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.
- too high an overclock
- a typo in cmdline.txt
- an incorrect kernel=<filename> parameter
- an invalid display mode
without requiring a PC. (Obviously it won't help if you've deleted start.elf...)
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.