"Tim Anderson" wrote in message ...
The cheaper motherboards (ie. most of them) for P4 or Athlon 64 support a
maximum of 4GB RAM, even if the motherboards and processors support the
AMD64 extensions that can address sqillions of GB. On the other hand, more
upmarket boards support more RAM - up to 24GB or maybe more - for Xeon and
Opteron.

If you buy one of these 4GB boards and install 4GB RAM, you don't get the
use of all of it. The top of the 4GB address space gets shadowed by system
functions such as PCI Express addressing. This is not just a small detail
- typically you lose 1GB of your 4GB. See:

http://www.itwriting.com/blog/?postid=152

I can't at the moment find a clear explanation of this. I understand about
the shadowing, but the question of course is why a modern board can't use
a higher range of addresses to make the full 4GB available to the OS. The
manufacturers mutter about "PC Architecture", but then again they also
make boards that *do* overcome this limit.

With PAE, PC processors have been able to address more than 4GB for years.
So why are we still running into this limit?

Because *applications* have to be written to be PAE-aware,
and have to be written in 64-bit address space to make use of
anything over 4GB. This was (and still is) very much a niche
market -- very few desktop/workstation users will come close
to using 2GB, let alone 4, which is why MS originally split the
4GB address range between 2GB user space and 2GB
system space.