On Nov 22, 2007, at 5:49 PM, Jon M. Hanson wrote:

> ll 64-bit processors (not Itanium) have "compatibility mode" where
> it can run in 32-bit mode and look like any other 32-bit processor.
> In compatibility mode you cannot run 64-bit code. Compatibility mode
> can only be set at boot time so it's not something you can switch
> back and forth while the system is running. This is why you can boot
> 32-bit Windows XP on these 64-bit processors.

This is not true. OS X and HPUX will run 32-bit and 64-bit  
applications simultaneously. They just won't run on Pentium D class  
machines. But my question relates to Optiplex computers which are  
Pentium D machines and do have 64-bit BIOS. Under your scenario, they  
must boot in 64-bit mode because of the 64-bit BIOS. So they can't run  
32-bit Windows. But they do. Hence my question.
>
>
> I'll reiterate that you cannot run 64-bit applications on 32-bit CPUs
> at all. It doesn't have anything to do with the kernel. You can't
> even run 64-bit applications when the 64-bit CPU is running in 32-bit
> compatibility mode.

Nobody asserted that you could run 64-bit applications on 32-bit CPUs.  
That would be silly. What I asserted was that you could support 64-bit  
applications on a 32-bit kernel. Which you clearly can on 64-bit  
PowerPC and Core 2 architectures.

I readily admit that what I can easily find on the web corresponds to  
the "Compatibility mode can only be set at boot time so it's not  
something you can switch back and forth while the system is running"  
model that you lay out above and that model is perfectly consistent  
with what Craig reports. Something is missing though. These machines  
can run either 32 or 64-bit Windows but only 64-bit Linux. It doesn't  
make sense.
--
Conscience is thoroughly well-bred and soon leaves off talking to  
those who do not wish to hear it.
-Samuel Butler, writer (1835-1902)



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