SIMSCRIPT II.5 Programming Language

SIMSCRIPT II.5 Programming Language
tialized to zero. If no expression list appears, all, if any, user-defined attributes are set to zero.
Thus, the preamble definition:
processes
every SERVICE has a SERVER, a CUSTOMER,
and may own a TRANSACTION.SET
followed in some routine by the statement:
schedule a SERVICE giving TELLER(N) and F.QUEUE at time expression
assigns the values of TELLER(N) and F.QUEUE to the attributes SERVER and CUSTOMER, respectively,
but leaves the set-ownership attributes set to zero. Following the conventional notation of
the call statement, a number of argument list forms may be used: the word given may be used,
or the argument list enclosed in parentheses. No yielded phrase may be used. Examples are:
schedule an ARRIVAL giving ORIGIN now
reschedule this SERVICE giving A and B in 2 days
As these attributes are initialized at the time of scheduling, their values are unchanged by filing and
removing them from the future event set. Thus, they provide a means for passing values to event
and process routines. There are some difficulties, however, attached to accessing these values within
the routines. Upon entry to an event routine, the associated event notice is usually destroyed, and
so the attributes are no longer accessible. This is not the case for process notices. A process has a
lifetime associated with the duration of an activity, whereas an event is instantaneous. Process notices
are not automatically destroyed until completion of all activity represented by the process. The
automatic destruction of an event notice is suppressed by appending the phrase saving the
event notice to the event routine definition, as in:
event ARRIVAL saving the event notice
In this way, the attributes can be accessed within the event routine, or even later. The global variable
with the name of the event is set by the timing routine to the event notice pointer before execution
of the event routine is initiated, and so may be used to subscript the attributes. The event
notice may be later destroyed as for any temporary entity. Care should be taken not to alter the value
of this variable, by schedule statements, for example, until all attributes have been accessed, or
before using it in destroying the event notice. It must not, of course, be redefined as a local variable,
as this makes access to the global value impossible. In the case of processes, a system-maintained
global variable, process.v, holds at all times either a pointer to the process notice of the currently
executing process, or a zero value if no process routine is executing. Thus, this variable may be
used within a process routine or within subprograms called from a process routine, to access the attributes
of the associated process notice. Such a subprogram can test process.v to determine
whether, in this instance, it has been called from a process or a nonprocess routine.
A second way in which the values of attributes passed to an event or a process routine may be made
available is to define a list of given arguments to the routine. At entry to the routine, and in the case
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