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6.3 / OPTICAL MEMORY 207

MIN SEC Sector Mode ECC
00 FF ... FF 00 Data Layered

12 bytes 4 bytes 2048 bytes 288 bytes
SYNC ID Data L-ECC

2352 bytes

Figure 6.11 CD-ROM Block Format

the center and spiraling out to the outer edge of the disk. Sectors near the outside of
the disk are the same length as those near the inside. Thus, information is packed
evenly across the disk in segments of the same size and these are scanned at the
same rate by rotating the disk at a variable speed.The pits are then read by the laser
at a constant linear velocity (CLV). The disk rotates more slowly for accesses near
the outer edge than for those near the center. Thus, the capacity of a track and the
rotational delay both increase for positions nearer the outer edge of the disk. The
data capacity for a CD-ROM is about 680 MB.
Data on the CD-ROM are organized as a sequence of blocks. A typical block
format is shown in Figure 6.11. It consists of the following fields:

• Sync: The sync field identifies the beginning of a block. It consists of a byte of
all 0s, 10 bytes of all 1s, and a byte of all 0s.
• Header: The header contains the block address and the mode byte. Mode 0
specifies a blank data field; mode 1 specifies the use of an error-correcting
code and 2048 bytes of data; mode 2 specifies 2336 bytes of user data with no
error-correcting code.
• Data: User data.
• Auxiliary: Additional user data in mode 2. In mode 1, this is a 288-byte error-
correcting code.
With the use of CLV, random access becomes more difficult. Locating a spe-
cific address involves moving the head to the general area, adjusting the rotation
speed and reading the address, and then making minor adjustments to find and ac-
cess the specific sector.
CD-ROM is appropriate for the distribution of large amounts of data to a
large number of users. Because of the expense of the initial writing process, it is not
appropriate for individualized applications. Compared with traditional magnetic
disks, the CD-ROM has two advantages:

• The optical disk together with the information stored on it can be mass
replicated inexpensively—unlike a magnetic disk. The database on a mag-
netic disk has to be reproduced by copying one disk at a time using two
disk drives.