linux-kernel - Re: memory remapping, 4gb memory on 945gt

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Message-ID: <477EAD37.4070903@zytor.com>
Date:	Fri, 04 Jan 2008 14:03:35 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Dr. David Alan Gilbert" <linux@...blig.org>
CC:	Andi Kleen <andi@...stfloor.org>, felix-linuxkernel@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: memory remapping, 4gb memory on 945gt

Dr. David Alan Gilbert wrote:
>>>
>>> Now I can understand this explanation for 32-bit mode, but I'm running
>>> in 64-bit mode.  There should be a way to use the fourth gig under
>>> Linux.  Is there?
>> The chipset limitation applies to 64bit mode as well as to 32bit mode
>> (which actually does not have a 4GB limitation with PAE)
> 
> Note that some system and memory vendors have incorrect statements
> stating that the 3.xGB limit is a 32bit OS issue on these machines
> and it'll all just work fine on 64bit OSs.
> 

Supposedly WinXP-32 doesn't use memory over 4 GB even if it is available 
(a market-segmentation decision of the part of Microsoft, to force 
people to buy WinServer 2003; WinXP SP2 does PAE so there is no 
technical reason.)  This probably has disincentivized hardware vendors 
from providing support for remapping the memory from the aperture above 
the 4 GB mark.  The logic isn't all that trivial, especially if the 
aperture size is configurable, so I can understand why they would punt.

	-hpa
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