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

11.4 Essential Specification Modelling 345
System Dictionary. This is an alphabetically arranged list of items from the various
models of a specification. The meaning of each item is denoted in this list.
Architecture Decisions Statement. This is a brief justification for the design decisions
of the architecture design. It gives considerations and reasons why alternatives
were rejected.
Implementation Decisions Statement. This is a brief justification for the design decisions
made for the design of implementation structure.
The starting point for the catalogue is the Initial Requirements Description (see Figure
11.3). In practice initial requirements are often an unstructured mix of various sorts of
information. Besides functional requirements, for instance, environmental conditions
such as temperature, humidity, etcetera should be specified. Environmental conditions
are typically denoted in the Listed Requirements in the catalogue of the model.
Besides the items we mentioned, additional information can be added to the Requirements
Catalogue. This depends on the kind of project and the kind of customer. Traceability
information, for instance, can be required by the customer. This means that each
item in the model must be related to an initial requirement.
Notice that the unified model is not completely formal. A lot of comment is preferably
appended to the formal text. This can cause some redundancy. For instance, the
meaning of items can be given both as comment in the Unified Model and in the System
Dictionary. This information must be kept consistent.
11.4.3 Essential Modelling Activities
Various models can be developed concurrently. The order of modelling activities is
flexible. The purpose of the framework is to give orientation in the modelling process.
Various sorts of systems may require an emphasis on activities in some part of the
framework. For instance, a system with complex data structures and relatively few
control aspects will be modelled with emphasis on object class modelling. Using Object
Class Diagrams the various relations between classes can be visualised. The emphasis
in our research, is on the modelling of hardware/software systems that have control
aspects besides the processing of information. The case in Chapter 12, for instance, is
typically a specification of a hardware/software system that controls a machine and
that processes information. For these systems we can give an impression of the flow
of activities. Figure 11.9 shows an impression of the development of various parts of a
specification over time. This picture does not show the iterations that always happen
in a development process. All modelling activities must be performed for each of the
following models in both phases:
1. Essential Specification which consists of
a. Essential System model
b. Essential Environment model