SD is quite correct, I got that wrong way round. You can run 32 on 64 but it doesn't work too well because it can only address half the registers or something. 64 won't run on 32.

I think that AMD did come out with a processor that would run 32 bit in native 32 but could also run 64 bit with full efficiency. I don't know if it ever went on sale significantly though?

So, Cider old chap, without a 64 bit processor your CD is only good for keeping coffee rings off your desk?

I also seem to recall that earlier AMD 64 bit processors we actually running at 40 bit. The idea was to allow access to more memory addresses than 32 bit permits?