(I wrote this post quite a long time ago, during the port of Waimanu Daring Slides to GBA. It has been sitting here for a while, sorry for the delay. You can find the result of my pretty hard work here, on Disjointed Studio blog)
When Nintendo were planning the DS, they decided that the new console would be compatible with the previous one, the Game Boy Advance (GBA for short). So they have put in the same processor - an ARM7TDMI, which is the only processor in the GBA and the 'secondary' processor in the DS when running DS native code. They have also put in an evolution of the same 2D core, giving it many new powerful features. So the GBA, from the point of view of a DS homebrewer, is not completely different, but there are lots of differences that you should keep in mind if you decide to adventure yourself into GBA homebrew land.
So here's an overview of the GBA 2D core features comparing them to the DS... of course, without mentioning things you may easily notice such as that the GBA has got one screen only with a resolution of 240x160 pixels, whereas DS has two 256x192 pixels screens and 2 separate 2D cores. Of course, the GBA has no 3D core at all.
Please note that the list isn't comprehensive, and I'm describing differences pertaining to the graphical 2D core only.
- The DS supports up to 4 backgrounds at the same time. Two of them can only be 'normal' backgrounds (no rotation and scaling is supported on these backgrounds), but you can choose how you want the other two backgrounds to be. So your options are to have all 4 normal backgrounds or you can have 3 normal backgrounds and 1 that supports rotation and scaling (known as 'rotscale' or 'affine' background) or even have 2 normal and 2 rotscale backgrounds. With GBA, you can have 4 normal backgrounds too, but if you need rotscale backgrounds, you have to give up two normal backgrounds for each rotscale background you want to use. So you'll eventually have 2 normal backgrounds and just one rotscale background or 2 rotscale backgrounds with no other backgrounds at all.
- The DS also features 'extended' rotscale backgrounds, which are rotscale backgrounds supporting up to 1024 different tiles, and each tile can be eventually flipped horizontally/vertically and/or use one of 16 separate 16-color or 256-color palettes. On the GBA there's no such 'extended' rotscale thing, and 'regular' rotscale backgrounds are 256 colors backgrounds that supports up to 256 different tiles only, with no flipping and of course no palette selection.
- Extended rotscale backgrounds on DS can also become bitmap backgrounds, making it possible to have bitmaps over (or under!) text/rotscale backgrounds, or even a bitmap over another. On the other hand, the GBA has very few bitmap oriented features. You can have only a single bitmap background, even if you have 3 choices of what to show in it. You can show a 240x160 15bpp (32 thousand colors) bitmap with a single framebuffer since there's not enough VRAM to have two of them - such a bitmap requires 75 KB. The second choice is a 240x160 256 colors bitmap with double framebuffer, and the last choice is quite a bizarre 160x128 15bpp double framebuffer bitmap background.
- The DS main 2D core (but not the sub core) has a 'large bitmap mode', featuring a single 1024x512 bitmap background. Of course, on the GBA that doesn't exists.
- Palettes: the DS has 16 256-color additional palettes (known as 'extended' palettes) for backgrounds plus another 16 for sprites, besides the regular 256 colors palette for the backgrounds and the regular one for the sprites. Both of these regular palettes also be used as if they were 16 separate 16-color palettes, for 16-color tiles and sprites. The GBA features the very same regular palettes, but there are no 'extended' palettes.
- The GBA has only a total of 96 KB of video RAM, of which 64 KB are dedicated to background maps and tiles, and 32 KB dedicated to sprites. This means, for example, that only 512 different 256-color tiles for sprites can be stored here, even if the GBA 2D core could use up to 1024 different tiles. Also, when choosing a bitmap mode, only 16 KB are left for sprites as the first 80 KB of VRAM are bound to the background bitmap framebuffer(s).
- On the GBA the sprites will always overlap with each other according to their order in the OAM (Object Attribute Memory). This means that sprite number 0 will be always 'on top' of sprite number 1, even if the latter is bearing higher priority than the former. On the contrary, on the DS the priority also works sprite-on-sprite, not simply sprite-on-background.
- Bitmap Objects (also known as 15bpp sprites) don't exist on GBA.
All that said, please don't let this scare you. It's really a lot of fun to code on that little neat machine, and it will surely give you lots of satisfaction.