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Date: Wed, 30 Apr 2008 17:55:10 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Fabio De Francesco <fabiomdf@alice.it>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [x86_32] With 4GB installed, in which cases low mem total is
 less than 896MB?
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> Why is it needed to activate the PAE mechanism in order to see and use the whole 4GB memory? Is it what it is expected to be done?
> 
> Thanks in advance to anyone who will reply.

The PCI hole has to be below 4GB so the Intel chipset maps some RAM above
4GB where PAE is needed to address it.