That's Andrew Tanenbaum's
Operating Systems Design and Implementation.
Now, yes, that shows you how to write a Unix-like microkernel OS; just skip everything except the file system chapter. And don't follow the advice about tree data structures. Just use flat tables, and don't bother to implement exact file sizes. Presto: CP/M.
(I prefer the 2nd edition. The 3rd edition needlessly complicates IMO, mostly so the demo Minix code will work on a late 1990s PC instead of a 1980s PC.)
Now, yes, that shows you how to write a Unix-like microkernel OS; just skip everything except the file system chapter. And don't follow the advice about tree data structures. Just use flat tables, and don't bother to implement exact file sizes. Presto: CP/M.
(I prefer the 2nd edition. The 3rd edition needlessly complicates IMO, mostly so the demo Minix code will work on a late 1990s PC instead of a 1980s PC.)