The available FDS (Famicom DiskSystem) docs are barely accurate. There are translated docs, a technical reference (almost useless for emulation) and some others with completly unclear details. For the emulators that supports FDS, we have "hacks" or "try-and-error" results, making the games to work somewhat... It's quite difficult to get some accuracy and I'd like to request a brand new FDS document. Yes, we already got something: the sound info in a doc. A very good doc. If you already have this system emulated, I'm sure you could cleanup it a bit and get more games working.
What about the rest of registers? Is there a FDS hardware to get analyzed? Please, post your opinions.