Manual booting between OSes

It is possible to boot directly into any of your installed OSes (from any other OS) using the following techniques:

1. sudo reboot <n> (where n is the boot partition number of the OS to boot)
2. Use a sysfs parameter, e.g.

sudo su -c 'echo <n> > /sys/module/bcm2708/parameters/reboot_part'
sudo reboot

(Replace <n> with the partition number. You may need to change the bcm2708 to bcm2709 for the Pi2 and bcm2710 for the pi3)

PINN uses the following partition layout:
1 - PINN recovery partition
2 - <Extended partition> containing logical partitions 5,6 etc...
3 - reserved
4 - reserved
5 - settings partition for PINN
6 - 1st OS boot partition
7 - ...

So partition 6 is the boot partition of the 1st installed OS. To find a list of installed OSes and which partitions they use, mount the settings partition (5) and examine installed_os.json. The first partition will be the boot partition for each OS.