Re: BIOS detects 4 GB RAM, but kernel does not

From: Robert Hancock
Date: Fri Jul 28 2006 - 22:04:49 EST

iforone wrote:
see why above (mostly)... That doesn't necessarily mean that some kind
of certain 'config' doesn't need to be compiled into the kernel. Yet
I'm not sure exactly which options are necessary, if any, nor am I sure
you can achieve your goal using what *seems* (Robert Hancock's
reponses) to be essentially an Intel DualCore 32bit CPU(s). I'm having
a bit of trouble understanding (or believing) that an EM64T system
isn't capable of seeing more than 4GB RAM (although the Mobo max for
that Desktop mobo is 4GB) -- BUT then again, that is not a Server grade
Mobo either -- it's a Desktop mobo (perhaps using an
unsupporting(cheaper) Chipset(?). Yet; the BIOS detects all 4GB (which
shoots down my chipset theory) - so maybe it's a kerenl specific issue
(harkens back to General S's response about 2.6.15+ kernels only).
In my earlier post, I hadn't realized you were using a 64bit kernel.

The BIOS can see that all 4GB of memory is there, but it has no way of moving it out of the way to make room for the PCI/PCI-E memory-mapped IO space, so it has to map that IO space over the RAM in that part of the address space, rendering it inaccessible. There's no way that the kernel can fix this, regardless of whether you are using 32 or 64 bit or what configuration options are set.

Athlon 64/Opteron CPUs have support for moving this part of the RAM above 4GB to allow it to be used. This is part of the CPU's on-die memory controller so no special chipset support is needed. On Intel systems this support has to be provided by the chipset, and on the desktop boards, it's not.

You can see what's going on from the BIOS e820 memory map that's printed in the dmesg output at the start of bootup. If you calculate out the amount of address space reported as "usable" then you will get your value of 3.2GB or so which is all the kernel has access to. If the system supported memory remapping then you would see another region starting at 0x0000000100000000 (4GB) which would account for this missing 800MB or so.

Probably the main reason Intel didn't bother including this support in the desktop boards is that current non-server versions of Windows (at least 32-bit) won't use any memory that is mapped above 4GB anyway even though PAE is enabled - a purely artificial limit that MS put in place to discourage using desktop Windows on such large memory machines..

Robert Hancock Saskatoon, SK, Canada
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