Wikipedia
DR-DOS
DR-DOS (DR DOS, without hyphen up to and including v6.0) is an operating system of the DOS family, written for IBM PC- compatible personal computers. It was originally developed by Gary Kildall's Digital Research and derived from Concurrent PC DOS 6.0, which was an advanced successor of CP/M-86. As ownership changed, various later versions were produced with names including Novell DOS and Caldera OpenDOS.