NES Graphics: how PPU works

[nerososft]

http://www.dustmop.io/blog/2015/04/28/nes-graphics-part-1/

http://www.dustmop.io/blog/2015/04/28/nes-graphics-part-2/

http://www.dustmop.io/blog/2015/04/28/nes-graphics-part-3/