Media Structure - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 169

Figure 7-2: The MBR sector contains a partition table that specifies the
location(s) of one or more primary partitions.
Executable Code
Bytes zero through 445 can contain executable code. When a PC boots, the
system BIOS jumps to the executable code in the MBR sector of a storage
device. The code searches the partition table for an active, or bootable, par-
tition, and on finding one, boots the computer by running code stored in
that partition's first sector. Like any low-level program code, the code stored
in the MBR is specific to a CPU family. Executable code for a PC is useless
if the system's CPU is a microcontroller with a different instruction set.
Embedded systems typically boot from a specific location in dedicated pro-
gram memory rather than from storage media such as a flash-memory card
or hard drive. An embedded-system host can ignore any executable code in
the MBR sector. Because the partition table is in the same location in the
MBR sector in all devices, firmware can read the information directly from
the table.
The Partition Table
The partition table enables defining one or more partitions, or logical vol-
umes, in the storage media. Many devices have just one volume. The parti-
tion table in the MBR sector has room for four 16-byte entries that each