Path: utzoo!attcan!uunet!nih-csl!lhc!ncifcrf!haven!uflorida!rex!wuarchive!decwrl!elroy.jpl.nasa.gov!ames!coherent!next!gk
From: gk@next.com (Gregg Kellogg)
Newsgroups: comp.sys.next
Subject: Re: Can the NeXT do real time A/D?
Keywords: NeXT, Realtime
Message-ID: <259@next.com>
Date: 10 Oct 90 01:03:39 GMT
References: <1990Sep27.222026.18112@wam.umd.edu>
Reply-To: gk@NeXT.COM (Gregg Kellogg)
Organization: NeXT, Inc.
Lines: 35

In article <1990Sep27.222026.18112@wam.umd.edu> rgc@wam.umd.edu (Ross Garrett Cutler) writes:
>Hello,
>The subject says it all...does the NeXT support real time A/D acquisition?
>If so, do you do it at the DSP level (since Mach, I believe, doesn't
>provide priority-driven pre-emptive task scheduling, as does QNX for example)?
>Thanks much for any info!
>--
>Please email -- I'll summarize.
>Ross Cutler
>University of Maryland, College Park
>Internet: rgc@wam.umd.edu

Most A/D acquisition is done using the DSP on NeXT.  The DSP provides a
great realtime environment allowing you full access to it's serial port
and quite a bit of processing power.  Higher-level, less time critical
software is then done on the 68000.  Doing this, we're able to sustain
some pretty good rates of data I/O with the DSP processing each byte
that's sent or received.

Mach does in fact to priority-driven pre-emptive task scheduling, as do
almost all versions of UNIX that I've ever worked with.  The problem is
that UNIX kernels tend to spend a lot of time in the kernel where they're
not preemptable.  Some versions of UNIX have fixed this, but it's an
inherently hard problem given the structure of the UNIX kernel.

Most realtime systems on the market don't do kernel preemption either, they
just ensure that they don't spend more than a fixed amount of time in the
kernel.  In my opinion, this is the answer for Mach as well.  Many groups
are moving to a "pure" or "micro" kernel, in which the UNIX functionality
is contained in a user-level task (or tasks).  This may not help tasks that
need to serialize through the compatibility task, but it does allow you to
create an environment in which you can control this, by simply not relying on
UNIX services in you time-critical task.

Gregg