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

264 Process Part of POOSL
We define
CName pc = CName p ¦ CName c C pc ¥¡ ¡ ¡
Var = IVar ¦ LVar p¥¡ ¡ ¡
Next we define the set Stat p of process statements. These statements are used to specify
the behaviour of process objects.
S p :: S
ch!m(E1 ¥¡ ¡ ¡ ¥ En) ¡
ch?m(p1 ¥¡ ¡ ¡ ¥ pm ¡ E) ¡
m(E1 ¥¡ ¡ ¡ ¥ En)(p1 ¥¡ ¡ ¡ ¥ pm)
¡
S ¡ p
; Sp
1
S ¡ p
1
2
or Sp
2
§ E S ¡ p
if E then S ¡ p
1 else S p
2 fi
1 abort S p
¡
2
S p
1 interrupt S p
2
do E then S ¡ p od
S ¡ p
The first type of process statement is a data statement defined in the language of data
objects of the previous chapter. These statements are used to model internal data
computations of a process.
The next two statements are the message-send and message-receive statements. A messagesend
statement ch!m(E1 ¥¡ ¡ ¡ ¥ En) indicates that a process is willing to send message m
together with parameters E1 ¥¡ ¡ ¡ ¥ En, which refer to data objects, on channel ch. A
message-receive statement ch?m(p1¥¡ ¡ ¡ ¥ pn ¡ E) indicates that a process is willing to
receive message m with parameters p1 ¥¡ ¡ ¡ ¥ pm from channel ch under reception condition
E. E is a boolean expression which may depend on the input parameters p1 ¥¡ ¡ ¡ ¥ pm. It
may, however, also depend on instance variables or on other local variables.
For a message to be transferred, exactly two process objects are needed. One of the
objects should be executing a message-send statement, and the other a message-receive
statement. These statements must refer to the same channel and the same message.
Further, the number of parameters has to be equal. Finally, the reception condition of
the receiving object should allow message reception.
When two process objects communicate, they perform a so-called rendezvous. A rendezvous
procedure is performed in the following way: First, the message parameters
E1 ¥¡ ¡ ¡ ¥ En of the message-send statement are evaluated from left to right. Then deep-
Copies of these parameters are bound to the input parameters p1 ¥¡ ¡ ¡ ¥ pm of the messagereceive
statement. After that, reception condition E is evaluated at the receiving process.
If the expression evaluates to true or bunk, the rendezvous is successfully terminated.
In all other cases, the states of both processes are restored to the states just before the
rendezvous procedure started and no message is passed. This basically implies that the
check whether a message may be passed is performed transparently for both processes.