How to Run Homebrew Games on Your Nintendo DS
(Obligatory Disclaimer: Needless to say, what you do to your DS is your responsibility and yours alone [though I hardly think an r4 cartridge can damage your DS or anything like that].)
Did you just get a new Nintendo DS and wonder what else you can do with it besides play games, or have you had one for awhile and wonder what kind of other functionality it can have, what other kinds of things you can possibly poke with your stylus? Just want to know what you can make your DS do?
There's actually a lot of things you can make your DS do besides the usual obvious function of playing commercial cartridge games. In fact, with the help of a special flash cartridge (the r4, in my case) you can run a different OS on your DS and turn your little console into a text editor, e-book reader, mp3 player, even video player, among many other things--anything for which a homebrew program has been coded (and there are a lot of them).
If this sounds like something you want to go ahead and do, then let's get started:
The Hardware You're Going to Need
You're going to want a flash cartridge, the one I use and know best is the r4. This is basically a special cartridge that can store and run our alternate OS, and can also store any homebrew programs, games, and files that you want to run on your DS.
The r4's price varies, but it's relatively cheap, especially when you consider all the things it allows you to do with your DS that was far beyond the limits before. It more than pays for itself, I'd say.
Your r4 should come with instructions, but basically the jist of what you do once you get it is this: Fitted into the cartridge is a tiny mini-SD card. You install MoonShell onto this card, pop it in the flash cart, stick that into your DS, and your DS should load MoonShell instead of its usual OS upon start-up.
Even without any other programs yet installed, you'll notice that MoonShell has some really nice functionality as-is. It has a nice e-book reader, so you can read text files on your DS, and even has a video player built-in (though you have to convert the video to a special format first).
At this point, you can save text files and the special video files onto your mini-SD card and run them on your DS through MoonShell. Very exciting.
Using Homebrew Games and Programs
Now that all that's set up, you can go to any site that hosts homebrew programs and ROM's (the kind you can run on emulators) for the Nintendo DS and have some fun. (I'm sure you can find plenty of such sites. Try google.) All sorts of programs exist for the DS made by users.
One of my personal favorites is ComicbookDS, which allows you to read comic books and manga on the Nintendo DS, provided you have scans of the pages.
Another one I recommend is DSOrganize. It has all sorts of personal organizer-type functionalities, but it's terribly useful even for just it's browser alone. Thought it's quite a bit limited still, you can browse the internet with this utility from your DS, if you can pick up a wireless signal with the DS's wifi capability.
Of course, you can also use ROM's from homebrew games and play those, too. And, yes, it's also possible to find ROM's for commercial games, but that's illegal unless you're using the ROM's as a backup. I don't recommend doing that at all, though, since it gives MoonShell a bad name. Stick to buying commercial games and downloading free homebrew ones.
What a DS With an r4 Cartridge Totally Can't Do
You can't run the ROM's of gameboy or gameboy advance games from the r4 cart. It's just basically physically impossible, hardware-wise. Yes, gameboy advance cartridges will work on a DS, but they will run from the gameboy slot, which is associated with a totally different processor than the one that runs the DS games. To run gameboy games from the DS slot would thus require some kind of software emulator, and of course that's just silly and impractical.
So if you want to run homebrew gameboy advance games on the DS, you'll need one of the gameboy advance flash carts.
Otherwise, though, the r4 is invincible.
All right then, I hope I revealed a world of awesomeness to those of you who were previously unfamiliar with the whole idea of running MoonShell on your DS. Have fun.