How compatable? If you go by the published API for
MS-DOS 1.x, then it
should be pretty easy, but if by "compatable" you mean "run arbitrary
MS-DOS
programs, especially those from Microsoft" you may have your work cut out
for you. You may want to check out
http://www.freedos.org/.
-spc (But remember, MS-DOS 1.0 is a something of a CP/M for the 8086
world ... )
There are binaries and some cbios listings that might give you a head start.
Dan