[russellsprouts]

I've done a bunch of NES development. You are correct that the framebuffer is not modified directly.

For the background, there are two layers -- the 2 pattern tables are 256 8x8 tile images each using 2 bpp color. The name tables are what is actually displayed. Each tile in the background is an index into one of the pattern tables. (Determined by a global selection bit).

However, in some cartridges, the pattern tables are mapped to RAM. This lets the game simulate drawing into the framebuffer directly. Some games use this for drawing variable width fonts, for example. Battletoads does some cool parallax effects. It turns out that the pattern tables have just enough bits to fill the whole screen with a monochrome image, so you could treat it as a framebuffer (it requires timing changes to midframe, though). I created a demo for that http://forums.nesdev.com/viewtopic.php?f=22&t=14884