Daigoro said:
DLDI is a system that was developed to allow standardized read/write access across all different flash carts. It is sort of like a driver, but that must be patched into the homebrew roms before they can be executed. There is a tool you can use for that (there is even one which allows you to DLDI patch NDS files by right clicking in windows explorer), and some cards (like the R4) automatically patch the roms when you load them.
Why is it needed?
Well, commercial games use standard Nintendo libraries for accessing data on original DS game cards. Since each flash cart brand uses different circuitry, they all require some sort of patching on the roms (either manual or automatic, depends on the card) to replace the file access routines by the ones which will work on the cart in question.
Homebrew roms aren't made using Nintendo's libraries, so flash carts can't replace the file access routines on them. In the early homebrew days, developers had to add code specifically for each flash cart, and they depended on the manufacturers releasing libraries for accessing their carts or reverse engineering. But whenever a new cart was released, the homebrew roms wouldn't work on it unless the authors updated the code to work with the new card.
DLDI came to fix those issues. A standard driver-like system was created, which reserves a small space on the ROM for the DLDI driver which can be rewritten by a patching tool. That way developers only need to make a single version of their homebrew that will work on any cart which has an available DLDI driver. The homebrew also becomes "future-proof" and will work on new carts, as soon as they have DLDI drivers available. And flash cart manufacturers who wish to support homebrew can do so without disclosing their source codes: they can just make a DLDI driver themselves and make it available for download (or in some cases, make the patching process automatic, like the R4 does).
