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File Systems: FAT
9-1-1 FAT12 / FAT16 / FAT32
The earliest version of FAT, the file system integrated into Microsoft’s DOS, was FAT12,
so-called because each entry in the File Allocation Table was 12-bits. This limited disk size
to approximately 32-MB. Extensions to 16- and 32-bit entries (FAT16 and FAT32) expand
support to 2-GB and 8-TB, respectively. As described in Appendix E, “Fat Configuration” on
page 532, support for FAT12, FAT16 and FAT32 can be individually disabled, if desired.
FAT32 introduces several new innovations above its predecessors. The root directory in the
earlier systems was a fixed size; i.e., when the medium is formatted, the maximum number
of files that could be created in the root directory (typically 512) is set. In FAT32, the root
directory is dynamically resizable, like all other directories. Two special sectors are also
included: the FS info sector and the backup boot sector. The former stores information
convenient to the operation of the host, such as the last used cluster. The latter is a copy of
the first disk sector (the boot sector), in case the original is corrupted.
9-1-2 SHORT AND LONG FILE NAMES
In the original version of FAT, files could only carry short “8 dot 3” names, with eight or
fewer characters in the main name and three or fewer in its extension. The valid characters
in these names are letters, digits, characters with values greater than 127 and the following:
$ % ‘ - _ @ ~ ` ! ( ) { } ^ # &
In μC/FS, the name passed by the application is always verified, both for invalid length
and invalid characters. If valid, the name is converted to upper case for storage in the
directory entry.
Eventually, in a backwards-compatible extension, Microsoft introduced long file names
(LFNs). LFNs are limited to 255 characters, stored as 16-bit Unicode in long directory entries.
Each name is stored with a short file name composed by attaching a numeric “tail” to the
original; this results in names like “file~1.txt”. In addition to the characters allowed in SFNs,
the following are allowed in LFNs:
+ , ; = [ ]
111
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