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A knowledge-based program defines the behavior of an agent by combining primitive actions, programming constructs and test conditions that make explicit reference to the agent’s knowledge. In this paper we consider a setting where an agent is equipped with a Description Logic (DL) knowledge base providing general domain knowledge and an incomplete description of the initial situation. We introduce a corresponding new DL-based action language that allows for representing both physical and sensing actions, and that we then use to build knowledge-based programs with test conditions expressed in the epistemic DL. After proving undecidability for the general case, we then discuss a restricted fragment where verification becomes decidable. The provided proof is constructive and comes with an upper bound on the procedure’s complexity.

Thesis (MSc (Mathematical Sciences)--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Binary Decision Diagrams (BDDs) are data structures that have been used to solve various problems in different aspects of computer aided design and formal verification. The large memory and time requirements of BDD applications are the major constraints that usually prevent the use of BDDs since there is a limited amount of memory available on a machine. One way of overcoming this resource limitation problem is to utilize the memory available on a network of workstations (NOW). This requires the distribution of the computation and memory requirements involved in the manipulation of BDDs over a NOW. In this thesis, an algorithm for manipulating BDDs on a NOW is presented. The algorithm makes use of the breadth-first technique to manipulate BDDs so that various BDD operations can be started concurrently on the different workstations on the NOW. The design and implementation details of the distributed BDD package are described. The various approaches considered in order to optimize the performance of the algorithm are also discussed. Experimental results demonstrating the performance and capabilities of the distributed package and the benefits of the different optimization approaches are given. / AFRIKAANSE OPSOMMING: Binêre besluitnemingsbome (BBBs) is data strukture wat gebruik word om probleme in verskillende areas van Rekenaarwetenskap, soos by voorbeeld rekenaargesteunde ontwerp en formele verifikasie, op te los. Die tyd- en spasiekoste van BBB-gebaseerde toepassings is die hoofrede waarom BBBs nie altyd gebruik kan word nie; die geheue van ’n enkele is ongelukkig te beperkend. Een manier om hierdie hulpbronprobleem te omseil, is om die gedeelde geheue van die werkstasies in ’n netwerk van werkstasies (Engels: “network of workstations”, oftewel, ’n NOW) te benut. Dit is dus nodig om die berekening en geheuevoorvereistes van die BBB bewerking oor die NOW te versprei. Hierdie tesis bied ’n algoritme aan om BBBs op ’n NOW te hanteer. Die algoritme gebruik die breedte-eerste soektegniek, sodat BBB operasies gelyklopend kan uitvoer. Die details van die ontwerp en implementasie van die verspreide BBB bilbioteek word beskryf. Verskeie benaderings om die gedrag van die biblioteek te optimeer word ook aangespreek. Empiriese resultate wat die werkverrigting en kapasiteit van die biblioteek meet, en wat die uitwerking van die onderskeie optimerings aantoon, word verskaf.

This doctoral thesis considers the automatic verification of parameterized systems, i.e. systems with an arbitrary number of communicating components, such as mutual exclusion protocols, cache coherence protocols or heap manipulating programs. The components may be organized in various topologies such as words, multisets, rings, or trees. The task is to show correctness regardless of the size of the system and we consider two methods to prove safety:(i) a backward reachability analysis, using the well-quasi ordered framework and monotonic abstraction, and (ii) a forward analysis which only needs to inspect a small number of components in order to show correctness of the whole system. The latter relies on an abstraction function that views the system from the perspective of a fixed number of components. The abstraction is used during the verification procedure in order to dynamically detect cut-off points beyond which the search of the state-space need not continue. Our experimentation on a variety of benchmarks demonstrate that the method is highly efficient and that it works well even for classes of systems with undecidable property. It has been, for example, successfully applied to verify a fine-grained model of Szymanski's mutual exclusion protocol. Finally, we applied the methods to solve the complex problem of verifying highly concurrent data-structures, in a challenging setting: We do not a priori bound the number of threads, the size of the data-structure, the domain of the data to store nor do we require the presence of a garbage collector. We successfully verified the concurrent Treiber's stack and Michael & Scott's queue, in the aforementioned setting. To the best of our knowledge, these verification problems have been considered challenging in the parameterized verification community and could not be carried out automatically by other existing methods.

<p>A major current challenge consists in extending formal methods in order to handle infinite-state systems. Infiniteness stems from the fact that the system operates on unbounded data structure such as stacks, queues, clocks, integers; as well as parameterization.</p><p>Systems with unbounded data structure are natural models for reasoning about communication protocols, concurrent programs, real-time systems, etc. While parameterized systems are more suitable if the system consists of an arbitrary number of identical processes which is the case for cache coherence protocols, distributed algorithms and so forth. </p><p>In this thesis, we consider model checking problems for certain fundamental classes of probabilistic infinite-state systems, as well as the verification of safety properties in parameterized systems. First, we consider probabilistic systems with unbounded data structures. In particular, we study probabilistic extensions of Lossy Channel Systems (PLCS), Vector addition Systems with States (PVASS) and Noisy Turing Machine (PNTM). We show how we can describe the semantics of such models by infinite-state Markov chains; and then define certain abstract properties, which allow model checking several qualitative and quantitative problems.</p><p>Then, we consider parameterized systems and provide a method which allows checking safety for several classes that differ in the topologies (linear or tree) and the semantics (atomic or non-atomic). The method is based on deriving an over-approximation which allows the use of a symbolic backward reachability scheme. For each class, the over-approximation we define guarantees monotonicity of the induced approximate transition system with respect to an appropriate order. This property is convenient in the sense that it preserves upward closedness when computing sets of predecessors.</p>

L'utilisation des méthodes formelles est de plus en plus courante dans le développement logiciel, et les systèmes de types sont la méthode formelle qui a le plus de succès. L'avancement des méthodes formelles présente de nouveaux défis, ainsi que de nouvelles opportunités. L'un des défis est d'assurer qu'un compilateur préserve la sémantique des programmes, de sorte que les propriétés que l'on garantit à propos de son code source s'appliquent également au code exécutable. Cette thèse présente un compilateur qui traduit un langage fonctionnel d'ordre supérieur avec polymorphisme vers un langage assembleur typé, dont la propriété principale est que la préservation des types est vérifiée de manière automatisée, à l'aide d'annotations de types sur le code du compilateur. Notre compilateur implante les transformations de code essentielles pour un langage fonctionnel d'ordre supérieur, nommément une conversion CPS, une conversion des fermetures et une génération de code. Nous présentons les détails des représentation fortement typées des langages intermédiaires, et les contraintes qu'elles imposent sur l'implantation des transformations de code. Notre objectif est de garantir la préservation des types avec un minimum d'annotations, et sans compromettre les qualités générales de modularité et de lisibilité du code du compilateur. Cet objectif est atteint en grande partie dans le traitement des fonctionnalités de base du langage (les «types simples»), contrairement au traitement du polymorphisme qui demande encore un travail substantiel pour satisfaire la vérification de type. / Formal methods are rapidly improving and gaining ground in software. Type systems are the most successful and popular formal method used to develop software. As the technology of type systems progresses, new needs and new opportunities appear. One of those needs is to ensure the faithfulness of the translation from source code to machine code, so that the properties you prove about the code you write also apply to the code you run. This thesis presents a compiler from a polymorphic higher-order functional language to typed assembly language, whose main property is that type preservation is verified statically, through type annotations on the compiler's code. Our compiler implements the essential code transformations for a higher-order functional language, namely a CPS conversion and closure conversion as well as a code generation. The thesis presents the details of the strongly typed intermediate representations and the constraints they set on the implementation of code transformations. Our goal is to guarantee type preservation with a minimum of type annotations, and without compromising readability and modularity of the code. This goal is already a reality for simple types, and we discuss the problems remaining for polymorphism, which still requires substantial extra work to satisfy the type checker.

We discuss a new approach to the construction of software systems. Instead of attempting to build a system that is as free of errors as possible, the designer instead identifies key properties that the execution must satisfy to be acceptable to its users. Together, these properties define the acceptability envelope of the system: the region that it must stay within to remain acceptable. The developer then augments the system with a layered set of components, each of which enforces one of the acceptability properties. The potential advantages of this approach include more flexible, resilient systems that recover from errors and behave acceptably across a wide range of operating environments, an appropriately prioritized investment of engineering resources, and the ability to productively incorporate unreliable components into the final software system. / Singapore-MIT Alliance (SMA)

L'utilisation des méthodes formelles est de plus en plus courante dans le développement logiciel, et les systèmes de types sont la méthode formelle qui a le plus de succès. L'avancement des méthodes formelles présente de nouveaux défis, ainsi que de nouvelles opportunités. L'un des défis est d'assurer qu'un compilateur préserve la sémantique des programmes, de sorte que les propriétés que l'on garantit à propos de son code source s'appliquent également au code exécutable. Cette thèse présente un compilateur qui traduit un langage fonctionnel d'ordre supérieur avec polymorphisme vers un langage assembleur typé, dont la propriété principale est que la préservation des types est vérifiée de manière automatisée, à l'aide d'annotations de types sur le code du compilateur. Notre compilateur implante les transformations de code essentielles pour un langage fonctionnel d'ordre supérieur, nommément une conversion CPS, une conversion des fermetures et une génération de code. Nous présentons les détails des représentation fortement typées des langages intermédiaires, et les contraintes qu'elles imposent sur l'implantation des transformations de code. Notre objectif est de garantir la préservation des types avec un minimum d'annotations, et sans compromettre les qualités générales de modularité et de lisibilité du code du compilateur. Cet objectif est atteint en grande partie dans le traitement des fonctionnalités de base du langage (les «types simples»), contrairement au traitement du polymorphisme qui demande encore un travail substantiel pour satisfaire la vérification de type. / Formal methods are rapidly improving and gaining ground in software. Type systems are the most successful and popular formal method used to develop software. As the technology of type systems progresses, new needs and new opportunities appear. One of those needs is to ensure the faithfulness of the translation from source code to machine code, so that the properties you prove about the code you write also apply to the code you run. This thesis presents a compiler from a polymorphic higher-order functional language to typed assembly language, whose main property is that type preservation is verified statically, through type annotations on the compiler's code. Our compiler implements the essential code transformations for a higher-order functional language, namely a CPS conversion and closure conversion as well as a code generation. The thesis presents the details of the strongly typed intermediate representations and the constraints they set on the implementation of code transformations. Our goal is to guarantee type preservation with a minimum of type annotations, and without compromising readability and modularity of the code. This goal is already a reality for simple types, and we discuss the problems remaining for polymorphism, which still requires substantial extra work to satisfy the type checker.

Thesis (Msc (Mathematical Sciences. Computer Science))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Model checkers are programs that automatically verify, without human assistance, that certain user-specified properties hold in concurrent software systems. Since these programs often have expensive time and memory requirements, an active area of research is the development of distributed model checkers that run on clusters. Of particular interest is how the communication between the machines can be reduced to speed up their running time. In this thesis the design decisions involved in an on-the-fly distributed model checker are identified and discussed. Furthermore, the implementation of such a program is described. The central idea behind the algorithm is the generation and distribution of data throughout the nodes of the cluster. We introduce several techniques to reduce the communication among the nodes, and study their effectiveness by means of a set of models. / AFRIKAANSE OPSOMMING: Modeltoetsers is programme wat outomaties bevestig, sonder enige hulp van die gebruiker, dat gelopende sagteware aan sekere gespesifiseerde eienskappe voldoen. Die feit dat hierdie programme dikwels lang looptye en groot geheues benodig, het daartoe aanleiding gegee dat modeltoetsers wat verspreid oor ’n groep rekenaars hardloop, aktief nagevors word. Dit is veral belangrik om vas te stel hoe die kommunikasie tussen rekenaars verminder kan word om sodoende die looptyd te verkort. Hierdie tesis identifiseer en bespreek die ontwerpsbesluite betrokke in die ontwikkeling van ’n verspreide modeltoetser. Verder word die implementasie van so ’n program beskryf. Die kernidee is die generasie en verspreiding van data na al die rekenaars in die groep wat aan die probleem werk. Ons stel verskeie tegnieke voor om die kommunikasie tussen die rekenaar te verminder en bestudeer die effektiwiteit van hierdie tegnieke aan die hand van ’n lys modelle.