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Mystic Pascal User Manual 22
7. Multi-tasking Support
Mystic Pascal is a multi-tasking system. The editor,
compiler, system display and other major components are processes
which execute concurrently. Pascal programs may also use multi-
tasking.
In Mystic Pascal any global procedure may be started as an
independent process by the START builtin procedure. Up to 50
Pascal procedures may execute concurrently. The system
automatically switches among the concurrent procedures as they
execute. Procedures are allowed to execute if they are not
in a wait state and based on their static priority and the length
of time they have been waiting. Even the lowest priority
procedure will execute.
Concurrent procedures will run until the final END is
reached. At that time the process will be deleted. The same
Pascal source procedure may be started more than once -- a single
source procedure may have many incarnations. Concurrent
procedures may be recursive, may call global procedures, may
access global variables and may START global procedures.
Section 7: Multi-tasking Support
Mystic Pascal User Manual 23
7.1 Message Passing
Concurrent procedures may communicate by passing messages.
Messages are processed by Mystic Pascal's queue management
system. This is not only for communication but also allows
efficient synchronization among the concurrent procedures --
attempting to receive a message from an empty queue causes the
procedure to enter a wait state until a message arrives.
Queues are first-in-first-out (FIFO) storage mechanisms.
The queue messages are stored in the dynamic storage area.
As an example, consider a concurrent procedure LOG whose
purpose is to write one line messages to a printer. LOG must
accept messages from several other concurrent procedures. LOG
has its own input queue from which it receives messages and to
which other procedures send messages. LOG can ensure that only
one user at a time (itself) does printing. If several procedures
could print at the same time, their messages could become
interleaved resulting in a garbled printout.
PROCEDURE LOG;
LABEL 100;
BEGIN
QUEUE('LOGQ ');
100: RECEIVE('LOGQ ', MSG);
{print the message}
GOTO 100;
END;
Careful planning is necessary when concurrent procedures may
access a printer, console or disk files at the same time. Having
one concurrent procedure handle all accesses is a common solution
to this issue. This is related to the concept of a "monitor."
Before a queue may be used, it must be created by the QUEUE
builtin procedure. Each queue must have a unique 8 byte name.
The builtin queue management procedures are:
QUEUE create a new queue
SEND store a message on a queue
RECEIVE get a message from a queue
Section 7: Multi-tasking Support
Mystic Pascal User Manual 24
7.2 START
START( procedure_name, process_name, stack_size );
The START procedure is used to initiate a concurrent
procedure. The procedure will execute as a separate process
until it terminates by reaching its final END.
The process_name is an 8 character ASCII field. This name
is displayed in the system display and is used in the PRIORITY
builtin procedure. A single Pascal procedure may be started
several times. Each start must use a unique process name.
The stack_size is an integer expression. It specifies the
amount of space to be allocated in paragraphs - multiples of 16
bytes. Enough space must be included for this procedure's local
variables, the local variables of all procedures it calls and
additional stack space for evaluating expressions.
If the concurrent procedure is recursive or calls any
recursive procedures, great caution should be used in estimating
the stack_size or an insufficient storage run-time error may
result.
The priority of a concurrent procedure is initially set to
20. This may be modified by the PRIORITY builtin procedure.
START( LOG, 'LOGPROC ', 30 );
START( PRINTER, 'PRINTER ', 100 );
START( DOWJONESPORT, 'DJP ', 200 );
Section 7: Multi-tasking Support
Mystic Pascal User Manual 25
7.3 PRIORITY ** 1.6 **
PRIORITY( process_name, priority );
The PRIORITY procedure is used to modify the static priority
of a concurrent procedure which has been started. The initial
priority is 20. The priority parameter is an integer expression
in the range 1 to 64.
PRIORITY( 'LOGPROC ', 10 );
PRIORITY( 'DJP ', 64 );
PRIORITY( 'CHESS A ', 40 );
PRIORITY( 'CHESS B ', 35 );
Section 7: Multi-tasking Support
Mystic Pascal User Manual 26
7.4 QUEUE ** 1.6 **
QUEUE( queue_name );
The QUEUE procedure creates a new queue for message passing.
Each queue is identified by a unique 8 byte queue name. This
name does not have to be ASCII.
QUEUE( 'LOGQ ' );
QUEUE( 'ERRORMSG' );
QUEUE( 'COMMANDS' );
Section 7: Multi-tasking Support
Mystic Pascal User Manual 27
7.5 SEND ** 1.6 **
SEND( queue_name, variable );
The SEND procedure stores a message on a queue. The queue
must have been created by the QUEUE procedure.
The concurrent procedure that issues the SEND will resume
execution immediately unless the space allowed for this queue is
full. Then the issuer is placed in a wait state until space is
released by another concurrent procedure removing messages from
the queue.
SEND('RED ALRT','EEG INPUT CORRELATES 1.0 WITH PREDICTION');
SEND( 'TREESORT', ROOTPTR );
SEND( 'COMPILER', SOURCEARRAY );
Section 7: Multi-tasking Support
Mystic Pascal User Manual 28
7.6 RECEIVE ** 1.6 **
RECEIVE( queue_name, variable );
The RECEIVE procedure removes the oldest message from the
specified queue. If no messages are present, the issuer is
placed in a wait state until a message arrives.
RECEIVE( 'PRINTER ', OUTPUTLINE );
RECEIVE( 'BROKER3 ', STOCKORDER );
RECEIVE( 'EDITOR ', COMMAND );
Section 7: Multi-tasking Support