From owner-freebsd-questions@FreeBSD.ORG Thu Mar 27 15:01:16 2008 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id DCFB61065676 for ; Thu, 27 Mar 2008 15:01:16 +0000 (UTC) (envelope-from erik@cepheid.org) Received: from mail.cepheid.org (aleph.cepheid.org [72.232.60.94]) by mx1.freebsd.org (Postfix) with ESMTP id C2B0F8FC1B for ; Thu, 27 Mar 2008 15:01:16 +0000 (UTC) (envelope-from erik@cepheid.org) Received: by mail.cepheid.org (Postfix, from userid 1006) id 7A8E59B4006; Thu, 27 Mar 2008 09:42:40 -0500 (CDT) Date: Thu, 27 Mar 2008 09:42:40 -0500 From: Erik Osterholm To: lhecking@users.sourceforge.net Message-ID: <20080327144240.GA69841@aleph.cepheid.org> Mail-Followup-To: Erik Osterholm , lhecking@users.sourceforge.net, freebsd-questions@freebsd.org References: <003501c89014$77ddf640$6799e2c0$@com> <20080327142605.6190DC33C@cork.irdesign.cypress.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080327142605.6190DC33C@cork.irdesign.cypress.com> User-Agent: Mutt/1.4.2.3i Cc: freebsd-questions@freebsd.org Subject: Re: FreeBSD-6.3 only detects 3GB of RAM X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 27 Mar 2008 15:01:17 -0000 On Thu, Mar 27, 2008 at 02:26:05PM +0000, lhecking@users.sourceforge.net wrote: > fred writes: > > Hello all, > > > > I am trying to fix an issue with my dual xeon ibm server, it only detects > > 3GB or RAM but I have 4GB: > [...] > > I have seen this problem under Linux on IBM Intellistations (6225). > IBM pointed me to a tech document (the number of which I do not > remember) that says when the machine detects more than 3GB, it uses > some of that memory internally for ... stuff. Still, I find stealing > 1GB quite unacceptable. It's not really stealing it. It has to do with how the computer hardware handles memory mapped IO. A certain amount of memory is allocated per device which needs MMIO; in a typical computer, this could be as much as 1GB. It's mapped from the highest portion of the address space that the computer can handle--in a 32-bit environment, that means that it's mapped from 4GB on down. I haven't looked to see if it's arcitecturally the same on 64-bit machines, but if so, we'll see the same problem once we start putting more than a few terabytes of RAM in computers. Luckily, I think that day is quite a ways off, and who knows what changes will be made in computing by then. An MSDN blog actually has a decent description of the issue, including some points I didn't mention here: http://blogs.msdn.com/hiltonl/archive/2007/04/13/tbhe-3gb-not-4gb-ram-problem.aspx Erik