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Physical Driver Number. This is related to the BIOS physical drive number. Floppy drives are numbered starting with 0x00 for the A: drive, while physical hard disks are numbered starting with 0x80. Typically, you would set this value prior to issuing an INT 13 BIOS call in order to specify the device to access. The on-disk value stored in this field is typically 0x00 for floppies and 0x80 for hard disks, regardless of how many physical disk drives exist, because the value is only relevant if the device is a boot device.
Computer Boot Sequence - The Mossywell Web Site
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