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

5.4 Essential Model and Extended Model 125
The phases divide the specification process in a sequence of two steps in time. In each
phase two models are developed simultaneously:
Essential Behaviour Model and Architecture Structure Model;
Extended Behaviour Model and Implementation Structure Model.
Simultaneous modelling is visualised by the two headed arrows in Figure 5.4.
The separation into four models determines the activity framework of the new method.
Behaviour modelling consists of two pieces. Firstly essential behaviour and secondly
additional behaviour that is needed because of implementation decisions. Additional
behaviour leads to an extended behaviour. This separation can be explained by a short
excursion to method heuristics.
Ward and Mellor [WM85] give some guidelines in the context of redevelopment versus
new development. An essential model must be derived from the existing design, in case
of redevelopment. An example of a heuristic for redevelopment is ’ignore implementation
technology verification’. This means: Try to find the essential behaviour in the existing
implementation. Remove behaviour that improves the imperfections of the implementation
technology. After all, the new implementation may have completely different properties.
We interpreted this heuristic for redevelopment as very important for new development.
We need apparently two separate phases. A first modelling phase where the essential
behaviour is defined and a second phase where behaviour that comes with concrete
implementation decisions is added.
Notice that the separation into phases is different from the sequential approach in Figure
5.3. In both phases of development the behaviour model must be created in parallel with
the structure of the implementation. Essential behaviour modelling is performed parallel
with essential implementation modelling. The essential implementation model defines
implementation decisions on a high level of abstraction and determines corresponding
structuring and modularity of the system. This defines a view of the system that reveals
structure, which influences the modularity of the system. This view is therefore called an
architecture structure. Consequently this high level model is called Architecture Structure
Model. Lower level, more detailed implementation structure decisions are defined in the
so-called Implementation Structure Model.
The precise difference between essential specification and extended specification depends
on the system to be designed. Large complex systems tend to have more levels
of abstraction than smaller systems. Therefore there may be different interpretations of
what is essence and what is extension, and also what architecture modelling is and what
implementation modelling is.
An essential behaviour model defines ’what’ communication happens. It abstracts from
’how’ communication between modules is performed. Behaviour, that defines how
communication is realised, is added in an extended model. An example of additional
behaviour is the extension with communication protocols. A simple message in the