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

6.3 Communication 175
There is a grain of behaviour where interrupts can take place. At the moment of an
interrupt request the object will normally be busy with the execution of some process
statement. Immediately after the execution of the present statement the rendezvous
for the interrupt message must be performed. The continuation behaviour can be
a method implementing the required interrupting behaviour, possibly followed by a
return to the original behaviour. The grain of behaviour where interrupts can take
place is the atomic process statement. These atomic statements are message-send and
receive statements, method calls, and data statements. These atomic statements cannot
be interrupted. Composite process statements can be interrupted between the atoms that
compose them. The semantics of POOSL (see Chapter 9) define that statements are not
taken into execution if they cannot be completed. So a message statement is not taken
into execution until a rendezvous can be performed. This means that there is no risk
that interrupts are blocked by the waiting for a communication.
Examples of the use of interrupt and abort statements are:
¢
aS1; ¡ ¡ aS2; aSn£ ; abort ch?m
¢
aS1; ¡ ¡ aS2; aSn£ ; interrupt ch?m
Their meaning is that message m on channel ch can respectively abort or interrupt the
behaviour that is specified as the sequence of atomic statements ¢
aS1; ¡ ¡ aS2; aSn£ ; .
Notice that we mentioned data statements as one of the possible atomic actions. They
are considered here because process statements can be data statements also. A data
statement may be a composite statement. This can mean that a composition of data
statements, being a data statement itself, cannot be interrupted at the instants between
the execution of the individual data statements. To model realistic real-time behaviour
it could be necessary to allow the interrupt of a composite data statement. It appeared
to be a matter of interpretation of the (data) statement separator ’;’ whether a composite
data statement can be interrupted or not. This was observed during the development
of the POOSL compiler. We decided to interpret the statement separator always as a
process statement separator [Kup96].
Summary:
An interrupt message symbol represents a synchronous message passing that can be
forced to happen after the current atomic process statement is finished. POOSL offers
two primitives for the description of the behaviour of an interrupt flow. An interrupt
specifies that the current behaviour can be resumed, after the interrupt behaviour has
finished. An abort specifies that the current behaviour is interrupted but cannot be
resumed. Alternative continuing behaviour can be specified.
6.3.6.6 Interrupt with Reply Symbol
During the work on our mailing machine case, the need of an additional symbol appeared.
This symbol is a combination of an interrupt message flow and a message with