I have recently figured out that Mac OS X actually CAN run 64bit (x64) applications even if x86 kernel is loaded. That was shocking for me for a first time.
But then I've realized that it is really weird if system is up and running under x64 compatible CPU can't run x64 applications, no matter what kernel managing the processes. Is it really so hard? Just load damn app into the memory and set up the CPU operation pointer to the first byte, easy as pie!
The one and only barrier to do that, as I could imagine, is some kind of "executable headers". Unfortunately, I am not very comfortable with Windows architecture and binary structure, so I need more explanation here.
De facto UNIX-like OS binary header standard ELF has it's brother ELF64, which (as the document here describes) doesn't have much differences with ELF32, but even though 32bit kernels aren't able to run x64 code. Yes, this program is likely linked to x64 libraries and lets imagine we just copied and pasted them right into /usr/lib64 folder. But I'm pretty sure that doesn't help, why?
And finally, what is so special about Mac OS X kernel so it doesn't worry about program instruction set used? Does Mac OS X have some universal and suitable for both kernels executables header, so it just can load app into the memory and say to CPU "execute right from here, I don't mind what stands for that"?
P.S.: I really thought much about where to place this question: on the stackoverflow.com or superuser.com, and decided to place here, because the topic is likely more OS-specific thing.