From owner-freebsd-questions@FreeBSD.ORG Thu Apr 9 13:17:06 2015 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 5D4DBBAE for ; Thu, 9 Apr 2015 13:17:06 +0000 (UTC) Received: from be-well.ilk.org (be-well.ilk.org [23.30.133.173]) by mx1.freebsd.org (Postfix) with ESMTP id 355877CF for ; Thu, 9 Apr 2015 13:17:06 +0000 (UTC) Received: by be-well.ilk.org (Postfix, from userid 1147) id 9DD7C33C24; Thu, 9 Apr 2015 09:16:58 -0400 (EDT) From: Lowell Gilbert To: Pierre-Yves =?iso-8859-1?Q?P=E9neau?= Subject: Re: How FreeBSD manage more than 4GB on 32 bits architecture References: <5526408F.1090005@asim.lip6.fr> Reply-To: freebsd-questions@freebsd.org Date: Thu, 09 Apr 2015 09:16:58 -0400 In-Reply-To: <5526408F.1090005@asim.lip6.fr> ("Pierre-Yves =?iso-8859-1?Q?P=E9neau=22's?= message of "Thu, 09 Apr 2015 11:04:15 +0200") Message-ID: <44k2xlcugl.fsf@be-well.ilk.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4 (berkeley-unix) MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Cc: freebsd-questions@freebsd.org X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 09 Apr 2015 13:17:06 -0000 Pierre-Yves P=E9neau writes: > For my internship, I have to find a way to manage more than 4GB of > memory with 32 bits processors. It seems that the FreeBSD kernel is the > perfect candidate for that. According to the architecture handbook(*), > "FreeBSD could theoretically handle memory configurations up to 8TB on a > 32 bit platform." This is quoting the Architecture Handbook's KVM memory mapping section. The following sentence reads "However, since most 32 bit platforms are only capable of mapping 4GB of ram, this is a moot point." > For the last two days, I was looking for documentation, articles, > thesis, or e-mails on that topic, but I can't find relevant > informations. Is anybody have informations on that topic ? Where should > I start to read the source code to find my answers ? I'm going to guess that the idea behind that comment is that FreeBSD's page tables can hold 2G (2^31) entries. At a 4k page size, that would give you 8TB of memory. But that only helps you if the hardware is physically capable of reading from more than 4G different locations. Therefore, the question becomes: what kind of hardware are you going to use that can keep track of those locations? As a side note, I would also recommend the book "The Design and Implementation of the FreeBSD Operating System," now out in a new edition. Good luck.