Verification and synthesis of asynchronous control circuits using petri net unfoldings

PhD ThesisDesign of asynchronous control circuits has traditionally been associated with application of formal methods. Event-based models, such as Petri nets, provide a compact and easy to understand way of specifying asynchronous behaviour. However, analysis of their behavioural properties is often hindered by the problem of exponential growth of reachable state space. This work proposes a new method for analysis of asynchronous circuit models based on Petri nets. The new approach is called PN-unfolding segment. It extends and improves existing Petri nets unfolding approaches. In addition, this thesis proposes a new analysis technique for Signal Transition Graphs along with an efficient verification technique which is also based on the Petri net unfolding. The former is called Full State Graph, the latter - STG-unfolding segment. The boolean logic synthesis is an integral part of the asynchronous circuit design process. In many cases, even if the verification of an asynchronous circuit specification has been performed successfully, it is impossible to obtain its implementation using existing methods because they are based on the reachability analysis. A new approach is proposed here for automated synthesis of speed-independent circuits based on the STG-unfolding segment constructed during the verification of the circuit's specification. Finally, this work presents experimental results showing the need for the new Petri net unfolding techniques and confirming the advantages of application of partial order approach to analysis, verification and synthesis of asynchronous circuits.The Research Committee, Newcastle University: Overseas Research Studentship Award

McMillan's complete prefix for contextual nets

In a seminal paper, McMillan proposed a technique for constructing a finite complete prefix of the unfolding of bounded (i.e., finitestate) Petri nets, which can be used for verification purposes. Contextual nets are a generalisation of Petri nets suited to model systems with readonly access to resources. When working with contextual nets, a finite complete prefix can be obtained by applying McMillan’s construction to a suitable encoding of the contextual net into an ordinary net. However, it has been observed that if the unfolding is itself a contextual net, then the complete prefix can be significantly smaller than the one obtained with the above technique. A construction for generating such a contextual complete prefix has been proposed for a special class of nets, called read-persistent. In this paper, we propose an algorithm that works for arbitrary semi-weighted, bounded contextual nets. The construction explicitly takes into account the fact that, unlike in ordinary or readpersistent nets, an event can have several different histories in general contextual net computations

Eco-Photography: Picturing the Global Environmental Imaginary in Space and Time

Engaging with art historical, visual cultural, and ecocritical analysis, this thesis asks the question: why has the environment-in-crisis become a central focus in contemporary photography? 'Eco-photography' visualizes the global environmental imaginary, both representing and contributing to the planetary awareness of environmental risk. Defining eco-photography as a category of images that participates in critical ecological and environmentalist practices by maintaining the ideal of an earth in 'balance', I reveal the conceptual underpinnings of this body of images as a continually shifting set of social values and relations. In Part I, I frame this category of eco-photography as a communicative genre that reflects and contributes to environmental discourse in public cultural spheres. The photographs I analyse employ realism as a rhetorical and aesthetic approach to envision the environmental imaginary in a direct and naturalizing manner. As such, eco-photography requires careful reading to understand how such images communicate, and especially the rhetorical, visual, and affective strategies that they employ. Part II focuses on the temporal dissonance of eco-photography and the problem of expressing concern for the future using a medium that is bounded in time. I argue that eco-photography is best understood as a mode of temporal slippage that offers valuable insights into environmental concerns as they are evolving. Looking at examples of repeat photography, I analyse the discourse of objectivity and witnessing in eco-photography. Nuclear photography is considered in this section for its impact on our global sense of anxiety for the future. Eco-photography is seen to be a source of hope as it records for the future images of a world at risk. Part III explores the deterritorializing impact of images and considers how the circulation of eco-photography is contributing to a sense of global cultural dislocation through the representation of local and global environmental justice issues. This section asks the question: how can photography help to visualize the complexity of humanity's relationship to the planet? I conclude by considering whether the cosmopolitan notion of a global citizenry of photography can be a positive force for promoting environmental change

On the computation of McMillan's prefix for contextual nets and graph grammars

In recent years, a research thread focused on the use of the unfolding semantics for verification purposes. This started with a paper by McMillan, which devises an algorithm for constructing a finite complete prefix of the unfolding of a safe Petri net, providing a compact representation of the reachability graph. The extension to contextual nets and graph transformation systems is far from being trivial because events can have multiple causal histories. Recently, we proposed an abstract algorithm that generalizes McMillan's construction to bounded contextual nets without resorting to an encoding into plain P/T nets. Here, we provide a more explicit construction that renders the algorithm effective. To allow for an inductive definition of concurrency, missing in the original proposal and essential for an efficient unfolding procedure, the key intuition is to associate histories not only with events, but also with places. Additionally, we outline how the proposed algorithm can be extended to graph transformation systems, for which previous algorithms based on the encoding of read arcs would not be applicable