Specification of Reactive Hardware/Software Systems - Electronic ...

2.4 Specification Languages, Formality and Tools 19
primitives. Informal languages allow a certain degree of freedom in the use of the
different model primitives. These languages allow designers to easily reason about
the different (often vague) problem domain concepts without being hindered by all
kinds of strict semantic rules. Informal languages further simplify the (necessary)
communication between problem domain experts, of the same field or of different fields
of expertise. Therefore informal models form an important, even indispensable, part of
any complex specification.
Complex system development requires the use of informal models.
Formal Models for Precision
A specification should be precise and verifiable [IEE84]. These requirements are not met
when specifications consist of informal models alone. The imprecision of informal models
allows context sensitive and discipline sensitive interpretations. Different designers
can interpret the same model in different ways. Inconsistent interpretations will lead to
incompatible subsystems, resulting in high costs for changes. Next to precise, a specification
should be verifiable. This means that one should be able to answer the question: ’Am
I building the system correctly?’. During the process of specification, verification boils
down to the checking of consistency between models 1 . A special form of verification 2 is
validation. Validation answers the question, ’Am I building the correct system?’. During
validation it is checked whether a model satisfies the requirements and suffices for its
purpose. The checking of consistency between models is terribly complicated if these
models are informal. First of all it is hard, if even possible, to define and to interpret
the notion of consistency in the first place. Secondly, consistency checking of complex
models becomes unmanageable without the availability of advanced software tools. The
development of such tools requires a considerable degree of formality.
Complex system development requires the use of formal models.
Informal Views and Formal Unified Models
We conclude that specifications may not consist of informal models alone. If a method
does not support the creation of a unified model, the informal models should be accompanied
by formal counterparts as much as possible. A method that reasonably satisfies
this requirement is Statemate [H
90]. This method produces three separate views that
are expressed in readable, graphical languages. These views are state-transition views,
functional views and structural views. Within the Statemate method, state-transition
views are the most important. These are expressed in the well-known formalism called
Statecharts [Har87]. A rigorous semantics of Statecharts was first defined in [H 87].
1 The term verification is often used to refer to the check of whether a model satisfies (or conforms
to) another model derived in a previous design phase (see also Section 2.6). Satisfiability checking and
conformance checking are in fact a form of consistency checking.
2 Note that since in our opinion, a requirements document is a model too, validation can be considered
a form of consistency checking.