is it possible to convert .fds to .nes?
It's been done a few times (SMB2j pirate comes to mind), but often involves hacks or even lossy methods. Some FDS routines don't really work without FDS hardware (the FDS sound comes to mind for games that use it, but I'm sure there's other routines that are FDS specific).
-Rob
No, it is not possible to "convert .fds to .nes". Any FDS games which you may have seen as .NES files were heavily modified to use a different mapper (instead of relying on switching disks).
Is there an existing NES mapper that can be made to use VRAM and to bankswitch 32 KB of WRAM into CPU $6000-$DFFF but can also bankswitch ROM into that same place using mapper writes? MMC5 looks close, but can it use VRAM? If so, then it might be possible to make a clone of the FDS BIOS that copies files from "disks" in ROM to RAM. The interrupt, on the other hand, might need to be manually hacked around.