Snow Leopard Not in 64-bit mode by Default

So, you bought Snow Leopard, and you thought everything would just work. Guess what, you were wrong.  By default, Snow Leopard boots in 32-bit mode, not 64-bit. This was a decision on Apple’s part, and after readying why, I understand why they did it.  However, I want full 64-bit mode.  First, you need to verify that your Mac can even boot into 64-bit mode.  In a terminal window, run this command:

ioreg -l -p IODeviceTree | grep firmware-abi

You should get back:

“firmware-abi” = <”EFI64”>

If yours says EFI32, there is no reason to keep reading because you can’t boot in full 64-bit mode.

Here’s the boot as you want method for 32 and 64-bit. At boot up, hold down the 6 and the 4 keys. This will boot into 64 bit, just for that boot.

You can verify that you’re in 64 bit mode by going to About this Mac -> More Info -> Software:

Look at 64-bit Kernel and Extensions, if it says No, then you are not in 64-bit.

If you want to always boot in 64-bit mode, here’s the easy and safe way to do so:

With a plain-text editor, edit: /Library/Preferences/SystemConfiguration/com.apple.Boot.plist

Insert arch=x86_64 into the Kernel Flags field.

<?xml version=”1.0” encoding=”UTF-8”?>
<!DOCTYPE plist PUBLIC “-//Apple Computer//DTD PLIST 1.0//EN” “http://www.apple.com/DTDs/PropertyList-1.0.dtd”>
<plist version=”1.0”>
<dict>
<key>Kernel</key>
<string>mach_kernel</string>
<key>Kernel Flags</key>
<string>arch=x86_64</string>
</dict>
</plist>

That’s all it takes. Reboot, and you’re in 64-bit mode.  If you have a problem with an app after doing this, just go to the Info for that app, and you will now see a check box that says: Open in 32 bit mode.

Update:

These are the only models that can boot in 64-bit mode:

Xserve - Early 2008 - Xserve2,1 MA882LL/A | Early 2009 - Xserve3,1 MB449LL/A

Mac Pro - Early 2008 MacPro3,1 - MA970LL/A | Early 2009 MacPro4,1 - MB871LL/A, MB535LL/A

iMac Early 2008 - iMac8,1 MB323LL/A, MB324LL/A, MB325LL/A | Early 2009 - iMac9,1 MB417LL/A, MB418LL/A, MB419LL/A, MB420LL/A

MacBook Pro - Early 2008 (Penryn) MacBookPro4,1 MB133LL/A, MB134LL/A, MB166LL/A | Late 2008 (Unibody) - MacBookPro5,1 MB470LL/A, MB471LL/A | Early 2009 (Unibody) - MacBookPro5,2 MB470LL/A, MB471LL/A, MB604LL/A, MC226LL/A