How to Tell if Mac is Running a 32-bit or 64-bit Kernel in Mac OS X

Sep 7, 2009 - 20 Comments

64 bit kernel mac os x Ever wanted to know if your Mac is using a 32-bit or 64-bit kernel? It may seem geeky and in the weeds, but it’s now relevant. 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? How do you know if you’re running 32 bit or 64 bit Mac OS X? Well, there’s a simple command to determine if you are using a 32 bit kernel or a 64 bit kernel and the command is actually the same for both, it’s the output that will tell you which kernel version you are using.

Determining if Mac OS X is 64 Bit or 32 Bit

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.

Keep in mind that all modern Macs and new versions of Mac OS X are going to be 64 bit, so this is really only relevant to older hardware.


Related articles:

Posted by: William Pearson in Command Line, Mac OS


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  1. […] developer release since it has web functionality built in.  I confirmed I had a 64 bit machine by running uname -a.  I also finally realized the clue that I’ve clicked download.  It’s that the […]

  2. Mark says:

    I just got a Mac Mini 4,1 it has 4GB of ram currently and 10.6.6. It was by default running 32 bit os. Loading of the OS seems to be a little quicker, but I am not sure how stable this will be. I just tried firing up my Apps, Logic, Reason and Serato. They all loaded up, but did not try extensively to start up projects. I plan on installing FCE on here.

    The question is, is there any real benefit to running 64Bit OS?

  3. david says:

    i have brand new 27″ imac, 2.93GHz i7, MacOSX v 10.6.5
    toggle works fine for me, numpad does not work for me, tried it several times just to make sure, only numbers on key pad work.

    I only get beeping (not a single beep though) if i hold the 6 and 4 down for to long, it seems to be a mac version of the default windows beeping when you accidentally depress a key for to long whilst a machine boots, or have a boot problem (I guess windows nicked the idea off of the macs).
    hope this helps.

  4. Anonymous says:

    just thought i should put this out there. everybody is having varying experiences with holding down the keys whille booting. here is what i’ve found… only the numpad numbers will change from 32 to 64 and vice-versa. there will be a fairly loud beep if it changed. that is my eperience… i’m running a fairly new imac.

  5. Ani says:

    In my case
    uname -a gives this

    “Darwin walkwait-maclt 9.8.0 Darwin Kernel Version 9.8.0: Wed Jul 15 16:55:01 PDT 2009; root:xnu-1228.15.4~1/RELEASE_I386 i386 i386”

    This would indicate it is 32 bit.

    However “Processor Name: Intel Core 2 Duo”

    so is it 32 bit or 64 bit?

  6. Matt says:

    Just booted up my 27* in 64 bit mode and i thought it was in 32 bit its just got quicker esp loading my iphoto and itunes libs. Thanks for the info on how to do this. Its sites like these that let the average pc user get the most and best experience from their machines

  7. […] system. If you are unsure about how to figure out if you are on a 64-bit system, please check this. Please be sure to read the entire article before proceeding. If you are still unsure, you can […]

  8. Jason says:

    I have the newest Core i7 MacBook Pro and its kernel is running in 32-bit also. I feel it is still dog slow running Aperture 3 and multiple apps at once. I dropped $2.2k in order to run Aperture 3 better but have not seen much performance increase, especially since my plugins require it to run in 32-bit mode still.

    So is running the 64-bit kernel safe/stable to do, and if so how?

    • fiorina says:

      Get more RAM, more RAM helps the most for Aperature speed. You can run the 64 bit kernel it won’t hurt anything, but if a plugin requires 32bit it will prevent the 64bit Aperature from loading (I think).

  9. Barclay says:

    ioreg test should say:
    “firmware-abi” = <“EFI64”>

  10. Barclay says:

    So, there are 3 tests you can run, according to the above article & comments. When I run them I get differing results.

    uname test:
    Darwin dunnb.local 10.3.0 Darwin Kernel Version 10.3.0: Fri Feb 26 11:58:09 PST 2010; root:xnu-1504.3.12~1/RELEASE_I386 i386

    ioreg test:
    “firmware-abi” =

    System profiler:
    64-bit Kernel and Extensions: No

    Also, it appears that holding down the “6” and the “4” keys while rebooting has no effect.

  11. Greg says:

    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.

    • Gaurav says:


      does this mean that if my mac shows a ‘No’ in the ’64 bit kernel extension’, i cannot toggle between 32 bit and 64 bit OS, or does the ‘No’ show me the current status of the OS?

      i am a newbie to Mac, and am having trouble running Win7 64 bit on the Parallel Desktop virtualization package.

  12. 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.

  13. Eric M. says:

    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?

  14. Bill says:

    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.

  15. Mike says:

    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.

    • daftpunker says:

      that very handy my macbook air 2010 4GB ram 120GB SSD i tried that command that system profile method it says no 64 bit kernel but the terminal command said it is

      | | “firmware-abi” =

      which means it is capable of running 64bit OS so did i install snow leopard wrong also im downloading lion right now is there a way of making it 64bit during install this is my first new apple PC since my 2007 Power PC so any help to set this up 64bit in lion would be a great help

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