On Thu, 25 Oct 2007 16:36:39 +0100 (BST) David Brownlee wrote:

>>> I have a AMD64 machine, then I setup the NetBSD i386 port on it, so I
>>> have all apps for i386 in my amd64 machine (indluding flash, java,
>>> etc.)
>> Right, of course i386 will run on an Athlon64.  But you can't then run
>> amd64 or access more than 3 GB ram, or give an app more than 2 GB VM.
>
>  	Not that amd64 doesn't massively increase those limits,
>  	but for reference the 3GB limit tends to float around a
>  	bit depending on your BIOS: eg from an i386 server here:

This "limit" is more of a theory. The 32bit limit would be at 4GiB. That
doesn't necessarily say that the CPU can't address any more. Just think of
the old 8bit CPUs that could count to 65535, which if FFFF and therefore
16 bits of length - damn, my English sucks!

The limit was probably "introduced" because nobody could ever imagine that
memory would ever be a affordable as it is now. I can remember buying 16MB
of RAM for my 80486 and I paid 800 German Marks for the *used* modules.
That's about 410 Euros and inflation doesn't even come into that. If we
take that into accound, we are nearly at the number 800 again - but this
time in Euros. :-)

I have no clue as to why a BIOS or a northbridge is designed with
limitations like these. The cost-savings may be there, but they are
certainly too small to really bother with. But these limitations go all
through computer history. I had another 486 on a mainboard that could take
48MB of RAM - in theory. If I went above 32, the system just went so damn
slow, that the gain of the extra RAM was eaten up - along with at least
half of the other installed RAM. The chipset couldn't cache more than
32megs. I ran Linux at the time and even a text console was unbearably
slow.

A while after that I spent almost two weeks looking for a shop that would
sell me a tag-RAM module for my Pentium computer, so I could actually
*use* all the memory I had on board.

I wonder when we are going to find the first limitations for our 64bit
CPUs, that are not by design but by designer.

Regards,
Chris