Visual Formalism Research Articles

PTD: Architectural system description support based on visual specification languages

Formal languages become more and more necessary if we want to assure the satisfaction of the requirements of a system during its development. Several formal methods are gaining wider acceptance in spite of their difficulties to be applied in large systems design. The availability of graphical workstations, give us the possibility to create system specifications in a graphical way enhancing the intuition during this process. Visual languages formally related with formal description techniques, make possible to combine the advantages derived from mathematical models and graphical design. In this paper we present a formal visual language, named Process Topologie Diagrams (PTD), close to the formal description technique LOTOS. It is defined mathematically with a set of operators to manipulate PTDs. The language is supported by a software tool (editor, conversion from/to LOTOS, textual and graphical). There are also outlined methodological guidelines to support the architectural description of a system. This allow the generation of LOTOS specifications from a subclass of PTDs. The notation was applied to the specification of a gateway between the Digital Subscriber Signalling System No. 1 (DSS1) and the ISDN user part (ISUP) of Signalling System No. 7 (SS7). We plan to extend this visual formalism to support other kinds of system specifications and to enrich the model to deal with time requirements and PTDs animation.

Stimulus-response machines

This paper describes Stimulus-Response (S-R) Machines, a new visual formalism based on the semantics of Ada. S-R Machines are SCOOP-3's [2] main medium for specifying and implementing critical units of behavior. They have the same motivation as Harel's Statecharts [3]: to manage the unstructured explosion of blobs and arrows that occur on complex State Transition Diagrams. S-R Machines add to Ada's semantics a named off-page abstraction mechanism called "boxes." Boxes are used to decompose S-R Machines into hierarchies of states (goals, steps, conditions, or modes), each of which can be easily verified. S-R Machines are similar to Harlan Mills' Box-Structured Systems [4]: our interfaces are identical to his black boxes; our bodies are similar to his state and clear boxes. Because S-R Machines combine the familiar features of dataflow diagrams, object-message diagrams, and structured flowcharts, its notation is easy to learn. S-R Machines are useful when developing state-rich safety-critical systems or whenever understanding or communication are problematical or critical (e.g., educational settings and software inspections).

Using statecharts for hardware description and synthesis

Statecharts have been proposed recently as a visual formalism for the behavioral description of complex systems. They extend classical state diagrams in several ways, while retaining their formality and visual nature. The authors argue that statecharts can be beneficially used as a behavioral hardware description language. They illustrate some of the main features of the approach, including: hierarchical decomposition, multilevel timing specifications and flexible concurrency and synchronization capabilities. The authors also present a VLSI synthesis methodology by which layer area and delay periods can be reduced relative to the conventional finite-state-machine (FSM) synthesis method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On visual formalisms

The higraph, a general kind of diagramming object, forms a visual formalism of topological nature. Higraphs are suited for a wide array of applications to databases, knowledge representation, and, most notably, the behavioral specification of complex concurrent systems using the higraph-based language of statecharts.

Statecharts: a visual formalism for complex systems

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 elements, 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.