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From: johns@rd.scitec.com.au (John Saunders)
Subject: Re: Real-time systems: Windows-NT or QNX
Message-ID: <1994Oct19.020316.12642@rd.scitec.com.au>
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References: <37kql1$e4d@vixen.cso.uiuc.edu> <oy08Tc2w165w@qnx.nacjack.gen.nz>
Date: Wed, 19 Oct 1994 02:03:16 GMT
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Xref: glinda.oz.cs.cmu.edu comp.realtime:7237 comp.os.qnx:2283 comp.os.ms-windows.advocacy:40856 comp.robotics:14541

Bruce Simpson (bruce@qnx.nacjack.gen.nz) wrote:

> There are a number of reasons that an OS such as QNX might be a much
> better choice:

>   * device drivers don't require special programming techniques or toolkits
>   * OS/2 has large amounts of non-preemptable code in its kernal
>   * context switching in OS/2 is significantly slower than most RTOSes
>   * even the TIME_CRITICAL priority under OS/2 has too much latency in
>     scheduling at times.

In the book "The design of the OS/2 operating system" can't remember the
authors name, I seem to remember a garantee of 400uS maximum latency for
the TIME_CRITICAL priority. The book was about OS/2 2.0 and I don't
know if this has gotten better or worse with later releases. I suspect
with the conversion of drivers from 16 bit to 32 bit code it has gotten
better.

> OS/2 can be *forced* to act like a poor realtime OS but why bother to
> try and make a silk purse out of sow's ear when there is a range of
> "off-the-shelf" silk purses available that will do the job faster, easier
> and more effectively?

I would only consider OS/2 for a relaxed "real-time" system. It would
handle the user interface and a network/serial link to the real system
just fine. However for 1000Hz to 2000Hz interrupt rates I would be more
that a little worried to use it.

I would suggest QNX as the better alternative. With the very low latency
times I would expect it to easily keep up with high interrupt rates. There
are a number of other RT execs that would be suitable as well. However the
modular approach of QNX appeals to me because rather than having a large,
possibly buggy, monolith, you have a number of smaller simpler and possibly
less buggy modules.
-- 
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