RDOS

from The Free On-line Dictionary of Computing (8 July 2008)

Realtime Disk Operating System
RDOS

   <operating system> A {Data General} {operating system}
   developed in the 1970s or 1980s.  When used in conjuction with
   a {BASIC} (e.g. {Business Basic}) it could support 16
   concurrent users at the {record locking} level and two
   printers all on 128K memory.

   Reputedly {IBM} wanted to license this for the first {IBM PC}
   but DG turned them down so they went to {Microsoft} instead.
   How different the world could have been.

   [Before or after IBM wanted {CP/M}?]

   (1997-11-23)