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From: johnl@ima.UUCP
Newsgroups: net.micro.pc
Subject: Re: IBM/AT and memory > 512k
Message-ID: <38700004@ima.UUCP>
Date: Sun, 28-Apr-85 11:58:00 EDT
Article-I.D.: ima.38700004
Posted: Sun Apr 28 11:58:00 1985
Date-Received: Thu, 2-May-85 05:38:28 EDT
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Nf-From: ima!johnl    Apr 28 11:58:00 1985


> Our AT has 1 Mbyte of memory in it, 512k on the mother board and one 
> 512k (IBM) expansion board.  Is there any way to get DOS to recognize 
> at least 640k of memory in the system?  

In a word, no.  The motherboard memory is at addresses 0-512K, and the
expansion memory is at addresses 1MB-1.5MB.  The IBM board doesn't have
split addressing which would allow part of the expansion memory to reside
between 512K and 640K -- IBM has a separate board (available at extra cost)
which you can buy.  The add-in boards from other manufacturers all allow
split addressing, but not IBM.

There appear to be 2 1/2 ways to get some use out of your board:

1) Run Xenix, which runs in protected mode and uses all of the memory.
2) Use the DOS 3.x VDISK driver and make the second 512K into a virtual disk.
2.5) Write your own programs which use the same BIOS call that VDISK does and
move stuff in and out of the high memory.

If you think that this proves once again that the structure of the 286 is 
suitable primarily only for controlling high-performance vending machines, 
you're right. 

John Levine, Javelin Software, Cambridge MA 617-494-1400
{ decvax!cca | think | ihnp4 | cbosgd }!ima!johnl, Levine@YALE.ARPA