PC Hardware A Beginner’s Guide

(^204) PC Hardware: A Beginner’s Guide
In order to store data, the CD-DA standard had to be modified. In 1984, Philips and
Sony issued the Yellow Book standard that defined the CD-ROM for storing computer
data. The Yellow Book defined two new kinds of content sectors: Mode 1 and Mode 2.
Mode 1 sectors store computer data and Mode 2 sectors are used to store compressed au-
dioorvideoandgraphicdata.ThisnewstandardrecognizedtheneedfortheCD-ROMto
store data more precisely than the audio CD.
Audio CD (CD-DA) has 99 accessible tracks on which music is stored. The Yellow Book
defined the CD-ROM with what amounted to a file system. Both Mode 1 and Mode 2 sector
formats have a few bytes at the front of each sector. Table 10-1 lists the contents of a Mode 1
sector, showing the space used for the header and error detection and correction.
The size of CD-DA and CD-ROM Mode 1 and Mode 2 sectors are the same, but the
amount of user data varies because of sync bytes, header bytes, and error correction and
detection. The CD-DA format uses all 2,352 bytes of a sector for user data (music).
CD-ROM Mode 1 blocks have 2,048 of user data and Mode 2 blocks provide 2,336 user
bytes. Because of the amount of data they transfer, the two modes have different transfer
speeds (about 1.22Mbps for Mode 1 and 1.4Mbps for Mode 2).
In the Mode 1 sector, the first 12 bytes of the header are sync bytes that are used for sector
separation. The sync bytes at the beginning of a sector are intended to identify the sector
mode, but since the value of the sync bytes could coincidentally appear in the user bytes, the
length of the sector is also used to identify the mode type. The next four bytes are the header
bytes, three of which are used for addressing. The fourth byte indicates the mode used to re-
cord the contents of the sector. The address stored in the header bytes contains the length of
any blocks in the sector in minutes and seconds, plus other identifying information.
The header byte mode indicator contains theCIRC (Cross Interleaved Reed-Solomon
Code), which is the standard error detection and correction method used by CD-DA and
Bytes Use Purpose
12 Sync bytes Sector separator
3 Header bytes Addressing (minutes, seconds, and tracks)
1 Header byte Mode indicator (CIRC)
2048 User bytes Data
4 EDC bytes Error detection
8 Unused
276 ECC bytes Error correction
Table 10-1. CD-ROM Mode 1 Sector Format