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lecture07-handout4 - The Intuitive Semantics Logical...

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The Intuitive Semantics Logical Correctness Modeling/Distributed RT Systems—Lecture 07 Prof. Dr. Reinhard von Hanxleden Christian-Albrechts Universit¨ at Kiel Department of Computer Science and Applied Mathematics Real-Time Systems and Embedded Systems Group 10 May 2004 Last compiled: 9th May 2005, 22:28 hrs Esterel—The Semantics Part I WS 2004/05 Modeling/Distributed RT Systems—Lecture 07 Slide 1 The Intuitive Semantics Logical Correctness Overview The Intuitive Semantics Introduction to Causality Analysis The Pure Esterel Kernel Language Derived Statements Logical Correctness The logical coherence law Logical reactivity and determinism Instantaneous Feedback WS 2004/05 Modeling/Distributed RT Systems—Lecture 07 Slide 2 The Intuitive Semantics Logical Correctness Introduction to Causality Analysis The Pure Esterel Kernel Language Derived Statements Causality Problems present A else emit A end abort pause ; emit A when A present A then pause end ; emit A It’s easy to write contradictory programs Unfortunate side-effect of instantaneous communication coupled with the single valued signal rule These sorts of programs are erroneous and flagged by the Esterel compiler as incorrect WS 2004/05 Modeling/Distributed RT Systems—Lecture 07 Slide 3 The Intuitive Semantics Logical Correctness Introduction to Causality Analysis The Pure Esterel Kernel Language Derived Statements Causality Problems [ abort emit A when immediate B ] || [ present A then emit B end ; ] Can be very complicated because of instantaneous communication WS 2004/05 Modeling/Distributed RT Systems—Lecture 07 Slide 4
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The Intuitive Semantics Logical Correctness Introduction to Causality Analysis The Pure Esterel Kernel Language Derived Statements Causality Definition has evolved since first version of the language Original compiler had concept of “potentials” Static concept: at a particular program point, which signals could be emitted along any path from that point Latest definition based on “constructive causality” Dynamic concept: whether there’s a “guess-free proof” that concludes a signal is absent WS 2004/05 Modeling/Distributed RT Systems—Lecture 07 Slide 5 The Intuitive Semantics Logical Correctness Introduction to Causality Analysis The Pure Esterel Kernel Language Derived Statements Causality Example emit A; present B then emit C end; Red statements reachable present A else emit B end; Analysis done by original compiler: After emit A runs, there’s a static path to emit B Therefore, the value of B cannot be decided yet Execution procedure deadlocks: Program is bad WS 2004/05 Modeling/Distributed RT Systems—Lecture 07 Slide 6 The Intuitive Semantics Logical Correctness Introduction to Causality Analysis The Pure Esterel Kernel Language Derived Statements Causality Example emit A; present B then emit C end; Red statements reachable present A else emit B end; Analysis done by later compilers: After emit A runs, it is clear that B cannot be emitted because A ’s presence runs the “ then ” branch of the second present B
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