Hardware Reference
In-Depth Information
not take long before some drive manufacturers allowed their drives to go beyond
the official limit and come perilously close to the physical edge of the medium,
giving about 700 MB instead of 650 MB. But as the technology improved and the
blank discs were manufactured to a higher standard, 703.12 MB (360,000
2048-byte sectors instead of 333,000 sectors) became the new norm.
Note that even a 32x CD-ROM drive (4,915,200 bytes/sec) is no match for a
fast SCSI-2 magnetic-disk drive at 10 MB/sec. When you realize that the seek
time is often several hundred milliseconds, it should be clear that CD-ROM drives
are not at all in the same performance category as magnetic-disk drives, despite
their large capacity.
In 1986, Philips struck again with the
Green Book
, adding graphics and the
ability to interleave audio, video, and data in the same sector, a feature essential for
multimedia CD-ROMs.
The last piece of the CD-ROM puzzle is the file system. To make it possible to
use the same CD-ROM on different computers, agreement was needed on CD-
ROM file systems. To get this agreement, representatives of many computer com-
panies met at Lake Tahoe in the High Sierras on the California-Nevada boundary
and devised a file system that they called
High Sierra
. It later evolved into an In-
ternational Standard (IS 9660). It has three levels. Level 1 uses file names of up to
8 characters optionally followed by an extension of up to 3 characters (the
MS-DOS
file naming convention). File names may contain only uppercase letters, digits,
and the underscore. Directories may be nested up to eight deep, but directory
names may not contain extensions. Level 1 requires all files to be contiguous,
which is not a problem on a medium written only once. Any CD-ROM confor-
mant to IS 9660 level 1 can be read using
MS-DOS
, an Apple computer, a
UNIX
computer, or just about any other computer. CD-ROM publishers regard this prop-
erty as a big plus.
IS 9660 level 2 allows names up to 32 characters, and level 3 allows noncon-
tiguous files. The Rock Ridge extensions (whimsically named after the town in the
Mel Brooks film
Blazing Saddles
) allow very long names (for UNIX), UIDs, GIDs,
and symbolic links, but CD-ROMs not conforming to level 1 will not be readable
on old computers.
Initially, the equipment needed to produce a master CD-ROM (or audio CD,
for that matter) was extremely expensive. But in the computer industry nothing
stays expensive for long. By the mid 1990s, CD recorders no bigger than a CD
player were a common peripheral available in most computer stores. These de-
vices were still different from magnetic disks because once written, CD-ROMs
could not be erased. Nevertheless, they quickly found a niche as a backup medium
for large magnetic hard disks and also allowed individuals or startup companies to
