Intra-Modular Structuring in Model-Oriented Speci cation: Expressing Non-interference with Read andWrite Frames

Compositionality provides the key to managing complexity in software systems and thus should be sought at all levels in the design process. In the development of code from model-oriented speci cations, compositionality can not only be achieved by the decomposition of system speci cations into modules but may also arise within the development of individual modules. This thesis considers the compositional development of operations that share state within modules. A key mechanism in the structuring of such developments lies in the de nition of non-interference between sub-operations. The read and write frames in VDM implicit operations de nitions can be understood as constraints on the read and write accesses which can be made by valid implementations. Their use as such permits reasoning about non-interference between operations and can thus be a source of compositionality within modules. However, existing semantic models of model-oriented speci cation and re nement do not provide a basis for such an interpretation of read and write frames. Investigations are made into alternative denotational semantic models for operation speci cations which do support read and write frames and a proof theory for reasoning about non-interference between sub-operations is founded upon them. The proposed semantic models provide progressively ner distinctions between implementations over and above the standard model. Thus new possibilities arise for discrimination between speci cations that would otherwise be considered equivalent, whilst existing results remain valid.