32 Ubuntu on 64 bit AMD CPU, how ?

[Vincent Trouilliez]

Hi list,

A friend just bought a few computer parts to upgrade his old machine.
The new motherboard is fitted with one of these "64bit" dual core AMD
CPU's. The actual descroption on the vendor's website is:

"AMD Athlon 64 X2 Dual-Core 4600+ Socket 939"

I think I gathered from this list, by skimming posts here and there
over the past few months, that it is possible to get these 64bit CPUs to
run in 32bit mode, so that we can avoid all the problems with 64 bit (or
lack of) desktop software, and run a 32 bit OS which will work just as
well as on a 32 bit CPU.

However the technical details escape me:

How do I configure the CPU to work in 32 bit mode ? Is there an option
in the BIOS, which will configure the CPU at power-up ?
Or, is it done automatically, in software ? I mean for example, if I
just install a 32 bit Ubuntu version, will the 32 bit kernel detect the
64 bit CPU and automatically set it up for 32 bit operation when
booting ?

I should visit my friend tomorrow to install Ubuntu on that new machine,
so I would like to know what I am talking about by then ;-)

Thanks in advance for shedding some light :-)

Regards,

--
Vince