Linked by Thom Holwerda on Tue 18th Aug 2009 18:46 UTC
Even though Apple has been hyping up the 64bit nature of its ucpoming Snow Leopard operating system, stating it will be the first Mac OS X release to be 64bit top-to-bottom, reality turns out to be a little bit different so far. With the current Snow Leopard seed, only Xserve users get the 64bit kernel and drivers - all other Macs default to 32bit. By holding down the '6' and '4' keys during boot, you can to boot into full 64bit mode - that is, if your Mac supports it. As it turns out, some Macs with 64bit processors cannot use the 64bit kernel because the EFI is 32bit. Note: I should have included in the article that 64bit applications will run just fine (including benefits) on a 32bit kernel in Mac OS X. Since this was already possible in Leopard, I assumed people were well aware of that. Turns out some were not, so my apologies for that.
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Member since:
2007-12-13
Wow I didn't know that was possible. Do you have a reference for that? "
That's an over simplification. The Mach kernel will switch to a 64-bit OS X if it finds a 64-bit capable processor. You'll note that this happens even if you have 32-bit EFI.
Even then, the Mach microkernel does use the 64-bit "long mode" which permits execution of "64-bit" and "compatibility mode" (32-bit) code. In Leopard, the majority of the OS X kernel itself runs predominantly 32-bit and various frameworks run in the 64-bit mode. In Snow Leopard, everything runs 64-bit save for hooks for 32-bit legacy device drivers. There's nothing peculiar about it, it's built right into the EM64T spec -- it's just that leveraging it requires a microkernel architecture rather than a monolithic kernel.
This article doesn't really go into that and concludes, somewhat erroneously, that Snow Leopard doesn't run 64-bit on many pieces of Mac hardware. In fact, it's more accurate to say that the boot-loader and microkernel entry point will start in 32-bit mode and switch to 64-bit mode if there's a 32-bit EFI.