Re: VAX/VMS in industrial automation

From: Phil (dooleys_at_snowy.net.au)
Date: 10/05/04 

Date: Tue, 05 Oct 2004 12:39:30 GMT

"asm" <arut@post.com> wrote in message
news:c8b782e3.0410040507.6ab70c5@posting.google.com...
> Hi All,
>
> I have come across Vax/VMS being used in steel plant automation,
> but couldn't quite understand if it is an RTOS or not.
>
> VMS - virtual memory systems
> Not normally encountered in realtime systems
>
> What kind of scheduling does VAX/VMS use and if it is an RTOS
> what features make it an RTOS.
>
> Please clarify.
If you do a $ show system on vms it lists processes,
each process has a "base" and "current" priority level
(by convention batch has base priority 3 and interactive 4)
and the vms scheduler can increase the "current" priority
in response to interrupts (amongst other things), this
provides good "timesharing" response and throughput.
However the swapper process runs at priority 15
so any other process can have its working set
reduced or be swapped out, so cannot be regarded
as "realtime". Processes started with a priority of 16 or more
cannot be pre-empted by the swapper, so can provide a
realtime response (with a few qualifications).
There was some marketing-speak around that
suggested that vms was "soft" realtime, rather
than "hard" realtime
Phil

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