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From: hd@world.std.com (HD Associates)
Subject: Re: Real-time systems:  Windows-NT or QNX
Message-ID: <Cy0qDp.AuM@world.std.com>
Organization: The World Public Access UNIX, Brookline, MA
References: <jfoxCxs677.K3q@netcom.com> <tsikesCxtpK5.ICM@netcom.com> <id.B2ZD1.QOA@nmti.com> <jfoxCxxs76.D3p@netcom.com>
Date: Fri, 21 Oct 1994 10:27:25 GMT
Lines: 79
Xref: glinda.oz.cs.cmu.edu comp.realtime:7285 comp.robotics:14608

In article <jfoxCxxs76.D3p@netcom.com>, Jeff Fox <jfox@netcom.com> wrote:
>In article <id.B2ZD1.QOA@nmti.com> peter@nmti.com (Peter da Silva) writes
>>In article <tsikesCxtpK5.ICM@netcom.com>,
>>Terry Sikes <tsikes@netcom.com> wrote:
>>> Would you care to address the issue of Forth's reputation as an
>>> unmaintainable, write-only language?  ;)
>>
>>I've maintained unmaintainable write-only code in lots of languages, and
>>if I had to maintain unmaintainable write-only code in any language I'd
>>choose Forth.
>>
>>Forth isn't inherently write-only, and there's no language out there that
>>is the least bit difficult to write unmaintainable code in.
>>
>>On the other hand, Forth doesn't seem to scale that well.
>                                          ^^^^^
>I agree if by scale you mean BLOAT!  Forth is good to scale down the
>required computing resources and scale up the performance.  But if
>you mean scale up the required resources, costs, development time and
>scale down the performance by a couple of orders magnitude then I
>AGREE!

Evaluate the requirements. You might need TCP-IP support, GPIB
support, DOS filesystem, user interface, and so on.

I'll never say you can't do
everything in Forth, but I'll bet your choices are more limited.
>
>It is clear from what has been said in this thread that Forth and
>QNX and some others can SCALE down well to improve performance and
>reduce cost and performance overheads.  Windows does not scale down.
>
>62 minutes response to events does not quality as real time... And
>it is NUTS to consider using a OS that can have a 62 minute half-life 
>between crashes for an application described as capable of producing
>a dissaster in the real world if it fails!  
>
>The range of recomendations here has been mind boggling!
>
>Imagine applying some of the stuff that has been suggested in this thread.
>
>"Your robot is doing a demonstration of autonomous laser welding at a
>crowded trade show when the part being welded fails and falls from the
>robot's grip sending the welding beam into the crowd."

(death and destruction deleted; ask your lawyer about an autonomous laser
welding demo at a crowded trade show)

You have to assume a minimal bit of  intelligence on the part
of the designer of any mission critical ap.

The person with the nonexistent error detection is unlikely to
do much better in Forth or QNX.

Failure analysis, risk analysis, and safety requirements don't
change based on the OS.

There will be hardware solutions, hardware/software solutions, and
"software outside of the usual OS" solutions.

I do know a company using Windows NT in a real time AP.  They had to
push more off into hardware than I  would ever want to do or think
they should have (I would have used a second processor on the ISA board,
but they had a one processor rule, so you can get a feel for the
decision making process).

The NT decision was, as is so often the case when Microsoft comes up,
a marketing dictate.  I don't know where this number comes from, or if
it is real, or anything else, but
I was told that on their 486DX2-66 they are using a 100 ms "interrupt to
ready to run high priority task" number.

And I also have a client who has used various major RTEs (VRTX and Irmx)
who now have "Windows at Work" penciled in for future products
because it is "obviously" the future.
-- 
Peter Dufault            Real Time Machine Control and Simulation
HD Associates, Inc.      Voice: 508 433 6936  FAX:   508 433 5267
Pepperell, MA            hd@world.std.com   
