So million thanks for your reply I think I made some mistakes in this topic 1 Mac OS X 10 4 Tiger is a 32 bit PowerPC OS later versions adding support for 64 bit PowerPC Console Applications and 32 bit Intel processor 2 Mac OS X 10 5 Leopard is a 32 bit Universal OS with additional support for 64 bit applications for PowerPC64 and Intel64 processors 3 Mac OS X 10 6 Snow Leopard is the apple s first 64 bit OS running only on Intel processors retaining support for 32 bit PowerPC Applications 4 Mac OS X 10 7 Lion is the last OS retaining 32 bit kernel for Intel64 processors Before I thought OS X Tiger also supported 64 bit application for Intel64 processor that is something I made confused But I ve also heard AIX 4 x support 64 bit application on 32 bit kernel so I think for PowerPC architecture that would be a common thing But for Intel64 processor that is quite rare at least as I know only Apple had gotten a try For days I also read something on Hackintosh on the Internet so many hackers crack OS X kernel to adapt them onto AMD platform even though they might hack the kernels with their patches but I don t believe they would re architecture for the XNU kernel I saw many legacy AMD kernels could force hacked kernel sitting on legacy mode applications on Long Mode But in that environment IA 32 applications would fail to start That might say there might be something different between these two architectures Intel64 and AMD64 I ve also heard someone could perfectly host OS X on some virtual machines as is known to me none popular platform virtualizing software could cluster virtualize a guest OS so that could tell Apple did not use VT to implement that trick So that way Apple went even further than Microsoft s WoW64 In my imagination 32 bit kernel might provide essential OS mechanism to host 64 bit applications Each time application loader aware of 64 bit application would be ready to run it sets up lot of hooks onto the long mode to ensure 64 bit application would believe they are just sitting in 64 bit OS when they requires the system all those 64 bit hooks would grasp the processor from Long Mode back to Legacy Mode kernel to the real jobs So in this situation a processor would switch between Legacy Mode and Long Mode much more frequently than usual In OS X Leopard only very few Applications such as Chess provides 64 bit images so most time the processor might never need to swift to the Long Mode But in Snow Leopard if Intel64 processor found at system initialization all applications running within 64 bit mode So in this situation I confused myself whether Compatibility Mode host the IA 32 applications or the Legacy Mode host them But I would prefer the latter Even though Apple ever linked these two architectures IA 32 and Intel64 together so seamlessly but I would differentiate IA 32 with x86 64 Anyway again thank you very much for your reply