Int 21 Fn F400  - Doubledos - Installation Check/program Status            [T]

   AX = F400h

Return: AL = program status
       00h if DoubleDOS not present
       01h if running in visible DoubleDOS partition
       02h if running in the invisible DoubleDOS partition

See Also: AX=E400h,AH=F5h"DoubleDOS"