Filenames in ISO-9660 include an uppercase, fixed-size base name, a delimiter (a period) to separate filenames from the extension, and a three-letter extension name (also uppercase). Directory names contain maximum 8 characters and do not have extensions.
With filenames the extension may be followed another delimiter (a semicolon) and a revision number of the file.
The choice of filename is thus restricted to allow for the vast number of different systems that existed at the time the standard was determined. While the directory entries allow much larger names than this, the characteristics and size of the filename were developed to achieve level-one compliance with original High Sierra format.
But many systems with ISO 9660 capability are not compatible with the naming conventions: on a UNIX system, a semicolon is used as a command delimiter in the shell interpreter.
To by-pass there problems the Rock Ridge Interchange Protocol (RRIP) was designed to allow users of POSIX and other UNIX like systems to remain much of the directory information that is in the native file system.
There systems use directory entries for much more than just pointing to files.
- can point to other entries (symbolic links) or to device drivers that are linked to peripheral devices such as hard disks,
- can tape drives and CD-ROM drives (device files),
- includes information that lets the system know what type of file it is dealing with, whether it is a regular file, directory, symbolic link, or device file,
- has information regarding who has permission to read, write and execute each file.
Most of these systems are multi-users systems, and you would not want just anyone to be able to write to the device file that contains your operating system, because they could accidentally erase the entire operating system. And permission may not set to tight, because it can make CD-ROM unusable for users when they have no read access to files.