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

58 Abstraction of a Problem Domain
In contrast to the world of software design, there is no modelling tradition yet in the
world of hardware designers. There is no ’hardware engineering’ counterpart of ’software
engineering’. Most designers follow an ad hoc top down functional approach
’straightforward’ to an implementation. Although digital hardware designers started
to adopt simulation and use of higher level languages such as VHDL [IEE88], there is
almost no tradition in abstract and implementation independent modelling of a problem
domain.
A ’system’ specification and design method must offer concepts for the creation of two
models concurrently:
1. a problem domain model;
2. an architecture/implementation structure model.
For the domain model we choose an object-oriented approach. In this chapter we
discuss what objects are and how they can be used to model a system. The phenomenon
object has many aspects. The focus in this chapter is on sorts of objects, composites of
objects, static properties of objects such as relations, services and attributes. The study
of these aspects formed the basis for the development of the method SHE. Dynamic
behaviour and dynamic properties of (collaborating) objects are described in Chapter
6. Architecture and implementation structure modelling must guarantee the physical
feasibility of the system. Structure modelling is described in Chapter 5.
4.2 Objects
4.2.1 An Object represents an Object
In this chapter we start depicting objects. We can look at objects form various points
of view. The phenomenon ’object’ will appear to be very complicated. A good understanding
of the new specification method requires a good understanding of the concept
of object. There are two ways to look at objects. A first practical approach is to look at
the use of objects for the purpose of system modelling. In this case the meaning of an
object is related to the thing that the object represents. Secondly objects can be studied
from a conceptual point of view. In this second case we really try to understand what
the concept of object means. How powerful is the concept of ’object’ for the analysis
and description of complex systems?
A study of the concept of ’object’ shows very quickly that there is not such a thing as ’a
concept of object’. There appear to be as many different concepts of object as there are
object-oriented methods and languages. A simple example is the existence of sequential
and parallel object-oriented languages. When the concept of object is chosen such that
it can perform parallel behaviour, it is completely different from a concept of object that
can only perform sequential behaviour. When a method has a concept of object that is
chosen to be sequential, the method can not be used to model parallel systems.