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

From: Robert Hancock
Date: Sat Jul 29 2006 - 14:25:09 EST

iforone wrote:
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..

Agreed -- I _was_ going to inquire about XP x64 versions, and PAE also,
and I've read a bit about it (as microshaft has written on it's site),
but not in quite a while. I recall the semi-related NoExec (NX) bit
stuff (though I forgot the mnemonic difference between AMD64 and Intel
concerning this "bit", which if set disallows execution of any code
above the 4GB boundary, IIRC).

to follow-up: Do the XP x64 versions do something else artificially to
enable addressing up to 16GB of RAM or thereabouts. Or - is it that PAE
(Physical Address Extensions) stuff again that allows or it?

PAE is what allows an OS in 32-bit mode to access memory located above 4GB in the address space. On a 64-bit OS the CPU can access all the memory directly so PAE is not needed or possible. Also, PAE is often enabled on 32-bit systems even with less than 4GB of RAM as it allows using the hardware NX bit, which was only added to the PAE versions of the 32-bit page tables.

That's a separate issue, though, from the original subject of the thread (memory being mapped over by IO space), and from the artificial limit on accessing memory above 4GB in at least 32-bit consumer Windows.

More importantly -- I have (an as-yet-to-be-assembled system) : AMD64
s754 3000+ with a crappy mATX mobo here (VIA KTm800/8237) Chipset --
The RAM limit is 2GB total (2 x 1GB DIMM slots only). Do you think this
el-cheapo mobo would have problems accessing over 4GB *if* the Mobo was
designed for 4GB ?? IOW-- a Mobo perhaps such as General S uses (MSI,
ASUStek, etc) -- or do you know if there's something different between
the s754 and s939 models that I'm unaware of (besides the No Dual
Channel RAM in s754, since it's only 64bit Single-Channel capable, not

I don't think most S754 boards support physically installing that much RAM, so it may be moot. If you could, you could likely access memory above 4GB, though I'm not certain if the S754 CPUs support the memory hole remapping to avoid some of that RAM being lost.

Thanks for the continuing discussion -- I'm glad I didn't follow Intel
recently and become deceived again - as I'm sure MANY have -- you'd
think buying a spanking new Pentium-D (8xx) and a 'decent' Intel
Desktop mobo would allow access to more than 4GB RAM ...but no.... :-(
(especially when the specs for the mobo claim "4GB" - heck - might as
well remove 1 x 1GB DIMM, you'd only lose 200MB (yikes)).

Yeah, there is not much point in installing more than 3GB in such a board..

Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@xxxxxxxxxxxxx
Home Page:

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at