INT 21h, function 37h could be used to get and set the switch character for MS-DOS, in addition to requiring the use of '/DEV/' for device filenames:

    * AL=0, return switch character in DL
    * AL=1, set switch character to character in DL
    * AL=2, return '/DEV/' setting in DL
    * AL=3, DL=0 if '/DEV/' required, DL<>0 if not.

Okay, why do I remember this stuff? I haven't programmed MS-DOS for over 25 years!