0:504h                   Phantom drive flag

 Length: 1 byte

    Reports the status of a single-drive system when mimicking a two-drive
    system.

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    Examination of this byte will reveal the current status of the single
    drive installed on a one drive system. By using this byte, a
    programmer can avoid the message 'Insert disk for drive B:' when
    working with a single-drive system.

                  Value              Meaning
                   00h           Acting as drive A
                   01h           Acting as drive B
                   FFh           Acting as drive A

    Although the last value, FFh, is undocumented, FFh means the drive
    letter has not been determined yet, so using the drive as drive A will
    work correctly.

    To determine if only one drive is installed, use the equipment list
    (see INT 11h).