0:504h                   Phantom drive flag

Length: 1 byte

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

                                                                          

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).

See also: INT 11h

0:504h Phantom Drive Flag