Troubleshooting - Raspberry Pi User Guide - page 46

he Linux kernel is a single ile located in the /boot directory called kernel.img . When
the Pi is irst switched on and begins to load the operating system, it looks for this ile, and if
the ile is missing, the Pi won't work. he emergency kernel is a second ile, again in the /
boot directory, called kernel_emergency.img .
he emergency kernel is, in most cases, almost identical to the standard kernel. When
changes are made to the standard kernel, to boost performance or add new features for
example, the emergency kernel is left unaltered. his way, if the changes to the standard
kernel cause stability problems, a user can simply tell the Pi to load the emergency kernel
instead.
here are two ways to boot into the emergency kernel, and both require the use of a PC and
an SD card reader if the Pi can't boot. Otherwise, the following can be carried out on the Pi
itself.
he easiest way to boot the emergency kernel is to rename the existing kernel.img ile to
kernel.img.bak , and then rename the kernel_emergency.img ile to kernel.img .
When the Pi loads, it will now load the emergency kernel by default. To go back to the stan-
dard kernel, simply reverse the process: rename kernel.img to kernel_emergency.img
and kernel.img.bak to kernel.img .
An alternative method to load the emergency kernel is to edit the cmdline.xt ile (located
in the /boot directory) by adding the following entry at the end of the existing command
line:
kernel=kernel_emergency.img
his tells the Pi that it should load the kernel named kernel_emergency.img instead of
the usual kernel.img . Reversing the process is as simple as opening cmdline.txt again
and removing the entry.
You'll learn more about cmdline.txt and how it afects the operation of the Raspberry Pi
in Chapter 6, “Coniguring the Raspberry Pi”.