Int 21H, AH=37H, AL=03H

2.x and 3.3+ only

set availdev flag

      AH = 37h
      AL = 03h set availdev flag
              DL = new state
                  00h         \DEV\ is mandatory
                  nonzero     \DEV\ is optional

AL = status

          00h successful
          FFh unsupported subfunction

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)

AX=3701h

Text based on Ralf Brown Interrupt List Release 61