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=3702h

Text based on Ralf Brown Interrupt List Release 61