Int 21 Fn 37  - DOS 2.x And 3.3+ Only - "availdev" - Specify \dev\ Prefix  [D]

   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 MS-DOS 3.3+ and DR-DOS 3.41+ accept these calls, they have no
     effect, and AL=02h always returns DL=FFh