There's an old an OS-free/BIOS-based implementation of FAT12 called MDCFS.C (Minimal Dos Compatible File System) in Dave Dunfield's EMBEDPC.ZIP (only in the Internet Archive now). You'll also need peek at DK86IO.C from that zip as well as come up with an implementation for those BIOS calls. Dave's been active on vogons lately.
https://web.archive.org/web/20210505023207/http://dunfield.c...