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From: brandis@inf.ethz.ch (Marc Brandis)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: extended memory
Message-ID: <17720@neptune.inf.ethz.ch>
Date: 10 Dec 90 12:38:43 GMT
References: <18374@rasp.eng.cam.ac.uk> <15090024@hpspcoi.HP.COM>
Sender: news@neptune.inf.ethz.ch
Reply-To: brandis@inf.ethz.ch (Marc Brandis)
Organization: Departement Informatik, ETH, Zurich
Lines: 24

In article <15090024@hpspcoi.HP.COM> dlow@hpspcoi.HP.COM (Danny Low) writes:
>>(Kevin A.Price) 
>>We have a 286 PC/AT with 640k conventional memory and 384k
>>extended memory which is located at address 1 Megabyte.
>
>This does not sound quite right. It looks like you have 1MB of 
>convention memory divided into the usual 640K of available
>and 384K of reserved memory and not 640K of conventional and 384K of
>extended memory. I do not believe you can add extended memory
>in anything less than 512KB chunks. The 384KB in the 640KB to

There are some machines that allow you to configure 1 MB as 640K of conventional
and 384K of extended RAM. All Olivetti machines (and therefore many AT&T and
some XEROX PCs) that I know can do this. Note that some remapping hardware
is required anyway, otherwise you would not be able to implement the shadowing
feature correctly. At boot time, references to the segment at F000 have to
be directed to the ROM, while they have to be directed to the RAM once the
contents have been copied.


Marc-Michael Brandis
Computer Systems Laboratory, ETH-Zentrum (Swiss Federal Institute of Technology)
CH-8092 Zurich, Switzerland
email: brandis@inf.ethz.ch