kedos is simple OS for raspberry pi 2.

Useful links:

https://github.com/dwelch67/raspberrypi : Many examples and articles.

https://github.com/jncronin/rpi-boot : Simple bootloader for raspberry pi with not bad file system implementation.

https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface : Description of mailbox interface.

http://wiki.osdev.org/Main_Page Must read for hobby's kernel developer.

https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/os/ Good tutorial for raspberry programming with assembler language.