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
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Originally Posted by samsc
TODD SCRUTCHFIELD 