Media Structure - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 170

specify the sectors that belong to a partition. The table is in bytes 446
through 509. An entry can begin at byte 446, 462, 478, or 494. Table 7-1
shows the contents of an entry.
Each partition entry has fields that define the partition's starting location
when addressing via the CHS and LBA methods. The LBA field at byte 8
specifies the starting sector of the partition expressed as an offset from the
beginning of the media (the MBR sector). The CHS values are ignored
when using LBA. If a partition is bootable, executable code in the MBR may
use CHS addresses to locate boot code in the partition.
The partition-type field at byte 4 specifies a file system and also indicates
something about the partition's size (Table 7-2). Over the years, Microsoft's
operating systems have expanded their support for file systems and for parti-
tion sizes and addressing methods within the file systems. For example, par-
tition type 04h was added in MS-DOS 3.0 for FAT16 partitions of less than
32 MB. MS-DOS 4.0 added partition type 06h for FAT16 partitions
between 32 MB and 2 GB. Partition types 0Ch and 0Eh must support LBA
to enable PCs to use BIOS interrupt 13h to access the media. If the parti-
tion type is 00h, the entry is unused and the partition doesn't exist.
The final item in a partition-table entry, byte 12, is the total number of sec-
tors in the partition. Most program code ignores this value and instead uses
an equivalent value stored by the file system.
Extended Partitions
Devices with multiple partitions can use extended partitioning, where one
of the partition-table entries is for an additional, extended, partition whose
first sector contains an extended boot record (EBR) structure with its own
partition table (Figure 7-3).
The partition table in the MBR contains information about the media's pri-
mary partition(s). The partition table in an extended partition's EBR can
store at most one entry for a secondary partition and one entry for an addi-
tional extended partition. The additional extended partition, if present, con-
tains its own EBR and partition table. The EBR sectors in extended
partitions contain a partition table and signature but not executable code.
Any device with more than four partitions must use extended partitioning.
Large-capacity FAT16 devices use extended partitioning when the available