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From: j@bonnie.heep.sax.de (J Wunsch)
Newsgroups: comp.unix.bsd.freebsd.misc,comp.periphs.scsi
Subject: Re: Adaptec 1542C for $125CDN -- comments?
Date: 3 Jul 1995 09:50:29 +0200
Organization: Private U**x site, Dresden.
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Message-ID: <3t87g5$2ro@bonnie.tcd-dresden.de>
References: <rrwood-2106951636420001@bpci.net3.io.org> <3smdbn$ml0@bonnie.tcd-dresden.de> <DAy5Et.8uD@watserv1.uwaterloo.ca> <3t01b9$m0i@panix.com>
Reply-To: joerg_wunsch@uriah.heep.sax.de
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John Brock <jbrock@panix.com> wrote:
[ISA busmaster DMA cards and > 16 MB RAM]
>I asked about this a little while ago (prompted by a comment in the
>Linux SCSI HOWTO), and came to the conclusion that this is a phony
>problem. There is a 16MB limit for DMA, but the problem is taken care
>of in the drivers, which is really what you would expect anyway. It
>hardly makes sense that Adaptec or anybody would make a high end SCSI
>card that wouldn't work with a high memory PC.
It's not Adaptec's fault. The ISA bus does only have 24 address
lines. Blame Big Blue for not thinking beyond the limitations of a
286 when designing the bus (and for designing a motherboard DMA that
is so sluggish that people need to do busmaster DMA at all).
The FreeBSD kernel can work around the limitation by using ``bounce
buffers'' below the 16 MB mark, but my argument has been that it's not
a good decision if somebody is going to buy a new system. He will be
better served with a PCI architecture.
--
cheers, J"org private: joerg_wunsch@uriah.heep.sax.de
http://www.sax.de/~joerg/
Never trust an operating system you don't have sources for. ;-)