Snow Leopard Seeds Use 32bit Kernel, Drivers by Default
Linked by Thom Holwerda on Tue 18th Aug 2009 18:46 UTC
Mac OS X 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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FellowConspirator
Member since:
2007-12-13

"Mac OS X can run 64-bit applications on 64-bit machines even with the 32-bit kernel.


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.

Reply Parent Score: 3

galvanash Member since:
2006-01-25

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.


This is just a pet peeve of mine so don't take offense, but OSX's kernel is called XNU. Mach proper has not had any serious development work done on it for over 15 years. XNU is a fork that goes back quite a few years and as such deserves its own name by this point.

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.


XNU is not a microkernel, it is monolithic. The entire kernel and all its drivers and even the BSD subsystem all run in kernel mode. Regardless, there is nothing special about a microkernel in the sense you are talking about it - as is obvious since OSX doesn't have one.

Reply Parent Score: 2

Bill Shooter of Bul Member since:
2006-07-14

The obvious question, is what impact does that mode of operating have on operating system performance? Does it limit the number of threads the kernel can handle? Or the memory it can address? How does a 32 bit OSX kernel manage more than 4 gb of memory, or is the memory management 64 bit? What parts of the kernel stay in 32bit mode? Anything other than device drivers?

If there is no difference, then why the different behavior with the Macs' listed in the chart? Is there a good technical reason for booting in 32 bit mode over 64 bit?

Reply Parent Score: 2