[wplug] questions and clarification regarding RAM upper limits

[Bill Moran]

In response to "Brie Gordon" <brie.gordon at gmail.com>:

> So, I've been reading about the maximum amount of RAM that a computer
> can handle and I've been seeing the same response come up over and
> over again.
> A machine with a 32-bit architecture can handle up to 4 GB of RAM
> (I've seen 'hacks' to get Windoze to address more memory through an
> edit of the boot.ini file but I'm skeptical (and also don't care as
> much as I no longer use it). A 64-bit processor can handle a lot more
> (16 TB). Is this across the board? I've seen information that puts the
> Red Hat kernel 2.4 at 64 GB.
> 
> So, can any one confirm 4 GB is the max for a 32-bit architecture?
> For Fedora, Ubuntu, and Red Hat (let's assume 2.6.x kernels) with
> 64-bit architectures what is the max?

The question is far less simple than you might think.

On i386 arch, the maximum addressable memory is 4G, _except_ that many
motherboards use the last 500M for specially mapping PCI stuff, which
means that address space isn't available for normal memory use ... so
on some motherboards, the limit is 3.5G ... _unless_ you use PAE, which
uses some tom-foolery to allow i386 to address a full 2^64 of memory,
albeit with some serious performance hits.  AFAIK, most distros ship
with PAE-ready i386 kernels.

The _theoretical_ limit for amd64 arch is 2^64, which I believe is 16T.
However, I don't think any CPUs/motherboards are actually able to do
that at this time.  AFAIK, the most you can get out of any CPU actually
in production is 2^40.  The spec allows 2^64, but the chip makers are
saving some $$$ by not putting all those registers in, on the assumption
that nobody out there will need a full 2^64 of memory for many years.

Of course, if any of this information is wrong, I'll be happy to be
corrected.

-- 
Bill Moran
http://www.potentialtech.com