Source : Free On-Line Dictionary of Computing

Realtime Disk 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)