Out of the box Snow Leopard defaults to running in 32 bit mode. This is so the drivers for things like printers, scanners, network cards, etc. that have not been ported to 64 bit can run. Applications are unaffected by this. A 64 bit app will run in 32 bit mode and vice versa. If you’re not sure what mode your machine is running Snow Leopard in check out this article at MacObserver on how to tell.
The average user is much better off staying with the 32 bit mode for compatibility and ease of use. However, there may be times, especially for those running scientific software, when you need to run in 64 bit mode. And some servers, as mentioned in this Knowledge Base article do boot directly into 64 bit mode and may need to be set back.
You can choose to hold down the “6″ and “4″ keys on startup to boot into 64 bit mode. This will boot you into 64 bit for that boot cycle. When you reboot you will fall back to 32 bit again. Likewise, holding down the “3″ and “2″ keys on boot will put you into 32 bit mode.
If you want to change the mode and make it stick you need to do it at the command line. Fortunately Apple has added a command in the
systemsetup tool for just that.
To check which mode you’re currently in run this command in Terminal:
To set your machine to boot into 64 bit mode enter this command and reboot:
sudo systemsetup -setkernelbootarchitecture x86_64
To set your machine to boot into 32 bit mode enter this command and reboot:
sudo systemsetup -setkernelbootarchitecture i386
One oddity I’ve found so far is that on some machines that were upgraded from Leopard to Snow Leopard this command doesn’t appear in systemsetup. Do a
man systemsetup before running it to make sure you have the Snow Leopard version of