DR DOS is an MS-DOS compatible operating system from Digital Research that evolved from their earlier CP/M-86 based products Concurrent DOS and DOS Plus.
Released shortly after MS-DOS 5.0, DR DOS added SuperStor drive compression, and TASKMAX task switcher, once again one-upping Microsoft.
Microsoft responded by placing the infamous "AARD" code in a December 1991 beta of Microsoft Windows 3.1, giving people the impression that DR-DOS might not be compatible with Windows.