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From: wbeebe@bilver.UUCP (bill beebe)
Newsgroups: comp.os.minix
Subject: Re: EXTENDED/EXPANDED Memory on PC's
Message-ID: <384@bilver.UUCP>
Date: 18 Jan 89 12:35:56 GMT
References: <6470@louie.udel.EDU> <4054@omepd.UUCP> <533@Terra.cc.brunel.ac.uk>
Reply-To: wbeebe@bilver.UUCP (bill beebe)
Organization: W. J. Vermillion, Winter Park, FL
Lines: 23

In article <533@Terra.cc.brunel.ac.uk> ralph@ccs.brunel.ac.uk (Ralph Mitchell) writes:
>
>What about the case where a PC has 3x256K ram chips.  Surely the whole
>768K is contiguous memory, at least in the chips.  Are you saying that
>the address selection hardware is deliberately stopping access above
>640K and that there's some more hardware to map the remaining 128K
>elsewhere ??  Sounds like the designer had a bad day...
>

In the original PC spec, IBM said in effect that there would be no more
than 640K of contiguous, program memory, starting from 0000:00 to 9FFF:00.
Memory starting from A000:00 to BFFF:00 belongs to video display(s). Some
systems can get away with an additional 64K starting at A000:00 because
MDA and Herc video cards actually start at B000:00. There is indeed hardware
in the new PCs (via chipsets and some newer discrete designs) that map
memory above 640K into extended memory (ATs) and/or as shadow RAM for
display BIOS, ROM BIOS, etc. And no, the designer did not have a bad day,
considering the design is vintage 1980, when the majority of personal systems
(CPM and Apple II) had 64K or less. I don't want to get into this, as this
properly belongs in one of the architecture conferences, so read it, yell
at it, even delete it, but don't clutter comp.os.minix with replies to it.
I just get a little tired of potshots being taken by armchair engineers at
a PC design approaching it's tenth birthday...