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How to tell if you’re running the 32 bit or 64 bit kernel in Mac OS X Snow Leopard

September 7th, 2009 - 10.6, Command Line, Mac OS X, Snow Leopard

64 bit kernel mac os x Snow Leopard 10.6 is the first Mac OS X version to ship with a 64 bit kernel, and obviously your Mac has to have a 64 bit processor to utilize that kernel, but some 64 bit Mac’s are not defaulting to the 64 bit kernel. So how do you know which kernel version your machine is using? The command to determine if you are using a 32 bit kernel or a 64 bit kernel is the same, it’s the output that will tell you which kernel version you are using.

Open the Terminal and type the following command:

uname -a

If you’re using a 32 bit Kernel in Mac OS X:
iMac:~ user$ uname -a
Darwin iMac.local 10.0.0 Darwin Kernel Version 10.0.0: Fri Jul 31 22:47:34 PDT 2009; root:xnu-1456.1.25~1/RELEASE_I386 i386

see the i386 on the end there? That indicates it’s the 32 bit kernel

If you’re using a 64 bit Kernel in Mac OS X:
iMac:~ user$ uname -a
Darwin iMac.local 10.0.0 Darwin Kernel Version 10.0.0: Fri Jul 31 22:47:34 PDT 2009; root:xnu-1456.1.25~1/RELEASE_X86_64 x86_64

the x86_64 at the end will let you know you are using the 64 bit kernel.

You can alternate between the two by holding down “6″ and “4″ during system boot to load the 64 bit kernel, or holding down ‘3′ and ‘2′ during boot to use the 32 bit kernel. Your machine should default into the kernel that is best supported.

Posted by: William Pearson

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Comments:

Comments: 5

Comment from Mike
Time: September 7, 2009, 6:50 am

Actually, all Macs except for the Xserve default to the 32bit kernel. This is justified since the Xserve is a specialized piece of equipment where you can be sure all device drivers are built as 64bit. For the consumer line, that can’t always be assumed, so its safer to play with the 32bit kernel even while running 64bit apps.

The other easy to tell which kernel you’re running is by opening System Profiler and clicking on Software. In the overview it will list 64-bit Kernel and Extensions. Odds are it will say no, signifying you’re using the 32bit kernel. Your machine needs to have 64bit EFI, which only the newest Macs have. If you want to check for that use: ioreg -l -p IODeviceTree | grep firmware-abi. The result will show either EFI32 or EFI64 depending on what you have.

Comment from Bill
Time: September 7, 2009, 7:54 am

Only the server version of OSX lets you switch to 64 bit, I tried the toggle above, it doesn’t work. The Apple tech notes say this very clearly. I have a 64-bit EFI on my v1 MacBook Air, but this makes no difference to booting.

Comment from Eric M.
Time: September 7, 2009, 1:34 pm

Is there a way to test if applications are truly running in 64 bit? Apple claims that “Nearly all system applications — including the Finder, Mail, Safari, iCal, and iChat — are now built with 64-bit code”. So the kernal runs in 32 bit but the apps run in 64 bit?

Comment from Khürt L Williams
Time: September 7, 2009, 1:56 pm

I tried the toggle and it does indeed work. The iMac booted OS X 10.6 into 64 bit mode and now some of my system preferences and kernel extension no longer work.

http://www.flickr.com/photos/islandinthenet/3897219777/

Comment from Greg
Time: September 19, 2009, 11:00 pm

look under About this Mac/More info…/Software to see if there is a Yes or No in the line about 64 bit Kernal extensions.

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September 7th, 2009