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From: yarvin-norman@cs.yale.edu (Norman Yarvin)
Newsgroups: comp.sys.3b1
Subject: Re: more on the HFC saga
Message-ID: <1991May22.221923.12768@cs.yale.edu>
Date: 22 May 91 22:19:23 GMT
References: <102086@becker.UUCP> <1991May22.041659.13189@fithp>
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mhw@fithp (Marc Weinstein) writes:
>I'm beginning to think that there *ARE* select PCs out there which may
>just be able to handle these higher throughputs.  I know of at least
>one 3b1 which can communicate with a Sun using V.42bis, 19200 port rate,
>and sees ~1800 Bps with no data corruption.  Mine can't do this, and I'm
>not sure what the difference is.  Perhaps the port rate is not exactly
>19200 on some systems - perhaps the CPU clock is just a bit faster.

The device driver guide (documents/DDDguide.mm.Z on osu-cis) mentions
something which might be a reason for this.  Interrupts are handled by
having a linked list of interrupt service routines for each priority level;
when an interrupt of that level occurs the routines are called one after
another.  The ordering of this list can make a big difference, and loadable
device drivers install their interrupt service routines at the head of the
list.

On the other hand, the list is presumably uniform among all copies of any
given version of the OS, and is only changed when loadable device drivers
are installed.  Furthermore, loadable device drivers might use different
interrupt levels than those used by the serial port.