Date: Thu, 27 Mar 2008 09:42:40 -0500 From: Erik Osterholm <freebsd-lists-erik@erikosterholm.org> To: lhecking@users.sourceforge.net Cc: freebsd-questions@freebsd.org Subject: Re: FreeBSD-6.3 only detects 3GB of RAM Message-ID: <20080327144240.GA69841@aleph.cepheid.org> In-Reply-To: <20080327142605.6190DC33C@cork.irdesign.cypress.com> References: <003501c89014$77ddf640$6799e2c0$@com> <20080327142605.6190DC33C@cork.irdesign.cypress.com>
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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
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