CHAPTER 13

716 Chapter 13 Optical Storage
Although based on High Sierra, ISO 9660 does have some differences and refinements. But any drivers
that can read ISO 9660 also can read discs formatted in High Sierra. ISO 9660 has three levels of interchange
that dictate the features that can be used to ensure compatibility with different systems.
ISO 9660 Level 1 is the lowest common denominator of all CD file systems and is capable of being
read by almost every computer platform, including Unix and Macintosh. The downside of this file
system is that it is very limited with respect to filenames and directories. Level 1 interchange restrictions
include
■ Only uppercase characters A–Z, numbers 0–9, and the underscore (_) are allowed in filenames.
■ 8.3 characters maximum for the name.extension (based on DOS limits).
■ Directory names are eight characters maximum (no extension allowed).
■ Directories are limited to eight levels deep.
■ Files must be contiguous.
Level 2 interchange rules have the same limitations as Level 1, except that the filename and extension
can be up to 30 characters long (both added together, not including the . separator). Finally, Level 3
interchange rules are the same as Level 2 except that files don’t have to be contiguous.
Note that Windows 95 and later versions enable you to use file and folder names up to 255 characters
long, which can include spaces as well lowercase and many other characters not allowed in ISO 9660.
To maintain backward compatiblity with DOS, Win95 and later associate a short 8.3 format filename
as an alias for each file that has a longer name. These alias short names are created automatically by
Windows and can be viewed in the Properties for each file or by using the DIR command at a commmand
prompt. To create these alias names, Windows truncates the name to six (or fewer) characters
followed by a tilde (~) and a number starting with 1 and truncates the extension to three characters.
Other numbers are used in the first part if other files that would have the same alias when truncated
already exist. For example, the filename This is a.test gets THISIS~1.TES as an alias.
This filename alias creation is independent of your CD drive, but it is important to know that if you
create or write to a CD using the ISO 9660 format using Level 1 restrictions, the alias short names are
used when recording files to the disc, meaning any long filenames will be lost in the process. In fact,
even the alias short name will be modified because ISO 9660 Level 1 restrictions don’t allow a tilde—
that character is converted to an underscore in the names written to the CD.
The ISO 9660 data starts at 2 seconds and 16 sectors into the disc, which is also known as logical sector
16 of track one. For a multisession disc, the ISO 9660 data is present in the first data track of each session
containing CD-ROM tracks. This data identifies the location of the volume area—where the
actual data is stored. The system area also lists the directories in this volume as the volume table of
contents (VTOC), with pointers or addresses to various named areas, as illustrated in Figure 13.6. A
significant difference between the CD directory structure and that of a normal hard disk is that the
CD’s system area also contains direct addresses of the files within the subdirectories, allowing the CD
to seek specific sector locations on the spiral data track. Because the CD data is all on one long spiral
track, when speaking of tracks in the context of a CD, we’re actually talking about sectors or segments
of data along that spiral.
To put the ISO 9660 format in perspective, the disc layout is roughly analogous to that of a floppy
disk. A floppy disk has a system track that not only identifies itself as a floppy disk and reveals its
density and operating system, but also tells the computer how it’s organized—into directories, which
are made up of files.