Path: utzoo!mnetor!uunet!husc6!necntc!ima!johnl
From: johnl@ima.ISC.COM (John R. Levine)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Extended Memory for PS2 Model 30
Message-ID: <943@ima.ISC.COM>
Date: 11 Apr 88 03:48:02 GMT
References: <4710@bcsaic.UUCP> <6853@j.cc.purdue.edu> <1119@maccs.UUCP>
Reply-To: johnl@ima.UUCP (John R. Levine)
Distribution: na
Organization: Not enough to make any difference
Lines: 42
Keywords: ps2 model 30

In article <1119@maccs.UUCP> rob@maccs.UUCP (Rob McDonald) writes:
>In article<6853@j.cc.purdue.edu>tim@j.cc.purdue.edu.UUCP(Timothy Lange)writes:
>>First, an IBM PS/2 model 30 has an 8086 cpu, so extra memory will
>>have to used as EXPANDED memory, not extend.....
>I don't understand.  Does Extended memory require use of protected mode?
>Somebody please enlighten me.

In short, yes.  Tiny summary:

 Extended memory -- memory addressed above 1MB that is addressed by 80286
 or 80386 in protected mode.

 Expanded memory -- memory addressed though a bank switching hack that will
 work on any 8088 or above.

To add to the confusion (beyond the foolishness of choosing two names that
sound almost the same):

Every expanded memory card I've seen for an AT lets you configure the memory
as expanded or extended or a combination, depending on switches.

There are programs around that attempt to use extended memory on a 286 to
simulate expanded memory. They work for some applications but not for others.
You cannot correctly implement expanded memory with reasonable performance
without some hardware assistance, BUT:

The 80386 chip can use its on-chip memory management hardware to correctly
implement expanded memory on top of extended.  Most 80386 operating
environments do this for you.

On a PS/2 mod 50 or above, the configuration hardware built into memory cards
is sufficient to implement expanded or extended memory. By default it comes up
as extended, but suitable software (available from several sources) lets you
use it as expanded. The motherboard memory doesn't have the hardware, so you
can't use any of it as expanded. Note that since the PS/2 mod 30 only has an
8086, you can install expanded but not extended memory cards.

Confused? Wait until someone tries to explain how this fits with OS/2.
-- 
John R. Levine, IECC, PO Box 349, Cambridge MA 02238-0349, +1 617 492 3869
{ ihnp4 | decvax | cbosgd | harvard | yale }!ima!johnl, Levine@YALE.something
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