SIMSCRIPT II.5 Programming Language

SIMSCRIPT II.5 Programming Language
tity is not available, the process is put in a passive state and filed in the queue of processes waiting
for the particular resource.
An optional with priority expression may be added to the request statement. The queue is
ranked on high priority. If the phrase is not present, the priority is treated as zero. An optional
comma may be placed before the with priority phrase. Examples of the request statement
are:
request 1 WORKER(2) with priority 2
request 2 MACHINE(JOB.TYPE)
request 3 UNITS OF MATERIAL with priority 5
As the resource name is in fact a permanent entity name, it should be subscripted. If it is not, the
variable of the same name is used as an implicit subscript. This variable is initialized to 1 at resource
creation, but care should be taken if it is subsequently altered, by for each resource
statements, for example. Note that some implementations use an implicit subscript value of 1. It is
recommended that explicit subscripting be used in all cases.
A process that has requested some units of a resource may relinquish some number of these, but
not necessarily all it has. The number of units of the resource being relinquished is added to the
total quantity available. If any processes are queued awaiting the resource, they are scanned from
the front of the queue. Each is reactivated, with a corresponding reduction in the quantity of available
units of resource, until one is found whose request cannot be satisfied. The scan is then terminated.
The process relinquishing the resource continues execution at the statement immediately following
the relinquish statement. The relinquish statements corresponding to the above resource
request statements would be:
relinquish 1 WORKER(2)
relinquish 2 MACHINE(JOB.TYPE)
relinquish 3 UNITS OF MATERIAL
By way of explanation, it was stated that processes may be filed in queues, either waiting for, or
executing with, resources. This is not quite correct. Since a process may, in practice, require several
resources concurrently, a special temporary entity (qc.e) is created at each request for a resource.
It is these qc.e entities that are filed both in the set of resources associated with this
process, (process), and also in either of the sets x.resource or q.resource, depending on
whether the request has or has not been satisfied. Each qc.e also has a pointer attribute (who.a)
pointing to the process notice of the requesting process. The attributes of the qc.e entity are shown
in table 5-3. The q.resource set is ranked by high pty.a, thus permitting preemptive queuing
prior to allocation of resources. The q.resource and x.resource pointers are equivalenced, as
a process is either queuing or executing with any given resource.
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