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From: jkh@whisker.hubbard.ie (Jordan Hubbard)
Newsgroups: comp.os.386bsd.questions
Subject: Re: 16 MB RAM and Adaptec 1542C
Date: 26 Jun 1994 14:04:11 GMT
Organization: Jordan Hubbard
Lines: 28
Message-ID: <JKH.94Jun26140411@whisker.hubbard.ie>
References: <2udhe3$ofk@toads.pgh.pa.us> <mldCrxqM7.7wp@netcom.com>
<mldCrzoI2.BLw@netcom.com>
NNTP-Posting-Host: whisker.hubbard.ie
In-reply-to: mld@netcom.com's message of Sun, 26 Jun 1994 05:27:37 GMT
In article <mldCrzoI2.BLw@netcom.com> mld@netcom.com (Matthew Deter) writes:
A FreeBSD system per se is not limted to 16 megs. A PC running the
ISA bus *is*. The issue revolves around the way data is xfered from
disk to memory. SCSI devices use DMA (direct memory access) to allow
data to flow directly from the SCSI host to the machine's DRAM without
CPU intervention. Other schemes ("programmed" or "polled" I/O require
the CPU to touch all the data as it goes from disk to main memory,
acting as an intermediary.)
[ Rest of a rather fine discourse on the limits of ISA DMA limitations
deleted ]
I'm happy to say that with the release of 1.1.5A this is no longer the
case. ISA DMA to/from an address >16MB is "bounce buffered" into the
lower 16MB address space. Due to the way this is implemented, it's
also a *very low overhead* operation - so low as to be unnoticable in
the context of transfering data to or from what's already a rather
slow SCSI device (comparatively speaking).
So with 1.1.5, you no longer need to know or care. 32MB systems with
1542 controllers work just fine.
Cheers..
Jordan
--
Jordan K. Hubbard FreeBSD core team Friend to mollusks