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

26 On Specification of Reactive Hardware/Software Systems
way. The LOTOS language supports interrupts, but behaviour can only be aborted and
not interrupted. In ROOM behaviour can both be aborted as well as interrupted. SDL
and Estelle do not have interrupt facilities. The grain of concurrency in ROOM, SDL
and Estelle is at the level of process objects 7 . LOTOS supports concurrent processes.
LOTOS processes are pieces of behaviour, but they are not objects that perform this
behaviour. Nevertheless, objects can be modelled by processes. This is exploited in the
ROOA method [Mor94]. Except for ROOM, none of the methods support data objects.
LOTOS and SDL use abstract data types and Estelle uses Pascal types to model data.
Abstract data type descriptions are elegant from a mathematical point of view, but are
difficult to understand for non-mathematicians. Pascal is a procedural language and
has insufficient support for data modularisation.
The Concept of Inheritance
The reader may wonder why we did not discuss the concept of inheritance. Inheritance
is often regarded as the key feature of object-orientation. We do not agree with this
statement and share the viewpoint of P. America [Ame89] who argues that the essence
lies in encapsulation and in the protection of individual objects against each other.
Inheritance is certainly a useful concept for structuring large systems. It is not the key
issue for the modelling of reactive hardware/software systems. In this thesis we do
not pay as much attention to inheritance as a number of other specification methods do
[R 91, C 94, CY91a, CY91b, J 92, Boo91, Mor94, BH93].
2.6 Activity Frameworks for Specification Development
The Role of Activity Frameworks
Creating a specification is a very complicated task that can hardly be performed without
an activity framework guiding the specification and design development. In theory,
an activity framework should determine the different modelling or design phases, the
order in which they are visited and the transition criteria (such as verification and validation
criteria) for progressing from one phase to the next 8 . It should further determine
the objectives of each design or modelling phase and give guidance to the navigation
through each phase in order to obtain these objectives. This point of view upon activity
frameworks is theoretically appealing since it allows the specification development to
be split into clear-cut modelling phases. In practice this cannot work, as will be shown
in the remainder of this section.
A specification method requires an activity framework that
guides the development of specifications.
7In ROOM these objects are called actors, in SDL process instances and in Estelle they are called
modules.
8In literature this part of an activity framework is often called a process model. Examples of process
models are the waterfall model [Roy70] and the spiral model [Boe88].