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From: ralf@B.GP.CS.CMU.EDU (Ralf Brown)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Extended/Expanded Memory
Message-ID: <221@PT.CS.CMU.EDU>
Date: Fri, 23-Oct-87 11:32:57 EST
Article-I.D.: PT.221
Posted: Fri Oct 23 11:32:57 1987
Date-Received: Sun, 25-Oct-87 16:01:24 EST
References: <7405@dartvax.UUCP> <7662@steinmetz.steinmetz.UUCP>
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Organization: Carnegie-Mellon University, CS/RI
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Keywords: Going from one to the other

In article <7662@steinmetz.steinmetz.UUCP> davidsen@crdos1.UUCP (bill davidsen) writes:
>In article <7405@dartvax.UUCP> merchant@dartvax.UUCP (Peter Merchant) writes:
>|I have a Zenith 248 with 3 megabytes of extended memory.  I need to take that
>|memory and turn it into expanded memory.  Does anyone know if such a thing is
>|possible?  Any public domain/shareware products?
>
>QEMM from Quarterdeck seems to work quite well. I have the 386 version,
>but I'm told that there is a 286 version as well. It's absolutely possible.
>-- 
>	bill davidsen		(wedu@ge-crd.arpa)

The QEXT.SYS driver which comes with DV 2.0 calls itself QEXTXXX0, and has
the startup message "QUARTERDECK EXTENDED MEMORY MANAGER".  If this is
what you are thinking of, it will *not* allow you to use extended memory
as expanded memory.  What it DOES do is allow DESQview to run about 60K of
itself in the first 64K of extended memory *without* switching to protected
mode.

Using the first 64K of extended memory from real mode is possible because of
a quirk in the 286, which allows it to generate addresses up to 1M + 64K - 16
in real mode (i.e. FFFF:FFFF).  Since the 286 has 24 address lines, addresses
of >1M are not wrapped back to 0 as they are on the 808x with 20 address 
lines.  The AT has circuitry which normally forces the  21st address line
low, to effect the wrapping.  The QEXT.SYS driver turns this off, allowing
the CPU to address more than one meg.

-- 
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