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

214 Modelling of Concurrent Reactive Behaviour
6.6.6.2 Method Calls
After initialisation of its instance variables a process starts automatically with its initial
method call. Methods can be called without the reception of a message. This is possible
because of the separation of the concepts of message and method. Methods have been
described in Paragraph 4.15.3.2. We summarise the most important remarks of that
paragraph:
The name of a message to be received corresponds to a service that is offered by the
process objects of the class for which the message is defined.
Names of messages to be sent correspond to results of operations to be performed.
A service may consist of one or more operations.
Process objects may be concurrent autonomous entities that perform operations on
their own initiative. The initial method call is a typical example.
Operations are performed by pieces of behaviour called methods. The method names
serve only for the internal behaviour description. They are in principle completely
independent of the message names. However for readability and comprehensibility
of a behaviour specification it is strongly recommended to create a correspondence.
6.6.6.3 Named States
Methods are grains of behaviour that can be used in a flexible way. When a process is
executing a method this can be interpreted as being in a behaviour state. This interpretation
makes the method a sort of named state. Of course this approach works solely when
the grains of behaviour are designed and named adequately. We call this modelling
style a state-oriented style. Such a style can lead to very clear descriptions that make
the communication during model reviewing easier.
6.6.6.4 Method Definition
The behaviour of a process starts with its initial method call typically defined as
m(E1 ¥¡ ¡ ¡ ¥ Eq)( ). The results of expressions E1 ¥¡ ¡ ¡ ¥ Eq are bound to the input parameters
of the initial method which is one of the instance methods. An initial method has
no output parameters.
The other instance methods are named pieces of behaviour that compose the internal
sequential behaviour of a process. POOSL is very powerful in the sense that it enables
very compact and flexible behaviour descriptions. Behaviour can be described as either
sequences of methods, or non-deterministic choices for alternatives. Methods can also
be used in a mutual recursive way so that non-terminating behaviour can be described
(see also Subsection 9.7.4). Besides that, interruption or abortion of a method can be
defined.