 Versions of DOS before 3.2 totally or partially relied on the media descriptor byte in the BPB or the FAT ID byte in cluster 0 of the first FAT in order to determine FAT12 diskette formats even if a BPB is present.Depending on the FAT ID found and the drive type detected they default to use one of the following BPB prototypes instead of using the values actually stored in the BPB.Originally, the FAT ID was meant to be a bit flag with all bits set except for bit 2 cleared to indicate an 80 track (vs. 40 track) format, bit 1 cleared to indicate a 9 sector (vs. 8 sector) format, and bit 0 cleared to indicate a single-sided (vs. double-sided) format, but this scheme was not followed by all OEMs and became obsolete with the introduction of hard disks and high-density formats.