SD3049 Formal Methods. Formal Methods Module Leader Dr Aaron Kans ([email protected])
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Transcript of SD3049 Formal Methods. Formal Methods Module Leader Dr Aaron Kans ([email protected])
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SD3049 Formal Methods
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Formal MethodsModule Leader
Dr Aaron Kans ([email protected])
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What is this module about?
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What is this module about?
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What is this module about?Ariane5 rocket crash
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What is this module about?Ariane5 rocket crash
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What is this module about?Developing software like an ENGINEER
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High Integrity Software DevelopmentBy the end of this lecture you should be able to:define the term high integrity software;distinguish between different types of critical software;identify the weaknesses of testing as an approach to software verification;identify the weaknesses of natural language specifications;distinguish between formal and informal methods of software development;
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Introduction Often software is integrated into a mechanical or electronic system Such software is known as embedded softwareCosts of software failure in these systems can be dangerously high Require a higher degree of confidence in the correctness of the software.Such software is known as HIGH INTEGRITY SOFTWARE.
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Critical Software business critical softwaremission critical softwaresafety critical software
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Integrity LevelsIntegrity level 5Integrity level 1
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TESTINGSPECIFICATIONThe importance of the specification
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Limitations of Testing Testing cannot take place until some implementation is available.Testing can only help to uncover errors - it cannot guarantee the absence of them. Testing is always carried out with respect to requirements as laid down in the specification.
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UML: a review BankAccount
accountNumber: StringaccountName: Stringbalance: Real
deposit (Real)withdraw (Real) : BooleancurrentBalance(): Real
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Weakness of natural language specificationsWithdraw:Receives a requested amount to withdraw from the bank account and, if there are sufficient funds in the account, meets the request.Returns a boolean value indicating success or failure of the attempt to withdraw money from the account.Natural language descriptions do not have a fixed meaning, they are ambiguous.
These notations do not have a fixed semantics
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Incomplete specificationsA specification can be considered incomplete when the behaviour is not completely defined. Withdraw:Receives a requested amount to withdraw from the bank account and, if there are sufficient funds in the account, meets the request.Returns a boolean value indicating success or failure of the attempt to withdraw money from the account.
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Inconsistent specifications A specification is inconsistent when it contains within it contradictions. Withdraw:Receives a requested amount to withdraw from the bank account and, if there are sufficient funds in the account, meets the request.Returns a boolean value indicating success or failure of the attempt to withdraw money from the account.OVERDRAFT?
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Formal languages It is desirable to use a specification notation with a fixed, unambiguous, semantics. Notations that have a fixed semantics are known as formal notations, or formal languages. A fixed semantics is achieved by defining a language in a completely unambiguous way using a mathematical framework.
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Formal Methods initial formal specification1st transformation2nd transformationnth transformationfinal programA formal method includes a proof system for demonstrating that each transformation preserves the formal meaning captured in the previous step.
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Advantages of formal methods Generates good test cases;increases confidence that the specification accurately captures the real system requirements;important properties of the initial specification can be checked mathematically;proofs can help uncover design errors as soon as they are made;a proof of program correctness can be constructed.
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Classifying formal methods Algebraic Model-based Sequential systems Larch Vienna Development Method (VDM) Z B Concurrent Systems Calculus of Communicating Systems (CCS) OBJ Prototype Verification System (PVS) Concurrent Sequential Processes (CSP)
