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

5.8 Boundaries 137
x
w w
x Object A
y z
Object C
a b
c
IMPL: i80960
5.8.3 Behaviour and Boundaries
5.8.3.1 Behaviour Styles
Object B
Cluster used as
implementation
boundary: this cluster
must be inplemented
with an intel 80960
microprocessor.
Figure 5.11: Implementation Boundary
In the previous subsection (5.8.2), the application examples of boundaries mainly
showed possible formalisation of various forms of structure in the object models. In
5.2.3 about Modules, we wrote: ’The style of a behaviour description of a module should be influenced
strongly by its properties.’ Boundaries specify properties such as implementation
technology, concurrent or sequential behaviour, or distribution. These design decisions
should be taken as early as possible because they should influence the behaviour description
style. For example: a composite that is specified to have sequential behaviour
must be described such that only one object is active. A way to achieve this is to make
one object responsible for the mode of operation of the composite. This can be achieved
by using a supervised composite, see 4.8.8.
Another example is the partitioning of a system into hardware and software. Implementation
boundaries can prescribe specific implementations. In case of a software
implementation in an object-oriented implementation language, it may be worthwhile
to put effort in additional object modelling, to find generalisation/specialisation structures.
Object classes should be modelled according to such structures. A hardware
implementation may be synchronised by one common clock. In contrast, distribution
will usually cause asynchronous execution. The communication and the behaviour of
the objects must be modelled accordingly. It may be clear that POOSL modelling of
behaviour must be postponed until a model is somehow getting mature. The modelling
process must be so far that most objects have been found and their communication must
have been modelled according to architecture structures. Postponing too long, however,
is disadvantageous. Behaviour modelling may give important additional understanding.
A structure which must be changed because it was chosen too early does not have to