Path: utzoo!bnr-vpa!bnr-fos!bmers58!schow
From: schow@bmers58.UUCP (Stanley Chow)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 640KB + 3072KB = 4 MB?
Message-ID: <240@bmers58.UUCP>
Date: 18 Sep 89 16:46:22 GMT
References: <8909170749.AA26779@euler.Berkeley.EDU>
Reply-To: schow%BNR.CA.bitnet@relay.cs.net (Stanley Chow)
Organization: Bell-Northern Research, Ottawa, Canada
Lines: 55
Summary:
Followup-To:
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In article <8909170749.AA26779@euler.Berkeley.EDU>, ndeng@EULER.BERKELEY.EDU (Nan Deng) writes:
> Recently I have seen a number of 386 motherboards all using PHEONIX BIOS.
> (MICRONICS, GENE, etc.) All boards have 4MB memory chips installed. All
> boards reported 640KB of base memory and 3072KB extended memory when boot up.
> 
> My question is: where is the additional 384KB?
>

The standard PC memory map has this 384KB reserved for BIOS ROM, I/O card
ROM/RAM, etc. What happens to it is complicated and depends on the chip
set, BIOS. I have AMI BIOS with the NEAT chip set so that is what I will
talk about.

The Chips & Technology NEAT chip set acts as a traffic cop for memory
references to that address range. It can map the references to the mother
board DRAM, mother board ROM, expansion I/O bus, etc. All this is  under
program control (by writing to registers on the chips).  The
intention is that on boot up, all references are to ROM, then BIOS copies
itself to DRAM, BIOS will then tell NEAT to map BIOS references
to DRAM. Simalar thing for EGA ROM. (This is known at ROM shadowing).

Note that Video RAM must be left alone and not mapped out, otherwise the
program will run real fast, but nothing will show on the screen! 

The NEAT chip set can map individual 16K blocks as well as write-protect
each block. AMI BIOS (at least mine) chooses to map the lower 640K to 
motherboard DRAM, and the upper 384K is mapped to a combination of:
  - Protected DRAM, for BIOS ROM shadowing
		    or Video ROM shadowing
  - I/O bus for Video RAM
	    or  other cards

Unfortunately, all the unused address range is protected. This means
eventhough the DRAM is sitting on the motherboard, no program will
find it. There is nothing to stop me from writing a program to fiddle
the registers so that NEAT will map the unused address range back to
unprotected DRAM. But looking at the address map, there is not that
much that can be recovered:
   64K  BIOS ROM Shadowing
   32K  Video ROM Shadowing (remember 16K block size)
   64K  EMS page frame
  128K  Video RAM (at least I think it is 128K)
--------
  288K  used.

So the most that can be recovered is around 96K. (Hmm, I remember the
days when 48K was the maximum for an Apple II).



-- 
Stanley Chow        BitNet:  schow@BNR.CA
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