[AT]
Expects: AH    16H
DL    diskette drive number: 00H thru 03H
                                                                  
Returns: AH    00H = diskette change line not active
01H = invalid drive number
06H = either change line is not supported or
disk change line is active (media was swapped)
80H = diskette drive not ready or not installed
else= BIOS disk error code if CF is set to CY
                                                                  
Info: This determines whether the door of a drive has been opened since
the last time the change-line was cleared (it is cleared by all
successful disk accesses).

When change-line is active, it does not necessarily mean a new
diskette is in place -- only that the drive door has been opened
at some point.

Notes:   Use INT 13H 15H to see if a drive supports change-line
detection.

- -

INT 13H 16H: Detect Media Change