From:	MERC::"uunet!CRVAX.SRI.COM!RELAY-INFO-VAX"  7-APR-1993 19:28:43.56
To:	INFO-VAX@SRI.COM, wpearson@kean.ucs.mun.ca
CC:	
Subj:	Re: Interrupt Based Inter Process Communications

Wayne (wpearson@kean.ucs.mun.ca) asked:

>Hi,
>   I wish to implement interrupt based communications between several
>   processes on a VAX 8250 running VMS version 5.1. A signalling process
>   would generate an interrupt causing several other processes to retrieve
>   and process data from a common global section.

To which both Jerry Leicther and Jim Agnew replied, each with a different
approach.  The first thing that came to my mind, which I didn't see
suggested elsewhere, is to use the ENQ/DEQ system services with blocking
(or notify) ASTs.

Here, each reader of the global section would obtain a, say, CR mode lock
on some resource name.  The writer would obtain a NL mode lock on the same
resource.  When the writer has data to pass to the readers, it simply
converts the NL mode lock to EX mode.  Since the readers are blocking the
writer, and since they have requested blocking notification, the ENQW
service will issue ASTs to all of the readers.  Each reader would then
convert their CR mode lock to NL, and then attempt to convert it back
to CR mode.  This would place all of the readers on the conversion waiting
to be granted queue *behind* the writer.  Once the writer gets the EX
mode lock, it may simply convert it back to NL mode.  (This is a matter
of *when* you decide the writer can write.  It may be that you use this
technique simply to send signals to the readers, and *not* to control
access to the global section.)

Voila.

This also gets around a potential problem of mailboxes for this type
of application.  Namely, that only one reader can read the mailbox.  *IF*,
however, you only want one reader to process each message, then the
mailbox will work fine.   Although this seems to be what one would
normally want to do, it doesn't match the scenario you describe.  (Perhaps
each receiver in your scheme does something different with the data?)

-Derek S. Haining
 University Computing Services
 University of Washington
 Seattle, Washington  98195 
 (206) 543-5579

 DEREK@MAX.BITNET
 DEREK@MAX.U.WASHINGTON.EDU