From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S267012AbUBMNhT (ORCPT ); Fri, 13 Feb 2004 08:37:19 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S267014AbUBMNhS (ORCPT ); Fri, 13 Feb 2004 08:37:18 -0500 Received: from lists.us.dell.com ([143.166.224.162]:32926 "EHLO lists.us.dell.com") by vger.kernel.org with ESMTP id S267012AbUBMNhQ (ORCPT ); Fri, 13 Feb 2004 08:37:16 -0500 Date: Fri, 13 Feb 2004 07:36:47 -0600 From: Matt Domsch To: Nagy Tibor Cc: xela@slit.de, mochel@osdl.org, bmoyle@mvista.com, orc@pell.chi.il.us, linux-kernel@vger.kernel.org Subject: Re: HIGHMEM Message-ID: <20040213073647.B24512@lists.us.dell.com> References: <402CC114.8080100@dell633.otpefo.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.2.5.1i In-Reply-To: <402CC114.8080100@dell633.otpefo.com>; from nagyt@otpbank.hu on Fri, Feb 13, 2004 at 01:20:36PM +0100 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 13, 2004 at 01:20:36PM +0100, Nagy Tibor wrote: > We have two Dell Poweredge servers, an older one (PowerEdge 6300) and a > newer one (PowerEdge 6400). Both servers have 4GB RAM, but the Linux > kernel uses about 500MB less memory in the newer machine. This is a FAQ on the Linux-PowerEdge mailing list, and is captured here. http://lists.us.dell.com/fom-serve/cache/68.html I have 4GB (or more) RAM in my system. How come Linux sees less than that? BIOS must reserve some address space below 4GB for PCI devices such as RAID controllers, SCSI controllers, NICs, etc. RAID controllers in particular may request and be given 256MB each. This is address space that would normally be occupied by RAM, but instead is used by PCI devices. RAM addresses start at 0 and grow up. PCI device addresses start at ~4GB and grow down. As long as there is no overlap, the OS will see all available RAM and make use of it. If there is overlap, the PCI devices win, and that RAM is not made available to the OS. This is working as designed per PCI, BIOS, and system chipset specifications. You may wish to subscribe to the Linux-PowerEdge@dell.com mailing list at http://lists.us.dell.com/. Thanks, Matt -- Matt Domsch Sr. Software Engineer, Lead Engineer Dell Linux Solutions linux.dell.com & www.dell.com/linux Linux on Dell mailing lists @ http://lists.us.dell.com