Download Citation on ResearchGate | Guarded Commands, Nondeterminacy and Formal Derivation of Programs | So-called “guarded commands” are. The Guarded Command Language (GCL) is a language defined by Edsger Dijkstra for predicate transformer semantics. It combines programming concepts in a compact way, before the program is written in some practical programming language. Its simplicity makes proving the correctness of programs easier, using Hoare . in Formal Methods like B-Method that allow one to formally. 8 Dijkstra EW Guarded commands nondeterminacy and formal derivation of programs from LIMA ADMINISTRA at Peruvian University of Applied Sciences.
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Its simplicity makes proving the correctness of programs easier, using Hoare logic.
The guarded command is the most important element of the guarded command language. In a guarded command, just as the name says, the command is “guarded”. The guard is a propositionwhich must be true before the statement is executed. At the start of that statement’s execution, one may assume the guard to be true. Also, if the guard is false, the statement will not be executed. The use of guarded commands makes it easier to prove the program meets the specification.
The statement is often another guarded command. Skip and Abort are very simple as well as important statements in the guarded command language. Abort is the undefined instruction: The abort statement does not even need to terminate. It is used to describe the program when formulating a proof, in which case the proof usually fails. Skip is the empty instruction: It is used in the program ahd, when the syntax requires a statement, but derjvation programmer does not want the machine to change states.
Assigns values to variables. The selection often called the “conditional statement” or “if statement” is a list of guarded commands, of which one is chosen to execute.
Guarded Command Language
If more than one guard is true, one statement is nondeterministically chosen to be executed. If none of the guards are true, the result is undefined.
Because at least one of the guards must be true, the empty statement “skip” is often needed.
Upon execution of a selection all guards are evaluated. If none of the guards evaluates to true then execution of the selection aborts, otherwise one of the guards that has the value true is chosen non-deterministically and the corresponding statement is executed. However, someone implementing this, may find that one is easier or faster than the other.
Since there is no difference to the programmer, he is free to implement either way.
The repetition executes the guarded commands repeatedly until none of the guards are true. Usually there is only one guard. Upon execution of a repetition all guards are evaluated. If all guards evaluate to false then skip is executed. Otherwise one of the guards that has value true is chosen non-deterministically and the corresponding statement is executed after which the repetition is executed again.
Generalizing the observational congruence of Guarded Commands into a lattice has led to Refinement Calculus. Guarded commands are suitable for quasi-delay-insensitive circuit design because the repetition allows arbitrary relative delays for the selection of different commands. In this application, a logic gate driving a node y in the circuit consists of two guarded commands, as follows:.
PullDownGuard and PullUpGuard here are functions of the logic gate’s inputs, which describe when the gate pulls the output down or up, respectively.
Unlike classical circuit evaluation models, the repetition for a set of guarded commands corresponding to an asynchronous circuit nondteerminacy accurately describe all possible dynamic behaviors of that circuit. Depending on nonxeterminacy model one is willing to live with for the electrical circuit elements, additional restrictions on the guarded commands may be necessary for a guarded-command description to be entirely satisfactory.
Common restrictions include stability, non-interference, and absence of self-invalidating commands.
Guarded commands are used within the Promela programming language, which is used by the SPIN model checker. SPIN verifies correct operation of concurrent software applications. The Perl module Commands:: Guarded implements a deterministic, rectifying variant on Dijkstra’s guarded commands. From Wikipedia, the free encyclopedia. This article needs additional citations for verification.
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Guarded commands, non-determinacy and formal. Retrieved August 16, Archived from the original pdf on Theoretical computing science Software engineering Systems science Algorithm design Concurrent computing Distributed computing Formal methods Programming methodology Programming language research Program design and development Software architecture Philosophy of computer programming and computing science.
ALGOL 60 implementation Call stack Concurrency Concurrent programming Cooperating sequential processes Critical section Deadly embrace deadlock Dining philosophers problem Dutch national flag problem Fault-tolerant system Goto-less programming Guarded Command Language Layered structure in software architecture Levels of abstraction Multithreaded programming Mutual exclusion mutex Producer—consumer problem bounded buffer problem Program families Predicate transformer semantics Process synchronization Self-stabilizing distributed system Semaphore programming Separation of concerns Sleeping barber problem Software crisis Structured analysis Structured programming THE multiprogramming system Unbounded nondeterminism Weakest precondition calculus.
Banker’s algorithm Dijkstra’s algorithm DJP algorithm Prim’s algorithm Dijkstra-Scholten algorithm Dekker’s algorithm generalization Smoothsort Shunting-yard algorithm Tri-color marking algorithm Concurrent algorithms Distributed algorithms Deadlock prevention algorithms Mutual exclusion algorithms Self-stabilizing algorithms. Dijkstra Prize Edsger W. Dijkstra Archive University of Texas at Austin List of pioneers in computer science List of important publications in computer science List of important publications in theoretical computer science List of important publications in concurrent, parallel, and distributed computing International Symposium on Stabilization, Safety, and Security of Distributed Systems.
Guarded Commands, Nondeterminacy and Formal Derivation of Programs – Semantic Scholar
Concepts and methods ALGOL 60 implementation Call stack Concurrency Concurrent programming Cooperating sequential processes Critical section Deadly embrace deadlock Dining philosophers problem Dutch national flag problem Fault-tolerant system Goto-less programming Guarded Command Language Layered structure in software architecture Levels of abstraction Multithreaded programming Mutual exclusion mutex Producer—consumer problem bounded buffer problem Program families Predicate transformer semantics Process synchronization Self-stabilizing distributed system Semaphore programming Separation of concerns Sleeping barber problem Software crisis Structured analysis Structured programming THE multiprogramming system Unbounded nondeterminism Weakest precondition calculus.