Sorry if I jump in into that discussion as an absolute newbie to the 64bit world :-) I've just bought 2 AMD64 systems with Asus A8V boards and 4 1GB DIMMs each. Indeed I stepped over that memory problem and now I almost understand what the problem is. But let me ask two things: 1) If that reserved memory is required for PCI devices, why don't have 32bit systems that problem? On all my systems with 2GB RAM and video cards with 256MB video ram, there is the full 2GB available running linux. Why do only the 64bit systems need this explicit reservation between 3.5 and 4GB? 2) The Asus bios allows for software *and* hardware memory remapping. In both cases I have almost all 4GB of RAM available. However, I don't have 3D acceleration anymore because the fglrx driver complains about those uncacheable mtrr space between 3.5 and 4GB. Andi wrote sth. about that problem in an earlier thread that I found in the archives. What I don't understand is: How can that memory be available suddenly, when using the bios option to remap that memory hole, if the memory must be reserved for PCI devices? Or is it that it is indeed no longer reserved when doing the remapping, and that's why the fglrx drivers cannot access it? Andi wrote sth. in that other thread about manually creating a write- combined mtrr space from the uncacheable space (which I didn't try yet), but I don't understand why this should be possible: If I could easily create a write-combined space in the uncacheable space, why can't I just turn all the uncacheable mtrr space into write-combined and all the problems are gone? What's the relation between that "reserved for PCI" memory and this uncacheable mtrr range between 3.5 and 4G? Sorry when I'm asking so much, but I would really like to understand the background of these 64bit secrets :-) cu, FRank -- Dipl.-Inform. Frank Steiner Web: http://www.bio.ifi.lmu.de/~steiner/ Lehrstuhl f. Bioinformatik Mail: http://www.bio.ifi.lmu.de/~steiner/m/ LMU, Amalienstr. 17 Phone: +49 89 2180-4049 80333 Muenchen, Germany Fax: +49 89 2180-99-4049 * Rekursion kann man erst verstehen, wenn man Rekursion verstanden hat. *