OS X.6 - Booting the 64bit-kernel:
Check this list if your Mac supports a 64bit-kernel or look in the terminal:
$ sudo ioreg -l -p IODeviceTree | awk -F'"' '/firmware-abi/{print $4}'
If the output is EFI64, you will be able to boot the 64bit-kernel.Type these two commands, to make sure your Mac boots in 64bit:
$ sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.Boot "Kernel Flags" -string "arch=x86_64"
$ sudo nvram boot-args="arch=x86_64"
$ sudo nvram boot-args="arch=x86_64"
Reboot and type this command in terminal to check if you're Mac boots in 64bit-kernel:
$ uname -a
You should find something like x86_64 in the output.To reset to 32bit-kernel, type these two commands:
$ sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.Boot "Kernel Flags" -string ""
$ sudo nvram boot-args="arch=i386"
$ sudo nvram boot-args="arch=i386"