RANGE is a range of BIOS drive numbers to be unmapped. BIOS drive number 0
is for the first floppy, 1 is for the second floppy; 0x80 is for the first
hard drive, 0x81 is for the second hard drive, etc; virtual cdrom (hd32)
corresponds to BIOS drive number 0xA0, (hd33) corresponds to 0xA1, etc.