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

9.3 Formal Syntax 265
The fourth statement is a method call. By means of such a method-call statement a process
object can call one of its methods. A method call m(E1 ¥¡ ¡ ¡ ¥ En)(p1¥¡ ¡ ¡ ¥ pm) is executed in
the following way: First, expressions E1 ¥¡ ¡ ¡ ¥ En are evaluated from left to right. Next,
the values of these expressions are bound to the input parameters of the method m and
the local variables and output parameters are initialised to nil. Then the method body is
executed. If this execution successfully terminates, the output parameters of the method
are bound to variables p1 ¥¡ ¡ ¡ ¥ pm.
The fifth sort of statement is sequential composition, which is indicated with a semicolon.
Sequential composition has its usual meaning.
Next we have the choice statement S p
1
or Sp2
, indicating that a process can choose between
two alternatives. A process always leaves both alternatives open, at least until one of the
alternatives can actually be executed. So, a choice is never made a priori. As soon as one
of the statements has performed an execution step, the other is discarded. In general,
the process environment will permit only one of the alternatives. If both alternatives
are open, the choice is made non-deterministically. The choice statement has the same
meaning as the summation combinator (+) of CCS.
§ E S p is a guarded command. E is a boolean expression, called the guard of statement S p . A
guarded command is executed as follows: First, expression E is evaluated. If it evaluates
to true or bunk, an attempt is made to perform an execution step of rest statement S p .
If this attempt succeeds this execution step actually takes place and the execution of
S p is continued. Otherwise, the state of the executing process object is restored to the
state just before the expression was evaluated, and the guarded command blocks, i.e.,
is not executed. In general, guarded commands are used in combination with a choice
statement. If a blocking guarded command is used in isolation, the executing process
runs the risk of blocking completely.
The if-statement and the do-statement have their usual meaning.
Next we have the abort statement. S p
1
abort Sp2
denotes that statement Sp1
is executed until
it is aborted by statement S p
2. Upon successful termination of S p
1, the complete abort
statement terminates successfully. If an execution step of S p
2 can be performed, this
step actually takes place and the execution continues with S p
2. Statement S p
1 is thereby
completely discarded.
interrupt Sp2
indicating that Sp1
is executed
until it is interrupted by statement S p
2. If statement S p
1 terminates successfully, the
The last statement is the interrupt statement S p
1
complete interrupt statement terminates successfully. If an execution step of S p
2 can be
performed it actually takes place and the execution control switches from S p
1 to S p
successful termination of S p
2
this statement is again interruptable by S p
2 .
2. Upon
, the execution control is returned to Sp.
The execution of
Next, we define a set Systems p with typical elements Sys p ¥¡ ¡ ¡ .
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