adam_smasher wrote:
Ehhhhh, you're giving gamedevs too much credit here. It's entirely believable that in many cases they looked at the size of the mono GB market vs. the GBC-only market and decided that it wasn't worth any extra effort (or ROM space) to build an engine that works for both - regardless of whether or not it would be possible on the former or if it'd take a lot of work to trim it down. It'd definitely be case-by-case but I can imagine that there are at least a few GBC-only games that could be ported without too much trouble.
In my experience poking and prodding various Game Boy games to figure out how they work, it seems a majority of "black" carts (dual DMG/GBC compatibility) did not use any of the new features of the GBC besides colorized palettes. They don't use the new VRAM DMAs and don't use the expanded VRAM or WRAM. At most, they do use the GBC's double-speed mode, since it's a nice freebie. So in this regard, I think nitro2k1's statement is accurate: most games that are easy to backport to the DMG are already DMG compatible. A lot of GBC exclusive games are heavily dependent on the new DMAs in conjunction with 2x CPU speed. From what I've analyzed over the years, stripping that combo out severely limits some of the things GBC-exclusives were capable of.
However, I do agree that it's a case-by-case basis. Most GBC-only games extensively rely on the GBC-only stuff, but I can think of a handful that don't really fully utilize them. For example, I'm pretty sure you could backport Pokemon Crystal to the DMG without sacrificing much. Underneath the hood, it's Pokemon Gold/Silver, which worked just fine on the DMG. Seems like most of the GBC's features were used to spice up the graphics, and all of that can be undone. Probably Pokemon Puzzle Challenge too; since it's just a puzzle game graphics aren't anything the DMG couldn't handle, you'd have to rewrite the engine's WRAM usage, probably.