CD and CD-ROM in Operating System

CD

• Optical disks have higher density then magnetic disks
• Used for distributing commercial software and reference works (books)
• Cheap because of high production volume and consumption (for music CDs)
• First used for playing music digitally
• Laser burns holes on  a master (coated glass) disk
• Mold is made with bumps where holes were Polycarbonate resin (sticking) poured into-has same pattern of holes as glass disk.
• Aluminum coated put on top of Polycarbonate resin
• Pits (depressions) and lands (unburned area) are arranged in spirals
• Laser is used to read the pits and lands and convert them into bits (o and 1)

• Recording structure of a compact disc or CD-ROM.

CD-ROM

• CD's can be used to store data as well as audio
• Enter the CD-ROM
• Needed to improve the error-correcting ability of the CD
• Encode each byte (8 bits) in a 14 bit symbol with 6 bits of ECC
• 42 symbols form a frame
• Group 98 frames into a CD-ROM per sector
• Extra error-correcting code is attached to sector

CD-ROMs Sector Layout

• Each symbol = 14 bits (8 data 6 sec)
• 42 symbols makes 1 frame (192 data bits, 396 ecc bits)
• 98 frames makes 1 sector (2352 Bytes)

CD-ROM Performance

• 650 MB capacity vs GB SCSI disk capacity
• 150 KB/sec in mode 1, (1 x)
• up to 5 MB/sec for 32 x CD-ROM
• SCSI-2 magnetic disk transfers at 10 MB/sec
• Bottom line:

           CD drives can't compare to SCSI disk drives

CD-ROM

• Added graphics, video, data
• File system standards agreed upon
• High Sierra for file names of 8 characters, file type 3 characters
• Rock ridge for longer names and extensions
• CD-ROM's used for publishing games, movies, commercial software, reference works
• why? Cheap to manufacture and large capacity

CD-Recordable

• Cheaper manufacturing process led to cheaper CD-ROM (CD-R)
• Used as backup to disk drives
• Small companies can use to make masters which they give to high volume plants to reproduce

•  Cross section of a CD-R disk and laser. A silver CD-ROM has similar structure, except without dye layer and with pitted aluminum layer instead of gold layer.