FAT File Systems - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 186

Table 8-5: The FSInfo structure contains information that can help the
file-system driver find free clusters quickly.
Byte Size (bytes) Description

0 4 FSI_LeadSig signature. Must equal 41615252h.

4 480 Reserved. Set all bytes to 00h.

484 4 FSI_StrucSig signature. Must equal 61417272h.

488 4 The number of the last known free cluster. Set to FFFFFFFh if

unknown.

492 4 The cluster number where the file-system driver should start looking

for free clusters. Set to FFFFFFFh if unknown

496 12 Reserved. Set all bytes to 00h.

508 4 FSI_TrailSig signature. Must equal AA550000h.

Formatting software can select a file system by setting the cluster size so the
number of clusters is in the desired range. Because some existing FAT imple-
mentations don't calculate the number of clusters correctly, Microsoft rec-
ommends formatting all volumes to have at least 16 clusters more than the
specified minimum and 16 clusters less than the specified maximum for the
file system. Also note that a few values are outside the recommended ranges
for any FAT file system.
Table 8-6 shows the cluster sizes Windows XP Professional uses for FAT16
and FAT32 volumes of different capacities. For maximum compatibility,
volumes smaller than 16 MB should use FAT12. Windows can't format vol-
umes greater than 32 GB using FAT32 but can read volumes of this size if
formatted in another operating system.
The FAT32 specification describes a method for calculating the number of
data clusters in a volume. (It's not as straightforward as you might think.)
Embedded systems that format storage media can do the calculations, or
they can just use the same cluster size as Windows for their volume size.

A Hardware Solution
Device firmware typically implements support for FAT file systems. Another
option is to interface to a chip that supports FAT communications. The
uALFAT TM chip from GHI Electronics can access media formatted for the
FAT12, FAT16, and FAT32 file systems. A microcontroller can communi-