Directories - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 207

tition's boot sector. The volume-label entry doesn't display in the root direc-
tory's directory listing.
Subdirectory Entries
A directory entry with the directory attribute equal to 1 and the volume
attribute equal to zero defines a subdirectory under the directory. A subdi-
rectory can use any available data clusters.
A small embedded system might support only the root directory, ignoring
any subdirectory entries in the root directory and any files stored in subdi-
rectories. Firmware that doesn't support subdirectories can avoid overwriting
any inaccessible files and directories created by another host because the
FATs identify the clusters as in use.
Handling Long File Names
In a file system that supports long file names, a file or directory name can be
as long as 255 characters including one or more dots and extensions. A file's
complete path has a maximum of 260 characters, however, so volumes with
many levels of directories must use shorter names.
Each entry for an item with a long file name (LFN) has an 8.3 entry pre-
ceded by one or more 32-byte LFN entries. Systems that don't support long
file names use the 8.3 entry and ignore the LFN entries. Under Windows,
typing dir /x at a command prompt shows both the short and long file
names.
A small embedded system might choose to support 8.3 file names only.
Because every file with a long file name also has an 8.3 file name, a system
can access any file using 8.3 file names. If a system that supports only 8.3
file names renames a file that had a long file name, the checksums in the
LFN entries will almost always be invalid so the 8.3 file name will be the
only valid name.
LFN Entries
Table 9-3 shows the fields in an LFN entry. Each entry stores up to 13 Uni-
code characters. Each character code is two bytes.