Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!zaphod.mps.ohio-state.edu!wuarchive!uunet!pilchuck!amc-gw!jwbirdsa
From: jwbirdsa@amc.com (James Birdsall)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: What's so special about the UMB?
Message-ID: <1991May31.174949.21576@amc.com>
Date: 31 May 91 17:49:49 GMT
References: <1991May30.213856.1@freke.claremont.edu>
Reply-To: jwbirdsa@europa.amc.com (James Birdsall)
Organization: Applied Microsystems, Redmond, WA
Lines: 27

In article <1991May30.213856.1@freke.claremont.edu> dhosek@freke.claremont.edu (Don Hosek) writes:
>What is it about the first 64K of extended mem that makes it so
>special to DOS 5.0, DESQview, et alia? It seems that there should
>be no more trouble accessing any other 64K block of extended mem
>than the UMB, but I'm acknowledgedly ignorant of such things...

   First, terminology: what you are talking about is normally referred to
as the HMA (high memory area); it is the first 64K - 16 bytes of extended
memory. UMBs (upper memory blocks) are random blocks of memory that may 
exist between 640K and 1M. The HMA, UMBs (if any), and extended memory
are all covered in the XMS specification.

   What's so special about it? Well, due to a bug in the 286, it is
possible to access the HMA from real mode. When adding a segment and
offset, the overflow (21st bit) is supposed to be thrown away in real
mode. On the 286, it isn't necessarily. Therefore, if the A20 line
(21st address line) is enabled and the segment and offset values are set
up properly, it is possible to get at nearly the first 64K of extended 
memory. I forget whether this bug was deliberately kept in the 386, or
whether it is emulated in another way, but nearly all 386-and-up systems
can perform this trick as well.

-- 
James W. Birdsall   WORK: jwbirdsa@amc.com   {uunet,uw-coco}!amc-gw!jwbirdsa
HOME: {uunet,uw-coco}!amc-gw!picarefy!jwbirdsa OTHER: 71261.1731@compuserve.com
"The OS shouldn't die every time the controller drools on a sector." -- a sysop
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