Stimulus-response machines

Abstract

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).