Media Structure - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 168

are in a data cluster. The boot sector can also contain program code used in
booting the computer.The FATs hold a record of the data clusters used by
files.
The document that defines all three FAT file systems is a hardware white
paper titled FAT32 File System Specification from Microsoft. Chapter 8 has
more about FAT file systems.
Drives without an MBR Sector
Some storage devices don't have an MBR sector. Media that requires only a
single volume might not want to waste 512 bytes on an MBR sector. Media
without an MBR sector begins with the volume's boot sector.
For maximum compatibility with hosts, however, a device should include
the MBR sector. A host can find it challenging to determine whether a
device's media contains an MBR. One approach is to read the locations that
would contain a partition table and attempt to determine if the contents
describe a valid partition. The first bytes in the media can also offer a clue.
In bootable FAT16 media without an MBR, the first three bytes are typi-
cally EBh 3Ch 90h. In bootable FAT32 media without an MBR, the first
three bytes are typically EBh 58h 90h. A MultiMediaCard or SD Card
straight from the package is formatted with an MBR sector.
Byte Order
The FAT file systems were developed for use on the x86 architecture in IBM
PCs and their derivatives. The architecture of x86 CPUs is little endian,
which means that multi-byte values are stored with the least significant byte
at the lowest address. For example, in the MBR sector, addresses 510 and
511 must contain the signature AA55h. Because the storage is little endian,
location 510 contains 55h (the least significant byte) and location 511 con-
tains AAh (the most significant byte).
The Master Boot Record Sector
Sector zero, the Master Boot Record (MBR) sector, contains three items: an
area for executable code, a partition table, and a boot signature (Figure 7-2).