http://raspberrycompote.blogspot.nl/2012/12/low-level-graphics-on-raspberry-pi-part_9509.html

Basic, get screen info

http://raspberrycompote.blogspot.nl/2013/01/low-level-graphics-on-raspberry-pi-part.html

PArt 2 goes into actually drawing

http://raspberrycompote.blogspot.nl/2013/01/low-level-graphics-on-raspberry-pi-part_22.html

The details of the mapped display memory

and so on

http://raspberrycompote.blogspot.nl/2013/04/low-level-graphics-on-raspberry-pi-part.html

All code here: https://github.com/rst-/raspberry-compote