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From: troy@mr_plod.cbme.unsw.oz.au (Troy Rollo, )
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: Extended and expanded memory?
Message-ID: <785@usage.csd.unsw.oz.au>
Date: 1 Aug 90 07:54:16 GMT
References: <260@oiscola.Columbia.NCR.COM>
Sender: news@usage.csd.unsw.oz.au
Reply-To: troy@mr_plod.cbme.unsw.oz.au
Lines: 59

From article <260@oiscola.Columbia.NCR.COM>, by salmassr@oiscola.Columbia.NCR.COM (Samer M. Almassri):
salmassr> Hi.  I always get confused between expanded and extended memory.
salmassr> Could anyone post or mail me a good explanation.

salmassr> 1- What is the difference? (Technically & from a user point of view)
salmassr> 2- Why there are two kinds?
salmassr> 3- Which one you should use?
salmassr> 4- Can the computer use expanded and extended memory at the same time?
salmassr> and other related information that you can think of...

Alright, but remember - you asked for this.

	Expanded memory was the first kind of memory beyond the 1Meg boundary
of the original PC - it works by swapping parts of memory on an expanded
memory card into the 1Meg address space at segment C000 (or thereabouts). More
recent implementations can splatter it all over the place like a dog's
breakfast. This is done in the same way it was done in the old Apple days -
bank switching - Yuk.

	Only programs which are specifically written for expanded memory can
use it. This means that you can travel to the ends of the Earth and still not
find one that does (Nethack 3.0i is an example of a program which can use it.
I would mention others, but that would entail getting commercial).

	Then there's Extended. Extended memory only exists on AT class machines
or better. Elsewhere it is meaningless. Even on those machines where it exists,
DOS programs can't use it directly. It's beyond the 1Meg address space. In
fact, it occupies the area from 1Meg to 16Meg, which is an altogether
civilised place for it to go, otherwise it may clash with 0Meg-1Meg.
	The catch is, Extended memory can only be used in the protected mode
of the 286/386/486. Dos cannot be used in protected mode. For this reason,
Ms/PCDOS should be taken out and shot. The problem is, that in order to use
the Extended memory properly, you need to use a protected mode operating
system, like the latest version of MsWindows, or Xenix/Unix, or OS/2 (bleuch).
W{hich one then? I suggest you toss a three sided coin, because when DOS is,
as it deserves to be, taken out and shot, we could end up on any of those three.
And I don't care to guess which.

	While DOS is still on death row, however, you can get access to some
of Expanded memory through XMS drivers like HIMEM.SYS. Unfortunately
somebody appears to have cut the balls off HIMEM.SYS, and it only offers
64K. Some software will use Extended memory anyway, by slipping into
protected mode while DOS isn't watching, and slipping out again. This is a
reasonable approach, especially if you don't want to talk to DOS very often.
If I had my way I would send DOS to Tasmania so nobody important would ever
have to talk to it at all.
	There are also some drivers somewhere out there wich will turn the
Extended memory into a RAM disk. A reasonable approach too, but it locks other
uses out.

	Which is the best? Well, if you're running Windows, Unix or OS/2,
you're laughing. Throw in a couple of meg of Extended. If you're using a 386
as well, throw in a gig and have some real fun. If you're using DOS, you're
probably better off with Expanded, because more programs use it. Of course,
DOS could be obsolete before long (with any luck).

___________________________________________________________
troy@mr_plod.cbme.unsw.oz.au	Make our greenies useful!
The Resident Fascist		Put them in the army!