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  In the PC/AT and PS/2 ROM BIOS, service 16H (decimal 22) reports whether
  the diskette in the drive specified in DL was changed. The status is
  reported in AH (Figure 10-5).

  Remember several important points about service 16H. First, before you use
  this ROM BIOS service, call service 15H to ensure that the diskette-drive
  hardware can sense when a diskette is changed. Also, you should follow a
  call to service 16H with a call to service 17H (Set Diskette Type)
  whenever you detect a diskette change.

  Keep in mind that the hardware can only detect whether the diskette-drive
  door was opened; it cannot tell whether a different physical diskette was
  placed in the drive. You must still read data from the diskette to
  determine whether a different diskette is actually in the drive. Data such
  as a volume label, the root directory, or a file allocation table can help
  to uniquely identify a diskette.

  Value                    Meaning
  --------------------------------------------------------------------------
  AH = 00H                 No diskette change.
  AH = 01H                 Service called with invalid parameter.
  AH = 06H                 Diskette has been changed.
  AH = 80H                 Diskette drive not ready.
  --------------------------------------------------------------------------

  Figure 10-5.  Status values returned in AH by diskette service 16H.