A weird thing about Xenix is that it used the Microsoft C compiler, including COFF format object files. I didn't try it, but I'm pretty sure you could cross compile to MS-DOS with it.

Anonther thing I remember is that floppy driver was terrible- the interleave was incorrect for it, so pretty much one block per disk rotation. People forget, but a big reason for early Linux success was addressing issues like this.

Oh, and the filesystem used a linked-list for the free blocks. Basically you had instant fragmentation.