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How to boot Mac OS X 10.6 64-bit kernel

August 29th, 2009 Romain Leave a comment Go to comments

Just received my copy of Mac OS X 10.6 Snow Leopard! :-)

One of the main new features of this version is its 64-bit kernel. However, there are a couple of catches: First, even if you have a shiny Intel Core 2 Duo Mac you must have a 64-bit EFI in order to boot the 64-bit kernel.

ioreg -l -p IODeviceTree | grep firmware-abi

If you get EFI64 you’re in luck, otherwise tough… I got lucky: I do have a 64-bit EFI. Nice.

Second, even then all Macs (except Xserve) boot the 32-bit kernel by default. To use the 64-bit kernel you can either press ‘6′ and ‘4′ everytime you boot your Mac, or you can edit com.apple.Boot.plist as follows:

1. Open /Library/Preferences/SystemConfiguration/com.apple.Boot.plist

2. Find the string:

<key>Kernel Flags</key><string></string>

3. Change it to:

<key>Kernel Flags</key><string>arch=x86_64</string>

That’s it, you’re set. (if you ever need to boot in 32-bit mode, just press ‘3′ and ‘2′ during boot).

_

Update (31st August):

After having made the modification above, I wanted to insure that the kernel used was indeed the 64-bit one. To do that:

uname -a

Which for me unfortunately gives:

Darwin Kernel Version 10.0.0: Fri Jul 31 22:47:34 PDT 2009; root:xnu-1456.1.25~1/RELEASE_I386 i386

Argh…

So beware: a 64-bit EFI is not a guarantee that you’ll be able to use the 64-bit kernel. Especially, apparently iMac 7,1 (my case) cannot use the 64-bit kernel in any case.

Categories: Mac, OS X Tags: ,
  1. Michael
    August 29th, 2009 at 11:17 | #1
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    Damn:

    | | “firmware-abi” =

    What does that mean in practice?

  2. Michael
    August 29th, 2009 at 11:19 | #2
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    @Michael

    Hmm, it was:

    | | “firmware-abi” = ,

    what obviously means i have 32 bit EFI, but still what are the drawbacks?

  3. Romain
    August 29th, 2009 at 15:29 | #3
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    Hi Michael,

    Well, I get:
    | | “firmware-abi” = <”EFI64″>

    That’s been said, whether the kernel is 32 or 64 bits applications properly compiled will be run in 64 mode. So mostly no difference.
    The 64-bit kernel might be a tiny bit faster, but mostly (as I understand it) the main benefit is to be able to address more than 32GB of RAM… which is anyway not supported by most Macs.

    Anyhow, I wanted Snow Leopard to get a full 64-bit system, so now I got it ;-)

    Still, my disappointment is that my iMac has an ATI Radeon HD2600 graphic card, so no OpenCL for me to play with… :-(

  4. Michael
    August 30th, 2009 at 14:12 | #4
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    Those two comments of mine didnt come out how they should cause that html tags thing, but whatever.

    I have a macbook with intel gma 950 so i guess you cant compete with me in that matter ;D.

    Anyway, thanks for blogging about macs and raytracing, thats what concerns me the most :) .

    Ive had to stop working on my baby cause the repeat exams thing, but soon im striking back, especially that there is that newer java version i was waiting for :) .

    Cheers.

  5. Romain
    August 30th, 2009 at 14:46 | #5
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    @Michael

    Cool. Looking forward to seeing more nice renders on your blog. :-)

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