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 = new state
           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); DOS 2.x has an AVAILDEV= option in CONFIG.SYS to make \DEV
     mandatory
   although MS-DOS 3.3+, DR DOS 3.41+, and Novell DOS 7 accept these
     calls, they have no effect, and AL=02h always returns DL=FFh (except
     for Novell DOS 7, which leaves AX unchanged for both subfunctions)