Int 21 Fn 73  - Ms-dos 7 - Drive Locking ???                               [D]

   AH = 73h
   DL = drive (0=current, 1=A:, etc.)
   CL = which flag to get or set
       00h drive flag???
       01h ???
   AL = subfunction
       00h get ???
       01h set ???
       CH = new values for ??? flags
           bit 1: ??? (CL=00h only)
           bit 3: ??? (CL=01h only)

Return: CF clear if successful
       for AL=00h:
       AL = value of CL on entry
       for CL=00h: AH = new flag and 06h (i.e. bits 1 and 2 used)
       for CL=01h: AH = new flag and 08h (i.e. bit 3 used)
       (flag being taken from a table of bytes)
   CF set on error
       AX = error code (01h,0Fh,etc.) (see #1366)
       7300h if function not supported

Note:  these two subfunctions are available even when only the MS-DOS kernel
     is running