Path: utzoo!attcan!uunet!husc6!rutgers!bellcore!texbell!bigtex!james
From: james@bigtex.cactus.org (James Van Artsdalen)
Newsgroups: comp.sys.ibm.pc
Subject: Re: extended vs. expanded
Message-ID: <10928@bigtex.cactus.org>
Date: 24 Nov 88 05:19:57 GMT
References: <1717I78BC@CUNYVM> <1090@esunix.UUCP> <1970@hoqax.UUCP> <7710@pasteur.Berkeley.EDU>
Reply-To: james@bigtex.cactus.org (James Van Artsdalen)
Organization: Dell Computer Corporation, Austin TX
Lines: 20

In <7710@pasteur.Berkeley.EDU>, c9c-ba@dorothy.Berkeley.EDU.UUCP (George Voon) wrote:

> When one buys a 286/386, the 1 Meg is not from 0-1024K, but from 0-640K
> plus 1024-1408.  The memory from 640-1024 is reserved for video, etc.
> Therefore, the extra 384K is extended memory.

This is not always the case.  Most of our (Dell Computer) machines
uses the "extra" 384K of memory for ROM shadowing, not for extended
memory.  Shadowing gives a couple of big benefits:

1. The RAM used for shadowing is either 16 or 32 bits wide, depending
   on whether the CPU is a 286 or 386.  Many VGA cards only have 8 bit
   ROMs, so the wider RAM speeds execution.

2. ROM in a slot gets many wait states per access, whereas the system
   board RAM gets few if any.  This alone can double the speed of code
   in a VGA BIOS.
-- 
James R. Van Artsdalen      james@bigtex.cactus.org      "Live Free or Die"
Home: 512-346-2444 Work: 338-8789       9505 Arboretum Blvd Austin TX 78759