Lecture 6 & 7. System Models. System models are abstract descriptions of systems whose requirements are being analysed. Objectives To explain why the context of a system should be modelled as part of the RE process To describe Behavioural modelling (FSM, Petri-nets) ,
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State machine model does not show flow of data within the system
Finite State Machines (FSM), also known as
Finite State Automata (FSA)
are models of the behaviours of a system or a complex object, with a limited number of defined conditions or modes, where mode transitions change with circumstance.
A model of computation consisting of
a FA with two extras.
1. It has TWO alphabets, an input and output alphabet.
2. It has an output letter associated with each state. The machine writes the appropriate output letter as it enters each state.
This machine might be considered as a "counting" machine.
The output produced by the machine contains a 1 for each occurrence of the substring aab found in the input string.
Transitions are labelled i/o where
The following Mealy machine takes the one's complement of its binary input. In other words, it flips each digit from a 0 to a 1 or from a 1 to a 0.
Place with token
Arc with capacity 1
Data dictionaries are lists of all of the names used in the system models.
Descriptions of the entities, relationships and attributes are also included