RPI safe mode

This results in config.txt being ignored (except for avoid_safe_mode). A default cmdline.txt is applied, and kernel_emergency.img is loaded (if not available, then just kernel.img).

It provides a means of recovering from, e.g. without requiring a PC. (Obviously it won't help if you've deleted start.elf...)
 * too high an overclock
 * a typo in cmdline.txt
 * an incorrect kernel= parameter
 * an invalid display mode

The busybox based kernel_emergency.img has /boot mounted, and vi installed so you can fix up config.txt or cmdline.txt. (You can also mount /dev/mmcblk0p2 and fix up the rootfs if you have problems there that stop you booting).

Short Jumpers 5&6. Be very careful with where you insert the jumper. Some pins carry power, so shorting those would not be a good idea. Third row from the end.

More info: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=29&t=12007