You might be wondering how you can change your boot kernal so that you can use different hardware and/or software or perhaps some other reason.
In OS X Lion, you can choose a default of either the 32-bit or 64-bit kernal, by typing these commands in Terminal:
To select 64-bit mode as default:
sudo systemsetup -setkernelbootarchitecture x86_64
To select 32-bit mode as default:
sudo systemsetup -setkernelbootarchitecture i386
You can also boot up in either of these modes for one startup only without assigning a default mode. You can
do this by holding the 6 and 4 keys during startup for 64-bit mode or you can hold the 3 and 2 keys during startup for 32-bit mode.