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systems as possible. Therefore there are also stringent rules as to
what file and directory names are allowed:
In a file or directory name, only the capital letters A through Z, the
numbers 0 through 9 and the underline character may be used. The
file name must contain exactly one dot, which comes between the
actual name and the file extension. The name and the extension
may not be missing at the same time. The maximum length of a file
or directory name may not exceed 31 characters.
Because many operating systems (MS-DOS, Windows 3.x) are not
capable of handling 31 characters, ISO 9660 defines a total of three
levels of compatibility: Levels 1, 2 und 3:
For a CD-ROM as defined by Level 1, the following limitations are
observed:
1. The file name may not be longer than 8 characters.
2. The file extension may not have more than 3 characters.
3. Directory names may consist of a maximum of 8 characters.
A CD-ROM as defined by Level 2 does not have any limitations for
file or directory names, but a file may not be fragmented.
However, this limitation does not exist at Level 3. For example,
Level 3 may be used for incremental backups when only parts of a
large file have been changed. It is not necessary in this case to
completely re-write the entire file. Instead, you can continue to use
the old parts of the file already on the CD which are still valid.
There are expansions for many operating systems which attempt to
store additional information in an ISO directory while not interfering
with other systems. Examples of these are the Rock-Ridge-
Expansions for Unix-Systems and the Apple-ISO Expansions.
Joliet, which is defined by Microsoft, has significance here. It
provides a second, complete directory structure in Unicode and is
currently being used by Windows NT and Windows 95.
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