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Int 13h, 16h            Change Line Status                               many

    This service reports whether or not a diskette has been changed
    (disk drive door opened) since the last disk access. This service
    can be performed on diskettes only.

        Entry   AH = 16h
                DL = Drive number (Bit 7 must be clear)

        Return  AH = Change Line Status (See below)
                CF   Set if AH is not 0; otherwise cleared

    ------------------------------------------------------------------

    This service is not available for PCs, PCjrs, and XTs dated before
    1/10/86.

    Use Service 15h to determine if a drive supports change line
    detection.

    If the Carry Flag is set and AH is 01h or 80h, then an error has
    occurred. If AH is 06h, the Carry Flag will be set, but that does
    not signal an error; instead it means the "Diskette change" signal
    is active (as shown below).

                Change Line Status
                AH      Meaning
                00h     "Diskette change" signal not active
                01h     Invalid diskette parameter         (ERROR)
                06h     "Diskette change" signal active
                80h     Drive not ready                    (ERROR)

    For the AT, XT-286, and PC Convertible, the BIOS executes Int 15h,
    Service 90h (Device Busy), for the diskette (Type = 01h) prior to
    waiting for the interrupt. Int 15h, Service 91h (Interrupt
    Complete), is executed upon completion. Diskette operations that
    require the diskette motor to be on will call Int 15h, Service 90
    (Device Busy), with the type equal to "Diskette Drive Motor Start"
    (Type = FDh). This allows the system to perform another task while
    the drive motor is waiting to get up to speed.