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

6.6 Dynamic Behaviour 221
but not the other way around. Interrupts are only defined for process statements. This
means that complicated calculations performed by data objects cannot be interrupted.
6.6.8.3 Collaborating Process Objects
We offer various forms of communication for process objects. Clustered collections of
objects can be specified to be sequential by a concurrency boundary. When collaborating
objects must perform sequential behaviour as a whole, all objects must use message
passing with reply and waiting just like data objects do. By using the message flows
with reply consequently we can describe sequential behaviour. In the behaviour description
of the objects a message-send statement must then be followed immediately
by a message-receive statement for the reply. This guarantees that the process waits for
a reply.
In contrast, the separation of a reply and the first rendezvous enables concurrency,
independence and autonomy of process objects. Tail recursion offers the power to
describe infinite behaviour of concurrent processes. Processes are more autonomous
than data objects because processes protect themselves against unwanted invocations.
Processes can determine the order in which they want to receive messages or other
conditions that must be fulfilled.
6.6.8.4 Collaboration in Distributed Systems
Physically distributed systems or subsystems can need complex behaviour to perform
communication. Channels between distributed modules usually need protocols. A
way to specify the relative independence of subsystems is the use of asynchronous
message flows. However, an abstract model can be build using synchronous message
passing. Subsequently a model is extended with implementation details. In this phase
asynchronous messages can be used to model the protocol.
6.6.8.5 Dynamic Processes Linking
A system model can be constructed in particular styles. As a model is refined and
extended towards specific implementations, the quality of style determines the ease of
the mapping on an implementation description language. For example, an appropriate
model directed towards a software implementation can have:
data objects with their dynamic links;
process objects that share one channel, perform weakly distributed communication
and use dynamic linking by passing names of acquaintances.
For hardware an appropriate model can have:
concurrent process objects that are statically interconnected by channels;
data objects used for decomposition of complex sequential internal behaviour of
hardware building blocks.