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From: michael@resonex.com (Michael Bryan)
Subject: Re: More Details on the 386BSD Release 1.0 CD-ROM
Message-ID: <1994May26.105349.14954@resonex.com>
Organization: Resonex Holding Corporation, Fremont CA
References: <jmonroyCq1qK0.5vJ@netcom.com> <VIXIE.94May19144247@office.home.vix.com> <2s19rb$77v@acmex.gatech.edu>
Date: Thu, 26 May 1994 10:53:49 GMT
Lines: 33
In article <2s19rb$77v@acmex.gatech.edu>,
Jeff M. Garzik <gtd543a@prism.gatech.edu> wrote:
>In article <VIXIE.94May19144247@office.home.vix.com>,
>Paul A Vixie <vixie@vix.com> wrote:
>>there are only 24 address lines on an ISA bus. if you know a chip set that
>>can address more than 16MB of memory using those 24 address lines, please
>>tell us all about it.
>
>My pure-ISA motherboard has 8 SIMM slots instead of the normal 4, and I
>ran 24MB just fine under OS/2. Of course, I don't know what it was
>doing... it may or may not have been using those 24 address lines, I
>dunno. ;-) Just stating a case...
Of course, you're both right. The ISA bus provides only 24 address
lines, so devices on an ISA bus cannot access memory above the 16MB
mark. However, memory (usually) does not sit on the ISA bus, it sits
on the motherboard, very close (electronically) to the processor, and
all 32 address lines are brought to it. So an ISA system can access
and utilize memory well above the 16MB mark, up to the design limits
of the motherboard. However, if you put any ISA bus mastering devices
in this system, such as an Adaptec 1542 SCSI controller, you have to
do one of the following things:
1) Reduce memory to 16MB.
2) Have software which employs "bounce buffers", to move any
data to the low 16MB before initiating a DMA transfer.
3) Trash the system and go to PCI. ;-)
--
____ Children of a future age ____
\ / Reading this indignant page \ /
\/ Know that in a former time \/ Michael Bryan
Love, sweet love, was thought a crime michael@resonex.com