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

5.4 Essential Model and Extended Model 123
multidisciplinary approach and 6.5 about scenarios). All these views can be used for
refining the specification with details belonging to the specific view, and integrating the
relevant details into the behaviour model.
5.4 Essential Model and Extended Model
5.4.1 Essence and Implementation
The separation of essence and implementation reflects the distinction between ’problem’
and ’solution’. SASD methods [WM85, HP88] distinguish between respectively Essential
& Implementation Models and Requirements & Architecture Models. Figure 5.3
shows two approaches towards the modelling process. An essential model describes
Essence
Implementation
Essence Implementation
Sequential Approach Parallel Approach
Figure 5.3: Sequential versus Parallel Modelling
the requirements of a system in an implementation independent fashion. In a sequential
approach, an essential model is developed before an implementation model. Such an
approach leads to unnecessary development costs, caused by unnecessary iterations
(see Chapter 2). This is especially true for hardware/software systems, because of the
important role of various forms of ’implementation’ structure.
In the parallel approach both models are developed concurrently. Ward and Mellor
give two related heuristics concerning essence and implementation [WM85]. The first is
already mentioned: separate essential model and implementation model. The second is:
minimise the essential model distortion. This distortion can occur when the contents of
the essential model is mapped into an implementation environment. The implementation
structure in the implementation model will usually be inconsistent with an essential
model, when both models are developed independently. The essential model must be
distorted (by changes) to achieve consistency of both models afterwards. The best way
to minimise distortion is to prevent it by developing both essential model and implementation
model simultaneously. This is illustrated in Figure 5.3 as parallel approach.
Both the development of the essential end implementation model of a complex system
require activities on various levels of abstraction. The iterations necessary to keep the