Evolution is a key problem in software engineering and exacts huge costs. Industrial evidence even hints that companies spend more resources on maintaining and evolving their software than on the initial development. In managing the change and guiding evolution, considerable emphasis is placed on the architecture of the software system as a key artifact involved. One of the major indicators of the success (failure) of software evolution is the extent to which the software system can endure changes in requirements, while leaving the architecture of the software system intact. We refer to the presence of this "intuitive" phenomenon as architectural stability. We highlight the requirements for evaluating architectural stability. We pursue an economics-driven software engineering approach to address these requirements. We view evolving software as a value-seeking activity: software evolution is as a process in which software is undergoing a change (an incremental) and seeking value. The value is attributed to the flexibility of an architecture in enduring likely changes in requirements. To value flexibility, we contribute to a novel model that builds on an analogy with real options theory. The model examines some likely changes in requirements and values the extent to which the architecture is flexible to endure these changes. The model views an investment in an architecture as an upfront investment plus "continual" increments of future investments in likely changes in requirements. The objective is to provide insights into architectural stability and investment decisions related to the evolution of software architectures. We support the model with a three-phase method for evaluating architectural stability. The method provides guidelines on eliciting the likely changes in requirements and relating architectural decisions to value. The problem of valuing flexibility of an architecture to change requires a comprehensive solution that incorporates multiple valuation techniques, some with subjective estimates, and others based on market data, when available. To introduce discipline into this setting and capture the value from different perspectives, the method outlines a valuation points of view framework as a solution. The framework is flexible enough to account for the economic ramifications of the change on both structural (e.g., maintainability) and behavioral (e.g., throughput) qualities of an architecture and on relevant business goals (e.g., new market products). We report on our experience in using the model and its supporting method with two case studies. In the first case, we show how the model and its supporting method can be used to assess the worthiness of re-engineering a "more" stable architecture in face of likely changes in future requirements. We take refactoring as an example of re-engineering. In the second case, we show how the model and its supporting method can inform the selection of a "more" stable middleware-induced software architecture in the face of future changes in non-functional requirements. We critically discuss and reflect on the strengths and the limitations of our contribution. We conclude by highlighting some open questions that could stimulate future research in architectural stability, relating requirements to software architectures, and architectural economics.

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Model-based specification languages provide a means for obtaining assurance of dependability of complex computer-based systems, but provide little support for modelling and analysing fault behaviour, which is inherently probabilistic in nature. In particular, the need for a detailed account of the role of continuous probability has been largely overlooked. This thesis addresses the role of continuous probability in model-based specification languages. A model-based specification language (sGCL) that supports continuous probability distributions is studied. The use of sGCL and how it interacts with engineering practices is also explored. In addition, a re nement ordering for continuous probability distributions is given, and the challenge of combining non-determinism and continuous probability is discussed in depth. The thesis is presented in three parts. The first uses two case studies to explore the use of probability in formal methods. The first case study, on ash memory, is used to present the capabilities of probabilistic formal methods and to determine the kinds of questions that require continuous probability distributions to answer. The second, on an emergency brake system, illustrates the strengths and weaknesses of existing languages and provides a basis for exploring a prototype language that includes continuous probability. The second part of the thesis gives the formal de nition of sGCL's syntax and semantics. The semantics is made up of two parts, the proof theory (transformer semantics) and the underpinning mathematics (relational semantics). The additional language constructs and semantical features required to include non-determinism as well as continuous probability are also discussed. The most challenging aspect lies in proving the consistency of the semantics when non-determinism is also included. The third part uses a nal case study, on an aeroplane pitch monitor, to demonstrate the use of sGCL. The new analysis techniques provided by sGCL, and how they fit in with engineering practices, are explored.

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It is recognised that normative systems, and in particular electronic institutions and contracts are a potentially powerful means for making agent interactions in multi-agent systems effective and efficient. However, correctly specifying the behaviour of such systems is a difficult problem. Designers are faced with two concurrent, complex tasks: firstly they must specify the relationships (over time) between agents’ actions and their effects, and secondly they must also consider how agents’ actions are to be regulated through the definition of agents’ permissions and obligations. Such systems are typi- cally complex, and given this complexity it may be difficult for a designer to determine whether their original objectives have been captured by the specification of the system. In this dissertation we seek to address some of the problems associated with institu- tional specification. In order to do this we present a model for specifying institutions based on the notion of socially constructed reality that accounts not only for how the action and events which constitute the institution are described, but also how they are regulated. Institutions may be used in a number of ways, and may account for concepts at varying levels of abstraction. Recognising this we also investigate how several insti- tutions, each accounting for a particular aspect of a society may be composed and how the relationships between these institutions may be expressed. Given this model, we then demonstrate how, using the answer set programming paradigm institutional spec- ifications based on our model may be checked for the absence or presence of certain (un)desirable properties.

Bibliography: leaves 237-251. / xxi, 251 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / The work in this thesis is devoted to the architecture, implementation and performance of a MATRISC processing mode. Simulation results for the MATRISC processor are provided which give performance estimates for systems which can be implemented in current technologies. It is concluded that the extremely high performance of MATRISC processors makes possible the construction of parallel computers with processing capabilities in excess of one teraflops. / Thesis (Ph.D.)--University of Adelaide, Dept. of Electrical and Electronic Engineering, 1994

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