Directories - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 201

9
Directories
This chapter explains how directories store information and shows how to
access and store information in directories and subdirectories. The code in
this chapter uses the FILE and DISK structures from Chapter 8.
The Contents of an Entry
The root directory contains a 32-byte entry for each file in the root directory
and each subdirectory directly under the root directory. These entries are
sometimes called DOS 8.3 entries or just 8.3 entries because each can store a
file name no longer than eight characters before the dot and three characters
after the dot (for example, MYFILE01.TXT). The limitation dates to the
MS-DOS operating system.
If the file-system driver supports long file names, any name that doesn't fit in
an 8.3 entry is stored in one or more additional 32-byte entries that precede
the 8.3 entry, and the 8.3 entry stores a short version of the file name. An
8.3 entry uses upper-case text only, so another use for long-file-name entries
is to support lower-case text.