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Embedded Systems September 2000 Vol13_10

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ROB OS H ANA Sequence Enumeration Sequence enumeration is a technique for analyzing systems. It can be used to turn a complex set of requirements into an easily implemented state machine. he criticality of correct, comple te, and testable requirements is a funda- men tal tenet in software engineering. Both func- tional and fin ancial success are affected by the quality of require- men ts. So what is a requirement? It may range from a high-level abstract statement of a se rvice or of a system constraint to a detailed mathematical and functional specifi cation. Requirements are needed for several reasons: • To specify external system behavior • To specify impleme ntati on co n- straints • To serve as reference tool for main- tenance • To predict future changes • To characterize responses to unex- pected events The system ~esign er must under- stand requirements and be able to organize them. A technical back- ground and an understanding of the user are both required. Before design can start, each requir·ement must be understood in terms of significance and priori ty in the solution strategy. Because both the developer and the customer must understand th e requiremen ts, they are usually written in natural language. But natural lan- guage is a poor medium for communi- cating requirements. The use of natural language to specify complex requirement has at 116 SEPTEMBER 2000 Embedded Systems Programming least two problems: those of ambigui ty and inaccuracy. Many words and phras- es have dual meanings and can be altered depending on the context in which they are used. A word that means one thing to one person can mean something entirely different to someone else. For example the inter- pretation of the word "bridge" can be comple tely diffe rent depending on whether you are a dentist, a civil engi- neer, an electrical engineer, or some- one who just retired early. Other disciplines require a more exact language for communicating specifications. For example, an archi- tectural blueprin t or a circuit board schematic are formal specifications that defin e what is needed but not necessarily the impleme ntatio n

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