Thanks. It's not harder than to code for anything else

Of course to get anywhere with the speed, a lot of parts were written in ARM assembler and heavily customized and optimized.
You'd have to make a lot of compromises and not have compatibility and accuracy as your main goal, except you want a slide show.
For a HW renderer one main problem is, that the DS doesn't have per tile priority. You can make up for that with its 4 layers, but then you loose the ability to scale in hardware (just 2 layers can). Another thing is, that the maximum layer dimensions are 64x64, so you get into trouble if a game uses 128 cell backgrounds.
The maybe biggest problem is, that the DS can "only" address 1024 tiles per layer, so you have to find a way to deal with that. You could use 2 layers to display the tiles with higher indices, but you already need them for the priorities..
Of course there are solutions to most problems, but sometimes they are not really obvious.
If you decide to work on the DS and have any specific questions, feel free to ask.
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