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G53FSP Formal Specification

G53FSP Formal Specification. Dr. Rong Qu. Introduction to Formal Specification. http://www.cs.nott.ac.uk/~rxq/g53fsp. Background. A specification may have many uses, in many forms Management Specification in English Programmers Written in a pseudo (or real) programming language

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G53FSP Formal Specification

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  1. G53FSPFormal Specification Dr. Rong Qu Introduction to Formal Specification http://www.cs.nott.ac.uk/~rxq/g53fsp G53FSP Formal Specification

  2. Background • A specification may have many uses, in many forms • Management • Specification in English • Programmers • Written in a pseudo (or real) programming language • CS & Management • Improve quality of software systems • Provide proof & verifications G53FSP Formal Specification

  3. Background • Need to be precise • Management • Must be able to agree what is to be implemented G53FSP Formal Specification

  4. Background • Need to be precise • Programming • Written in language is probably no use • Expensive to create • Long • Include un-necessary details • Performance not effect of operation • Difficult to understand G53FSP Formal Specification

  5. Background • Precise definition of • Effects of various operations rather than its performance details • Information to be displayed to users • No need of • How the operations are to be done • How the data is to be stored Etc The details is no use of specify the effect of the system G53FSP Formal Specification

  6. Background • Also may need a specification to • Prove certain properties • Prove that certain combinations of states never occur • Value of a given variable never go outside certain bounds • Correctness of system can be proven G53FSP Formal Specification

  7. Purpose of Formal Specification • To state what system should do without describing how to do it • To reduce faults in systems • Invest more effort is early stage of system development • Requirement errors can be discovered as early as possible and resolved G53FSP Formal Specification

  8. Specification Parts • A specification need to include • Details of the system • The states it can occupy • Invariants which will always hold • Dynamic aspects • All operations which are possible • The relations of inputs to outputs • Changes of state that can occur G53FSP Formal Specification

  9. Specification Parts • A specification will include • Functional requirements • The effect of xxx will be • The output of command yyy will be as specified in standard zzz • The system will produce a report on salesman effectiveness G53FSP Formal Specification

  10. Specification Parts • A specification will include • Non-functional requirements (properties) • All data access should be via company supplied subroutines • The system should be immune to power failures • The response time must be … G53FSP Formal Specification

  11. Specification Parts • A specification will include • Design directives • The system will collect data from … • The VDU display will be in the form … • The designer will use SSADM G53FSP Formal Specification

  12. Specification Parts • A specification will include • Goals • Response times should be minimised • It should run in 748kb of memory • Data statements • The system must maintain the average temperature over the preceding … G53FSP Formal Specification

  13. Definition – Formal Specification • The specification will be • A strict mathematical definition of the effect of the required operation • Usually expressed in mathematical notions with precisely defined vocabulary, syntax and semantics • Definition is not necessarily in the form in which it can be programmed G53FSP Formal Specification

  14. Definition – Formal Specification • In computer science, formal methods refers to mathematically based techniques for the specification, development and verification of software and hardware systems. From Wikipedia, the free encyclopedia. http://en.wikipedia.org/wiki/Main_Page G53FSP Formal Specification

  15. Definition – Formal Specification • The approach is especially important in high-integrity systems, for example where safety or security is important, to help ensure that errors are not introduced into the development process. From Wikipedia, the free encyclopedia. http://en.wikipedia.org/wiki/Main_Page G53FSP Formal Specification

  16. Definition • Formal methods are particularly effective early in development at the requirements and specification levels. From Wikipedia, the free encyclopedia. http://en.wikipedia.org/wiki/Main_Page G53FSP Formal Specification

  17. Formal Methods Parts • Program specification • Program verification • Automated theorem proving • Model checking G53FSP Formal Specification

  18. Program Specification • A program specification is the definition of what a computer program is expected to do • It can be • informal, in which case it can be considered as a blueprint or user manual from a developer point of view, or • formal, in which case it has a definite meaning defined in mathematical or programmatic terms. G53FSP Formal Specification

  19. Program Verification • In computer science, program verification is the process of formally proving that a computer program does exactly what is stated in the program specification it was written to realize • Program verification is more specific in that it aims to verify the code itself, not only some abstract model of the program. • Intel, AMD: verify chips • BMW: automotive system G53FSP Formal Specification

  20. Automated Theorem Proving • Automated theorem proving is the proving of mathematical theorems by a computer program. Depending on the underlying logic, the problem of deciding the validity of a theorem varies from trivial to impossible. G53FSP Formal Specification

  21. Model checking • Model checking is a method to algorithmically verify formal systems. This is achieved by verifying if the model, often deriving from a hardware or software design, satisfies a formal specification. The specification is often written as temporal logic formulas. G53FSP Formal Specification

  22. Formal vs. Informal Methods • Formal specification vs. SSADM • SSADM • now government standard • Widely used in industry • Semester 5 of CSiT G53FSP Formal Specification

  23. Formal vs. Informal Methods • Lots of possible inconsistencies • Decision tables To prove that, if you use “don’t care” entries, the result is completely and uniquely defined? • Computer tools to help overcome the difficulties, but are still basic problems G53FSP Formal Specification

  24. Mathematics vs. Natural Language for System Specification • Deficiencies of natural language • Can be vague • Can be ambiguous • Can be self-contradictory • Can be incomplete • Encourages imprecise thinking • Cannot easily handle abstractions G53FSP Formal Specification

  25. Mathematics – good features • Easily handle abstractions • Can be used for reasoning about, and describing a system • Is concise • Is non-ambiguous G53FSP Formal Specification

  26. Mathematics – good features • Is applied widely to the real world • Can approximate where exactness is unnecessary • Changes slower than computing G53FSP Formal Specification

  27. Formal Notation – drawbacks • The customer cannot easily understand the specification • The mathematics used is unfamiliar to many of the staff • In real world, getting user requirements document is a problem G53FSP Formal Specification

  28. Z • A formal specification technique developed at Oxford • Uses very mathematical notation to provide exact definitions of a system • System is described in a number of small Z modules, which can cross-refer each other • Each module is expected to have some descriptive English text to help users to understand it G53FSP Formal Specification

  29. Summary • Background of formal specification • Parts of specification • Formal specification vs. • SSADM • Natural language • Formal method advantages and drawbacks • Z specification language G53FSP Formal Specification

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