To make boot loader options permanent, edit /etc/boot.conf. The boot loader reads and runs entries from this file before giving you the
boot> prompt, which means you can use it to automatically run boot loader commands every time your computer boots. (Although if you would rather sit at your computer and enter your settings every time you reboot, don’t let me stop you.)
Any command you might give at the loader prompt is a valid boot.conf entry. For example, if the default boot speed is too slow for your liking, you can set your boot timeout to two seconds by adding this line to boot.conf:
set timeout 2
You can also tell the system to boot a different kernel with the correct boot.conf command.
set image /bsd.mp ...