that form it, which are generated from their corresponding descriptors by assigning concrete values to all their parameters according to the specific configuration of the system.

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