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

366 SHE Framework
11.4.9 Requirements Catalogue
A Requirements Catalogue completes a system specification with information that lacks
in the other parts of a model, and also contains redundant information for the purpose
of easy retrieval. Figure 11.8 showed an approach where the Requirements Catalogue
consists of four items:
Listed Requirements;
System Dictionary;
Architecture Decisions Statement;
Implementation Decisions Statement.
The purpose of these items has been described in Paragraph 11.4.2.3. The further detailed
definition of the prescribed structure of a Requirements Catalogue is subject for future
research. The development of tools for our method will strongly influence our view on
the function of a catalogue. A System Dictionary may very well be distributed. Various
forms of information can be stored connected to graphical objects in various diagrams.
Automated documentation facilities can retrieve and present this information.
11.5 Extended Specification Modelling
11.5.1 Extension of the Essential Specification
In the previous section we described the development of the various views that form an
Essential Specification. An Extended Specification is developed by taking the Essential
Specifications as a starting point. This Essential Specification must be gradually refined
and extended with the purpose of bridging the gap between the Essential Specification
and the actual implementation of a system. Figures 11.3 and 11.4 show the place
of extended modelling relative to other development activities. Figure 11.5 shows
that both an Essential Specification and an Extended Specification consist of the same
sorts of views. Notice that all essential representations have a corresponding extended
representation. For example, Essential Message Flow Diagrams correspond to (are
the basis for) Extended Message Flow Diagrams. An Architecture Structure Model of
the Essential Specification corresponds to an Implementation Structure Model of the
Extended Model. Most of the approaches described in the previous section hold also for
the creation of an Extended Specification. Therefore we will limit the description to the
differences in the modelling approaches.
The milestone between essential modelling and extended modelling can be placed when
we pay attention to the purpose of both models. An Essential Specification is used
for extensive communication with users and other experts. The focus is mainly on
what the system must do. In contrast to other methods we allow to perform various
design activities in this essential modelling phase. These activities concern system level
design (architecture structure), design for reuse and early incorporation of definitive