OS X: Starting up with the 32-bit or 64-bit kernel

Mac OS X v10.6 and later include a 64-bit kernel. On hardware that supports the 64-bit kernel, you can choose whether to start up (boot) your Mac using the 64-bit kernel or the earlier 32-bit kernel.

You can use either of these methods:

Method 1: Startup key combination (for current startup only)

• If your Mac uses the 32-bit kernel by default, but supports the 64-bit kernel, you can start up using the 64-bit kernel by holding the 6 and 4 keys during startup.
• If your Mac uses the 64-bit kernel by default, you can start up with the 32-bit kernel by holding the 3 and 2 keys during startup.

Your Mac will revert to the default kernel the next time you reboot it.
 

Method 2: On-disk setting (persistent)

To select the 64-bit kernel for the current startup disk, use the following command in Terminal:

sudo systemsetup -setkernelbootarchitecture x86_64

To select the 32-bit kernel for the current startup disk, use the following command in Terminal:

sudo systemsetup -setkernelbootarchitecture i386

Note: This setting is stored in the /Library/Preferences/SystemConfiguration/com.apple.Boot.plist file and will take effect every time you start up from this disk. If you start up from a different disk, the setting on that disk, or the hardware default, will take effect.