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From: feg@moss.ATT.COM (Forrest Gehrke,2C-119,7239,ATTBL)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: hard 80386 questions
Keywords: 80386 (80286)
Message-ID: <1991Mar29.182908.14124@cbnewsl.att.com>
Date: 29 Mar 91 18:29:08 GMT
References: <5468@archive.BBN.COM> <1991Mar29.091522.18681@mailer.cc.fsu.edu>
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Reply-To: feg@moss.ATT.COM (Forrest Gehrke)
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In article <1991Mar29.091522.18681@mailer.cc.fsu.edu> you write:
>  [concerning bios address]
>They are located at the absolute addresses C000:0000 and F000:0000, which on
>286 or better machines, is converted to a 20 bit segment and a 16 bit offset,
>or 0C000:0000 and 0F000:0000.
>
>Since people expanding their memory to 1 megabyte STILL couldn't use the RAM
>between 640 and 1024k (384k of dead space), motherboards started to take ad-
>vantage of this by using the "unusable" RAM to reflect the contents of ROM,
>which are NOTICEABLE slower for this reason: RAM has 0-1 wait states, if the
>proper chips are put in... ROM has 3-4!  Therefore, the ROM is copied into the
>faster RAM at boot-up, and the RAM is made to emulate the ROM in write-protect
>and addressing.
>
>Therefore, the term SHADOW ram, is RAM in this "dead space" that is reallocated
>to take the place of system ROMs and BIOS.  Current development on the 286/386
>allows programs like QEMM by Quarterdeck to reassign the address of all remain-
>ing memory in this area (Shadow only takes up 128k) for program use.
>
Reassign where?

I am not sure--but your explanation sounds as though shadow ram is still
in the 384k upper memory.  But so is the ROM bios.  When using shadow ram,
is the address range of the bios still the same as the ROM?  How is
this address substitution trick accomplished?

You did not answer another question of the original poster which also
puzzles me: I happen to have a Micronics ascic motherboard and supplied
with it is their software called SETSYS.  With this I can set three
different bus speeds as well as turn on shadow ram of the bios.
Software tests with shadow ram show significantly higher write speed
to the screen.  However, with my Diamond controller of the 
non-interlaced monitor is supplied software called FASTBIOS.  The
manual explains that this is the same as shadow ram.  However,
tests show that crt write speed is nearly double the rate of
the SETSYS shadow ram bios.  Obviously, something is different and I
am wondering if somewhow the memory cache (I have 64kb of sram)
is now getting into the act?  But, this raises the question
why the SETSYS shadow ram doesn't.

BTW, in spite of all this speedup of write speed, I don't see 
this in real life with real programs.  Is this because most
programs are writing directly to the screen and not through
bios?  Also, can you name any popular applications which do
not write directly?  What about Windows?

Thanks for your time.

Forrest Gehrke feg@dodger.att.com