****Re: ****What's up with 64 bit Linux

[Jon M. Hanson]

On Nov 22, 2007, at 5:42 PM, Chris Gehlker wrote:

>
> On Nov 22, 2007, at 2:17 PM, Craig White wrote:
>
>> easy answer...Dell Optiplex 320
>>
>> i386 (32 bit) had multiple issues on this hardware including but most
>> especially the inability to work properly with the drive controller
>> (SB
>> 600) and a heavy hammer of requiring kernel parameter of acpi=off.
>> 86_64
>> didn't have those issues which made the choice of installing 64 bit
>> rather easy.
>
> Thanks  Craig.  That was the clue I needed to figure this out enough
> to satisfy my  curiosity. It appears that the notion I had picked up
> that it was reasonable to support 64-bit applications on a 32-bit
> kernel applies only  to the Core 2 and comparable generation of CPUs
> in the Intel/AMD world. For the Pentium D chips used in the Optiplex,
> 64-bit mode is pretty much all or nothing, though, as  you point out,
> there are wrappers for things like plug-ins.
>
> I think this answers my question about why Linux is went down the dual
> track mode. The developers couldn't ignore a generation of CPUs.
>
> Apparently these Optiplex machines have 64-bit BIOS and require 64-bit
> drivers, which leads me to wonder how you can get 32-bit  Windows to
> run on them.
>
>

All 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.

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.