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

298 Process Part of POOSL
and
(ch1?n1; ch2?n2; ch3?n3) interrupt ch?n
Both statements can be expressed, without the explicit use of the abort operator respectively
the interrupt operator:
and
(ch1?n1; (ch2?n2; (ch3?n3 or ch?n)) or ch?n) or ch?n
(ch1?n1; m2) or (ch?n; m1)
where m1¥ m2 and m3 are methods defined as
m1()() (ch1?n1; m2) or (ch?n; m1)
m2()() (ch2?n2; m3) or (ch?n; m2)
m3()() (ch3?n3) or (ch?n; m3)
It is clear that modelling of aborts and interrupts this way gets very cumbersome and
leads to unreadable specifications. Now consider statements
and
( do true then m()() od ) abort ch?n
( do true then m()() od ) interrupt ch?n
In case of the abort, the executing process repeatedly executes method m until it is
aborted by the arrival of message n. An occurrence of the abort does not only abort
the infinite loop, it can even abort the execution of method m. In case of the interrupt,
the execution of the loop as well the possible execution of method m are both resumed
after message n has been received. Further, each interrupt occurrence can be followed
by another one. It is quite impossible to specify such complex behaviour in a reasonable
fashion without the use of the abort operator or the interrupt operator.
The formalisation of the abort operator did not cause any difficulties. On the other hand,
finding an elegant semantic interpretation of the interrupt operator appeared to be a lot
more difficult.
The interrupt operator is formalised by rules (m’) ¡ ¡ (r’) of the labeled transition system.
Of these, rules (o’) ¡ ¡ (r’) may appear to be more complicated than necessary. Consider
for example rule (o’):