What relation to Graphic Modes have with Background Modes

[soulcollector]

So the GBA has 6 Graphics Modes and 4 Background Modes, fair enough. I was going through the Tonc documentation, what I don't understand is when we're using bitmap mode, say 3, we need to enable background mode 2 specifically? Aren't background modes for tiled modes?

 

[Coto]

You have 4 background registers, and depending on each render mode (Text, Affine, Text+Affine, Bitmap) there is a number of backgrounds available to them. Also each background features a priority bit, so if you want to use a single background, say 3, then the 3rd background should get the highest priority, mapped to a base address or tile address. (set up through the background controller), so the CPU can see and read/write pixels to that address, given a specified object format (either tile mode or direct RGB15 pixel format if Bitmap mode).

How to set up the screen:

- Set VRAM Bank you want to use (A,B,C or D)
- Tell the Display Control which background mode you'll use
- Set Background Rotation/Scaling bits (these are a two dimensional matrix transforming the output pixel ratio to screen buffer), only required if you're using Background 2 or 3
- Set the specific target Background controller, this tells the background how pixels should be processed (if tiles conform a set of pixels, if pixels are made independently (bitmap) , background size, etc)

These bits are for NintendoDS, but bits shouldn't change that much from GBA mode since the hardware was extended.

GBA Docs