INT 21 - DOS 2.x and 3.3+ only - "AVAILDEV" - SPECIFY \DEV\ PREFIX USE
        AH = 37h
        AL = subfunction
            02h get availdev flag
                Return: DL = 00h \DEV\ must precede character device names
                           = nonzero \DEV\ is optional
            03h set availdev flag
                DL = 00h        \DEV\ is mandatory
                   = nonzero    \DEV\ is optional
Return: AL = status
            00h successful
            FFh unsupported subfunction
Notes:  all versions of DOS from 2.00 allow \DEV\ to be prepended to device
          names without generating an error even if the directory \DEV does
          not actually exist (other paths generate an error if they do not
          exist).
        although DOS 3.3+ accepts these calls, they have no effect, and
          AL=02h always returns DL=FFh