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School of Computer ScienceG53FSP Formal Specification1 Dr. Rong Qu Introduction to Formal Specification http://www.cs.nott.ac.uk/~rxq/g53fsp

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School of Computer ScienceG53FSP Formal Specification2 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

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School of Computer ScienceG53FSP Formal Specification3 Background Need to be precise Management Must be able to agree what is to be implemented

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School of Computer ScienceG53FSP Formal Specification4 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

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School of Computer ScienceG53FSP Formal Specification5 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

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School of Computer ScienceG53FSP Formal Specification6 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

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School of Computer ScienceG53FSP Formal Specification7 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

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School of Computer ScienceG53FSP Formal Specification8 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

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School of Computer ScienceG53FSP Formal Specification9 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

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School of Computer ScienceG53FSP Formal Specification10 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 …

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School of Computer ScienceG53FSP Formal Specification11 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

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School of Computer ScienceG53FSP Formal Specification12 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 …

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School of Computer ScienceG53FSP Formal Specification13 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

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School of Computer ScienceG53FSP Formal Specification14 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

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School of Computer ScienceG53FSP Formal Specification15 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

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School of Computer ScienceG53FSP Formal Specification16 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

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School of Computer ScienceG53FSP Formal Specification17 Formal Methods Parts Program specification Program verification Automated theorem proving Model checking

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School of Computer ScienceG53FSP Formal Specification18 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.

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School of Computer ScienceG53FSP Formal Specification19 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

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School of Computer ScienceG53FSP Formal Specification20 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.

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School of Computer ScienceG53FSP Formal Specification21 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.

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School of Computer ScienceG53FSP Formal Specification22 Formal vs. Informal Methods Formal specification vs. SSADM SSADM now government standard Widely used in industry Semester 5 of CSiT

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School of Computer ScienceG53FSP Formal Specification23 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

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School of Computer ScienceG53FSP Formal Specification24 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

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School of Computer ScienceG53FSP Formal Specification25 Mathematics – good features Easily handle abstractions Can be used for reasoning about, and describing a system Is concise Is non-ambiguous

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School of Computer ScienceG53FSP Formal Specification26 Mathematics – good features Is applied widely to the real world Can approximate where exactness is unnecessary Changes slower than computing

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School of Computer ScienceG53FSP Formal Specification27 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

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School of Computer ScienceG53FSP Formal Specification28 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

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School of Computer ScienceG53FSP Formal Specification29 Summary Background of formal specification Parts of specification Formal specification vs. SSADM Natural language Formal method advantages and drawbacks Z specification language

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