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

222 Modelling of Concurrent Reactive Behaviour
In general complex systems require a mix of styles corresponding to a mix of implementation
technologies. The idea to connect all objects to one channel as we suggested for
software is a not very realistic extreme that gives little structure to a model. In general
we need a mix of system parts with different dynamic properties. Some system parts are
statically interconnected and active when the system is active. Other parts are needed
dynamically. We would like to create them and remove them.
6.6.8.6 Mimicking Process Creation
A POOSL model describes dynamic behaviour of a future implementation in for instance
hardware or software. The semantics of POOSL are defined in terms of an operational
model. One of the aspects is that the operational model explains how a system is instantiated.
Instantiation is a hierarchical construction process that in general will start
from a single cluster. For details see Chapters 8 and 9. Process objects, clusters and
channels of a system are instantiated as static structure. This structure is predetermined
at ’compile-time’. When we decide for an implementation in hardware this instantiation
approach is natural. A hardware structure simply exists before the system starts executing.
For software, however, it would be convenient to have a notion of process creation.
We do not offer this explicitly. A system is always initiated with a static number of
process objects and clusters. However, there is a way to mimic process creation. A process
can be claimed from a stock of processes when it is needed. Subsequently it can be
released which means that it is added to the stock again when its use is finished. This
approach requires instantiation of at least the maximum number of processes that is ever
needed simultaneously. Processes can be claimed and released if their internal behaviour
is constructed properly. These processes must be dynamically linked temporarily to
other processes that require the services they offer.
An example of a piece of behaviour that mimics process creation is the following
initial method call
free
instance methods
free
a?claim(requestorId);
a!granted(requestorId,myId);
claimed abort (a?release(id1,id2 id1=myId and id2=requestorId); free)
claimed
’Space for the specification of all services offered.’
The previous behaviour description can be part of the class description of the objects
in a multiple. It can be used to claim and release an object of such a multiple. Any
object of the multiple is connected to channel a and can receive a claim message if it
is free. With the claim message the object receives the identifier of the sender denoted
as requestorId. The object uses this identifier as destination indication in its reply and
puts its own identifier myId in this reply. The object is then claimed and can perform
its normal course of behaviour. This course of behaviour is modelled by the method