Q. Block Format and Disk Layout on CD-ROM?

A typical block format is displayed in Figure (a). It comprises the subsequent fields:

• Sync: Sync field identifies beginning of a block. It comprises a byte of all 0s, 10 bytes of all 1s and bytes of all 0s.
• Header: The header comprises block address and mode byte. Mode 0 specifies a blank data field and mode 1 specifies use of an error-correcting code and 2048 bytes of data as well as mode 2 specifies 2336 bytes of user data with no error correcting code.
• Data: User data.
• Auxiliary: Extra user data in mode 2. In mode 1 this is a 288-byte error correcting code.

Figure: Block Format and Disk Layout on CD-ROM

However what is Min (Minute), Sec (Second) and Sector fields in Header field?  The sectors of CD-ROM aren't arranged like sectors in hard disks (Please see Figure (b)). Rather they are all equal length segments. If we rotate CD drive at constant speed the linear velocity of disk surface movement will be higher at the outer side than that of centre portions. To offset this liner speed gap either we store less data on outer sectors or we decrease speed of rotation while reading outer tracks. CD follows later approach which is instead of moving CD drive at constant velocity. It's rotated at variable velocity. Speed or rotation of disk decreases as we move away from centre such that sector's can be read in constant time. This method of reading is known as Constant Liner Velocity (CLV).