We present a broad extension of the conventional formalism of state machines and state diagrams, that is relevant to the specification and design of complex discrete-event systems, such as multi-computer real-time systems, communication protocols and digital control units. Our diagrams, which we call statecharts, extend conventional state-transition diagrams with essentially three olements, dealing, respectively, with the notions of hierarchy, concurrency and communication. These transform the language of state diagrams into a highly structured' and economical description language. Statecharts are thus compact and expressive--small diagrams can express complex behavior--as well as compositional and modular. When coupled with the capabilities of computerized graphics, statecharts enable viewing the description at different levels of detail, and make even very large specifications manageable and comprehensible. In fact, we intend to demonstrate here that statecharts counter many of the objections raised against conventional state diagrams, and thus appear to render specification by diagrams an attractive and plausible approach. Statecharts can be used either as a stand-alone behavioral description or as part of a more general design methodology that deals also with the system's other aspects, such as functional decomposition and data-flow specification. We also discuss some practical experience that was gained over the last three years in applying the statechart formalism to the specification of a particularly complex system.

“The hardest single part of building a software system is deciding precisely what to build. No other part of the conceptual work is as difficult as establishing the detailed technical requirements...No other part of the work so cripples the resulting system if done wrong. No other part is as difficult to rectify later. ” [Brooks 87] Deciding precisely what to build and documenting the results is the goal of the requirements phase of software development. For many developers of large, complex

In this paper we present a design tool for complex computerized systems, that is based on the experience gained in the active database research area. CODES, a COntrol and Data Engineering of Systems, extends the capabilities of contemporary design tools by enabling a top-down design and reasoning facilities. The model combines the notions of hierarchy, concurrency and communication, and enables the representation of data flow as well as control flow. These properties form a highly structured and accurate specification language. The model is given using a formal and a graphical presentation. keywords: Design, Software engineering, Information modeling 1 Introduction The problem of specification and design of large and complex systems is well known in the system engineering literature [11], [8], [10]. The design of communication networks, computer operating systems, and human-computer interfaces of software products are several examples of systems that are difficult to define in a clear...