Int 2F Fn 4A00  CU - Dos 5+ - Floppy-disk Logical Drive Change Notificati  [D]

   AX = 4A00h
   CX = 0000h
   DH = new drive number
   DL = current drive number

Return: CX = FFFFh to skip "Insert diskette for drive X:" message

Note:  called by MS-DOS 5.0+ IO.SYS just before displaying the message
     "Insert diskette for drive X:" on single-floppy systems