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

216 Modelling of Concurrent Reactive Behaviour
corresponding to the reception of one of a collection of alternative messages or
events that can happen. Another example of the need of non-determinism arises if
a non- deterministic choice cannot be made because the details of events that cause a
transition are not known yet. For instance, real-time properties of a subsystem may
cause that a specific event happens always after another specific event. However,
as long as we do not consider particular implementation decisions we cannot
determine timing properties and consequently we cannot know such an order
of events yet. Non-determinism offers the possibility to describe the behaviour
alternatives without the need to know the order.
6. The rule § E S p denotes a so-called guarded command. The meaning is that the process
statement S p will be taken into execution if the guard E evaluates to true. Execution
waits if the guard evaluates to false. There could be confusion about the interpretation
of the meaning of the evaluation of a guard when a guarded command is used
as an alternative in an or statement. When the guarded command is selected the
guard must be evaluated. The rest statement of the guarded command is executed
if the result is true. However, when the guard is false than the rest statement cannot
be executed. This means that the preceding select statement is taken into execution
again and can make another choice. The fact that the guarded command is not
executed means that the guard must be considered as not evaluated.
7. The statement m(E1 ¥¡ ¡ ¡ ¥ En)(p1 ¥¡ ¡ ¡ ¥ pm) represents a method call. The execution
of the method is prepared by evaluation of the expressions E1 ¥¡ ¡ ¡ ¥ En from left to
right. Next the values are bound to the input parameters of the method m. All local
variables and output parameters of the method are initialised to nil. Subsequently
the method body is executed. If the body terminates successfully then results are
stored in output parameters of the method that are bound to variables p1¥¡ ¡ ¡ ¥ pm.
The behaviour of the process continues with the next statement after the method
call.
The method call can be used to construct infinite behaviour. The practical execution
problem of stack overflow, that can be caused by recursive calls, is solved by
offering tail recursion. To construct infinite behaviour a method with no output
parameters is called from another method with no output parameters, and vice
versa. Such a call must be the final statement, the tail, of such methods. (See
Subsection 9.7.4 for an example of tail recursion).
8. The message-receive statement ch?m(p1 ¥¡ ¡ ¡ ¥ pm ¡ E) makes that a process object can
receive messages from other process objects.
9. The message-send statement ch!m(E1 ¥¡ ¡ ¡ ¥ En) makes that a process object can send
messages to other process objects. Message modelling has been described extensively
in Section 6.3.
10. The abort statement S p
1 abort S p
2 enables the interruption of S p
1 with alternative
statement S p
2. The abort happens if S p
2 can be taken into execution and the execution
arrived at a transition between two statements where interruption is allowed
of S p
1