Optical Storage - Upgrading and Repairing PCs - page 728

arator). 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 as lowercase and many other char-
actersnotallowedinISO9660.Tomaintain backwardcompatibility withDOS,Windows
95 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 command prompt. To cre-
ate 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 files are recorded to the disc, meaning any long fi-
lenames 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
firstdatatrackofeachsession.Thisdataidentifiesthelocationofthevolumearea—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 11.17 . 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.
Figure 11.17 A diagram of basic ISO 9660 file organizational format.