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

24 On Specification of Reactive Hardware/Software Systems
Information exchange between process objects and clusters must
be based on message passing.
Reactive Behaviour
Processes in reactive systems, have to exhibit very complex communication behaviour.
They exchange messages in various ways: synchronous, asynchronous, continuous and
interrupt-driven. Further, they can direct a message towards a specific other process,
which they know by name. They can also put a message onto a channel, without
knowing the explicit name of the destination processes. Process objects must be able to
perform all these forms of communication behaviour.
Process objects must be able to perform various forms of complex
communication behaviour.
Data Objects
Processes in reactive systems are able to perform complex manipulations on their private
data. Because private data can have complex structures of their own, they should be
represented by objects too. We will call these objects data objects. Data objects can be
exchanged as parameters of messages between processes. To make data objects reusable,
they have to be organised into classes.
Private data of process objects must be represented by data objects.
Data objects must be grouped into classes.
A Comparison with Statemate
Statemate [H 90] is a well-known method for the modelling of complex reactive systems.
Statemate is consistent with many ideas of (real-time) structured analysis and structured
design methods [WM85, HP88]. These methods typically incorporate data flow models
and state machine models. Both [WM85] and [HP88], however, use conventional
finite-state machines and do not offer appropriate machine interaction primitives. Conventional
state diagrams are inappropriate for complex behaviour descriptions since
they suffer from being flat and unstructured, are inherently sequential in nature and
give rise to an exponential blow-up in the number of states [H 90]. These problems are
overcome in Statemate by incorporating the Statechart formalism [Har87]. Statecharts
extends conventional machines by offering decomposition of states, and broadcast communication.
A major disadvantage of Statemate is that it lacks good support to define
object-like entities. Modularisation in Statemate is achieved mainly through data-flow
transformations, called activities. Activities do not provide any strong form of encapsulation.
Concurrency in Statecharts is based on the synchrony hypothesis [BG85]
implying that all concurrent components of a system proceed in lock-step mode. In
addition, state machines communicate by data-less broadcast signals, by using shared
variables and by monitoring each others states. These forms of concurrency and communication
conflict with the requirement of objects to be independent weakly coupled
entities running at their own speed.