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contain executable code. A computer that boots from the storage device
runs the executable code on boot up. In Windows systems, the Fdisk tool
can perform the low-level formatting that stores the MBR structure in the
first sector. Fdisk deletes all programs and data previously stored in the
media.
Each volume begins with a boot sector that contains information specific to
the volume's file system. (Don't confuse this boot sector with the mas-
ter-boot-record sector.) File-system drivers in embedded systems often use a
FAT file system. The term FAT refers to a family of file systems as well as the
file allocation tables that every FAT system contains. The two most com-
mon FAT file systems are FAT16 and FAT32. A third FAT system, FAT12,
is suitable only for smaller-capacity media and is uncommon these days even
in embedded systems. (Floppy drives use FAT12.)
Every FAT volume contains the following elements: a reserved region that
includes a boot sector, a FAT region that stores the FATs, and a file and
directory data region that contains data clusters for storing files and directo-
ries. FAT12 and FAT16 volumes store the root directory in a dedicated
root-directory region that precedes the data clusters, while FAT32 volumes
store the root directory in any available data clusters.
A word about licensing: due to patent protections, manufacturers of some
types of devices that implement FAT file systems may need to obtain a
license from Microsoft. The devices include removable solid-state media and
some consumer electronics devices. Microsoft provides source code and test
specifications
to
licensees.
For
more
information,
see
Microsoft's NTFS is an alternative to FAT file systems for PCs. Because
Microsoft hasn't publicly released a specification for NTFS, the file system
isn't practical for use in embedded systems that don't use an operating sys-
tem with NTFS support.
FAT file systems store files and directories in data clusters. Each cluster con-
sists of one or more sectors. All clusters in a volume are the same size. Data
clusters can provide larger-capacity units for program code to work with
when storing and retrieving files.
The boot sector specifies what sectors are available for storing files and direc-
tories, what sectors contain the file allocation tables, and how many sectors
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