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Digital computers are increasingly being used in safety-critical applications (e.g., avionics, chemical plant and railway systems). The main motivations for introducing computers into such environments are to increase performance, flexibility and efficiency. However, the cost to safety in achieving these benefits using computing systems is unclear. The general class of systems considered in this thesis are process control systems. More specifically the thesis examines the class of safety-critical computing systems which are a component of a process control system that could cause or allow the overall system to enter into a hazardous state. This thesis investigates the role oiformal methods in safety-critical computing systems. The phase of system development considered is requirements analysis. Experience in safety-critical systems has shown that errors in the identified requirements are one of the major causes of mishap. It is argued that to gain a complete understanding of such computing systems, the requirements of the overall system and the properties of the environment must be analyzed in a common formal framework. A system development model based on the separation of safety and mission issues is discussed, which highlights the essential specifications that must be produced during requirements analysis. A formal model for the representation of these essential specifications is presented. The semantics of this formal model are based on the notion of a system history. To structure the specifications expressed by this formal model the concept of a mode is introduced. This thesis suggests that for a formal model to be useful during requirements analysis a related systematic methodology, which provides comprehensive guidelines for the analysts who use the model must be made available. An appropriate methodology, based upon the system development model, which incorporates some traditional system safety techniques is described. Overall, the thesis presents a framework for requirements analysis by providing a system development model, formal model and related development methodology. An example of how this framework can support requirements analysis is presented in the appendices Band C.

This thesis is concerned with the analytical modelling of computing and other discrete event systems, for steady state performance and dependability. That is carried out using a novel solution technique, known as the spectral expansion method. The type of problems considered, and the systems analysed, are represented by certain two-dimensional Markov-processes on finite or semi-infinite lattice strips. A sub set of these Markov processes are the Quasi-Birth-and-Death processes. These models are important because they have wide ranging applications in the design and analysis of modern communications, advanced computing systems, flexible manufacturing systems and in dependability modelling. Though the matrixgeometric method is the presently most popular method, in this area, it suffers from certain drawbacks, as illustrated in one of the chapters. Spectral expansion clearly rises above those limitations. This also, is shown with the aid of examples. The contributions of this thesis can be divided into two categories. They are, • The theoretical foundation of the spectral expansion method is laid. Stability analysis of these Markov processes is carried out. Efficient numerical solution algorithms are developed. A comparative study is performed to show that the spectral expansion algorithm has an edge over the matrix-geometric method, in computational efficiency, accuracy and ease of use. • The method is applied to several non-trivial and complicated modelling problems, occuring in computer and communication systems. Performance measures are evaluated and optimisation issues are addressed.

Thesis (M.S.)--West Virginia University, 2009. / Title from document title page. Document formatted into pages; contains xi, 138 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 136-138).

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