Recent Post

CD and CD-ROM in Operating System

  • 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'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
  • 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
  • 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.

No comments