0050h:0004h           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).