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

2.3 Specifications, Models and Views 13
given examples is often far from the real world. On the other hand there are a lot of
available books about methods. These methods are in some way mature and therefore
more or less old. The process from new method to published book is time consuming.
Especially because good books contain cases and experience, that took time to be gathered.
Methods have the problem of being very complex entanglements of entities. One
cannot simply implant a ’good part’ of a method into a new method. Together, frameworks,
notations, concepts, heuristics, languages and tools form a strongly connected
very complex artefact. Getting to know a method is extremely complicated and takes
a considerable amount of time. This makes it very hard to assess the weaknesses and
strengths of various methods. Therefore, designing a new method is not just a matter of
reading a few books and joining the nice parts. The gap has to be bridged between two
sorts of material. This leads to the objective of establishing a solid base by analysing
methods at the level of concepts. The proper and useful concepts have to be combined
into a new method. In order to get a consistent unity we will use formal approaches in
an either direct or indirect way.
Method design requires a fundamental and conceptual approach.
In this chapter we highlight the critical aspects we discovered to be relevant for the
definition of requirements for a new method.
2.3 Specifications, Models and Views
Models of Systems
A specification is a document that prescribes, in a complete, precise, verifiable manner, the
requirements, design, behaviour and other characteristics of a hardware/software system
or a hardware/software system component [IEE84]. This definition is rather broad.
We ’specify’ that a specification must contain a model of the physical system to be developed.
A model of a system is an abstract representation of this system. It provides a
means to reason about a system, i.e. to provide answers to certain questions about this
system. From each model a collection of properties concerning the system involved can
be deduced. A model of a system is then called adequate if this system satisfies (within
a certain degree of acceptability) each of these properties. Vice versa, this system is said
to satisfy the model. In general there exists more than one system satisfying the model.
So in fact, each model implicitly defines a collection of systems. This collection will
be called the realisation space of the model and each element of this space will be called
a realisation. Notice that the more properties the model allows to be determined, the
smaller the realisation space will be.
Development of complex systems requires the creation of models.
Expressing Properties
Development of a specification should end up with a model that is implementable by