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

11.3 SHE Framework 341
the system. The transition between the phases will depend on the sort of problem and
system to be developed. Once the transition took place, a return to the previous phase
must be considered as an unwanted iteration. Iterations can be necessary but must be
prevented as much as possible.
In Chapter 5 we explained why structure and implementation are essential elements in
a specification process. The framework gives an explicit place to the development of
structure and to the incorporation of design decisions. Both the Essential Specification
and the Extended Specification blocks in Figure 11.5 contain dotted blocks: a behaviour
model and a structure model. The architecture structure model and the implementation
structure model incorporate the structure and implementation aspects of a system.
During modelling of the essentials of the system only high level design issues may
be taken into account, together with prescribed implementation and topology aspects.
Essential Specification Models must guide the discussion to the essence of the required
functionality. The models must be rather abstract and easy to understand for this
purpose. This enables the communication with various experts and users about the
model.
The extension of the model in the next modelling phase can be performed with experts
that focus on an efficient and robust implementation. It will appear that more in depth
knowledge about languages and description methods for hardware and/or software
implementation is necessary to guarantee a fast and reliable implementation. In addition
knowledge of communication protocols will in general be necessary.
The extended modelling phase must prevent that non realistic abstractions cause too
many implementation problems. A thorough analysis of the essential model is not
enough to guarantee a smooth implementation process. For complex systems, it is
absolutely necessary to extend the model with all processes that appear to be necessary
for enabling chosen implementation technologies. Technology can and must strongly
influence the detailed formal behaviour description. Especially the description style
must be adjusted such that the mapping of the specification to an implementation
(language) is feasible. The gap between specification and implementation must be made
as small as possible. A reason for this is not only that it saves time and money but that
it has a large influence on the quality of the system as well. When an implementation
deviates strongly, it contains processes with deviant communication behaviour. This
behaviour is however precisely what we model and investigate. The problems with
the design of complex systems have mainly to do with a poor understanding of the
communication between concurrent processes. It is simply impossible for a designer to
keep an overview on a complex system without tools. The role of the unified model is
to verify the behaviour of collaborating processes by simulation and verification. So the
final model of the Extended Specification phase is a carefully designed robust model.
Deviations from this model caused by the transformation from model to implementation
must be prevented as much as possible. The proximity of both extended model and
implementation determine whether deviations appear or not.