This special issue is devoted to the synchronous approach to reactive and real-time programming. This introductory paper presents and discusses the application fields and the principles of synchronous programming. The major concern of the synchronous approach is to base synchronous programming languages on math-ematical models. This makes it possible to handle compilation, logical correctness proofs, and verifications of real-time programs in a formal way, leading to a clean and precise methodology for design and programming. 1. INTRODUCTION: REAL-TIME AND REACTIVE SYSTEMS It is commonly accepted to call real-time a program or system that receives external interrupts or reads sensors connected to the physical world and outputs commands to it. Real-time programming is an essential industrial activ-

Statecharts, popular for modelling system behavior in the structural analysis paradigm, are part of a fully executable language set for modelling object-oriented systems. The languages form the core of the emerging Unified Modelling Language.

While message sequence charts (MSCs) are widely used in industry to document the interworking of processes or objects, they are expressively weak, being based on the modest semantic notion of a partial ordering of events as defined, e.g., in the ITU standard. A highly expressive and rigorously defined MSC language is a must for serious, semantically meaningful tool support for use-cases and scenarios. It is also a prerequisite to addressing what we regard as one of the central problems in behavioral specification of systems: relating scenario-based inter-object specification to state-machine intra-object specification. This paper proposes an extension of MSCs, which we call live sequence charts (or LSCs), since our main extension deals with specifying "liveness", i.e., things that must occur. In fact, LSCs allow the distinction between possible and necessary behavior both globally, on the level of an entire chart and locally, when specifying events, conditions and progress over time within a chart. This makes it possible to specify forbidden scenarios, for example, and enables naturally specified structuring constructs such as subcharts, branching and iteration.

This paper describes an approach to writing requirements specifications for processcontrol systems, a specification language that supports this approach, and an example application of the approach and the language on an industrial aircraft collision avoidance system (TCAS II). The example specification demonstrates (1) the practicality of writing a formal requirements specification for a complex, process-control system and (2) the feasibility of building a formal model of a system using a specification language that is readable and reviewable by applications experts who are not computer scientists or mathematicians. Some lessons learned in the process of this work, which are applicable both to forward and reverse engineering, are also presented.

The paradigm of simulated annealing is applied to the problem of drawing graphs "nicely." Our algorithm deals with general graphs with straigh-line edges, and employs several simple criteria for the aesthetic quality of the result. The algorithm is flexible, in that the relative weights of the criteria can be changed. For graphs of modest size it produces good results, competitive with those produced by other methods, notably, the "spring method" and its variants.

This paper surveys the design of embedded computer systems, which use software running on programmable computers to im-plement system functions. Creating an embedded computer system which meets its performance, cost, and design time goals is a hardware-software co-design problewhe design of the hard-ware and software components influence each other. This paper emphasizes a historical approach to show the relationships be-tween well-understood design problems and the as-yet unsolved problems in co-design. We describe the relationship between hard-ware and sofhvare architecture in the early stages of embedded system design. We describe analysis techniques for hardware and software relevant to the architectural choices required for hard-ware-software co-design. We also describe design and synthesis techniques for co-design and related problems.

Requirements engineering (RE) is concerned with the identification of the goals to be achieved by the envisioned system, the operationalization of such goals into services and constraints, and the assignment of responsibilities for the resulting requirements to agents such as humans, devices, and software. The processes involved in RE include domain analysis, elicitation, specification, assessment, negotiation, documentation, and evolution. Getting highquality requirements is difficult and critical. Recent surveys have confirmed the growing recognition of RE as an area of utmost importance in software engineering research and practice. The paper presents a brief history of the main concepts and techniques developed to date to support the RE task, with a special focus on modeling as a common denominator to all RE processes. The initial description of a complex safetycritical system is used to illustrate a number of current research trends in RE-specific areas such as go...

1.1 Background: system architecture for system development? The characteristic approach in mature engineering disciplines (e.g. civil and chemical engineering) is to build systems (e.g., buildings or chemical plants) from known solutions such as

This paper addresses the design of reactive real-time embedded systems. Such systems are often heterogeneous in implementation technologies and design styles, for example by combining hardware ASICs with embedded software. The concurrent design process for such embedded systems involves solving the specification, validation, and synthesis problems. We review the variety of approaches to these problems that have been taken.

The process of understanding the system under analysis, the services required of it, its environment and associated constraints involves the capture, analysis and resolution of many ideas, perspectives and relationships at varying levels of detail. We believe requirements methods based on global reasoning lack the expressive framework to adequately articulate this distributed requirements knowledge structure. This paper describes the problems faced in trying to establish an adequate and stable set of requirements and proposes a novel ViewpointOriented Requirements Definition method (VORD) as a means of tackling some of these problems. This method structures the requirements engineering process using viewpoints which are associated with sources of requirements. The paper describes VORD in the light of current viewpoint-oriented requirements approaches and shows how its improves on them. A simple example of a bank auto-teller system is used to demonstrate the method. 3 1.0 Introduction...