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From: vrh@mhres.mh.nl (Michael Verhey)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Extended Memory
Summary: Description of Extended memory
Message-ID: <3164@mhres.mh.nl>
Date: 17 May 89 06:26:07 GMT
References: <1939@ucsfcca.ucsf.edu> <1319@westmark.UUCP> <18358@cup.portal.com>
Organization: Multihouse NV, the Netherlands
Lines: 24

In article <18358@cup.portal.com>, rwp@cup.portal.com (Roger William Preisendefer) writes:
> I recently bought a 286 machine with 1M of memory and Phoenix BIOS.  My
> programs are still limited to 640K.  How can I use the remaining memory?
> 
> Also, what is the difference between extended and expanded memory?

The 8086/88 processor can address 1Mb of RAM. The 80286 can address 16 Mb.
Everything above the 1Mb barrier is called extended memory. So the 80286 can
address up to 15Mb of extended memory. This memory is only accessible when
thhe processor runs in protected mode.

Expanded memory is not directly mapped by the processor, but accessed through
a special expanded memory driver.

Now about the 640k barrier: the processor runs under DOS in real mode. That
implies that the processor cannot access any extended memory above 1Mb.
The 1Mb it split up into 2 pieces: one part of 640k and one part of 384k.
The 384k is assigned to the ROM BIOS, display adapters and cartridges. So
that leaves only 640k for DOS itself.
-- 
Michael Verheij (using Netnews)		Multihouse B.V., the Netherlands
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"Experience comes with the amount of equipment ruined."