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

208 Modelling of Concurrent Reactive Behaviour
The concept of global state is helpful when we want to reason about the state of the
system and must be aware of the variety of aspects that form the state, such as behaviour,
variables and internal side effects. All notions of state we defined so far are helpful for
informal communication about a system. However, none them grasps the complete
complexity of state as defined in the formal semantics of POOSL. This notion of state is
defined in the next paragraph.
6.6.3.6 Execution State
Besides the sorts of state we distinguished so far we need another sort that is related
to the semantics of POOSL. Chapter 9 defines the transition system of the semantics
of process objects. It determines the finest grain of transitions between global states,
called configurations. On the level of the transition system we deal with recursive
operations, and infinite behaviour. We define the execution state of a (sub-)system as the
behaviour state together with the variables state and the local variables state. Execution
state transitions are transitions between successive so-called configurations. Notice that
the level of detail is so fine that local variables play their role too. The execution state
space is in general not finite 8 .
6.6.3.7 Summary of concepts of state
Besides the formal notion of execution state we gave various definitions that enable to
reason about a system’s state:
behaviour state of a system: a mode of behaviour at some level of abstraction;
behaviour state of an object: a mode of behaviour of an object;
variables state of an object: Cartesian product of the value ranges of the variables
of an object;
global state: Cartesian product of the set of behaviour states and the value ranges
of all variables.
Named states are useful for discussions about a system and for test and verification
purposes. Methods as well as variables can be used to achieve explicit naming of
states. Methods names can be chosen to denote modes of behaviour. We recommend to
choose expressive names that reflect the meaning of a behaviour state. A variable can
be used to store a named state. Other objects can ask for the state when appropriate
corresponding accessors have been defined. Methods can update a state that is stored in
a variable. This approach typically disconnects the direct correspondence between state
and method. The execution of correlated methods that perform some named operation
can be specified as the same state.
8 See Section 9.6 where an example of a graph with a finite number of nodes appears to be an infinite
transition graph.