Secondary memory

Under Construction

DeviceHard DriveCompact DiscDVDBlu-ray
Media Magnetic Optical
infra-red
780nm		 Optical
red-orange
650nm		 Optical
blue-violet
405nm
Surface One or more discs (platters).
Both sides used.
Only one surface read at a time.		 Single surface Single Sided Single Layer SS/SL
Single Sided Double Layer SS/DL
Double Sided Single Layer DS/SL
Double Sided Double Layer DS/DL 		Single and Dual layer
commericially available

16 layer possible

Size 10 Meg to Terabyte range 650 Meg SS/SL 4.7 GByte
SS/DL 8.5 GByte
DS/SL 9.4 GByte
DS/DL 17 GByte
 SL 25 GByte
DL 50 GByte
Track Layout Parallel concentric tracks
called cylinders. 		Single track starting from hub.
Burnable discs have a wobble pre-groove for write timing.		 Single track starting from hub.
Dual layer - 2nd layer loops back to hub		 Single track starting from hub.
Dual layer - 2nd layer loops back to hub.
Burnable discs have a wobble pre-groove for write timing.
Data layout Data layed out in sectors.
(Legacy)All tracks have same number of sectors.
(Modern)Disk broken into zones.
Zones on outer part of disc have more sectors. 		Data layed out in sectors. There is only one track but the number of sectors in each rotation of the disc vary continuously. Same as cdrom Same as cdrom
Addressing Sectors Data accessed by the cylinder (track), head (surface), and sector. (CHS)
On newer designs, sector adresses mapped on to a logical block address (LBA) scheme.
Hard drive still uses CHS adressing and hardware level. 		Sectors sequentially numbered from start of track.
Same as cdrom Same as cdrom
Data encoding Sector consists of :
  • Preamble - includes sector ID.
  • Data
    • 512 byte/sector - legacy.
    • 4096 byte/sector - newer drives.
    • Other possible but limited use.
  • ECC block
  • Inter-sector gap
    		• Audio
    Sector consists of :
  • Whole sector sampled data, 2352 bytes.
  • Sector ID embedded in data.
  • No ECC block

    Data
    Sector consists of :

  • Sync bits
  • 3 byte sector address
  • mode flag
  • Data, 2048 bytes.
  • error detection bytes
  • ECC block
  • Inter-sector gap

    Mode 2 data

  • Sync bits
  • 3 byte sector address
  • mode flag
  • Data, 2336 bytes.
    		• Data
    Sector consists of :
  • 12 byte header
  • including a 3 byte ID.
  • Data, 2048 bytes.
  • 4 byte EDC
  • Inter-sector gap

    ECC

  • Separate ECC block for each 16 data sectors.
    		•  
    Accessing Data Read/write head placed over appropriate cylinder.
    Selected head activated.
    Seequentiallly reads until target sector found.     		Audio - sequentally read from start of track/disc.

    For data sector or specific audio session(song) targeted.
    Track skip-read until close to target sector.
    Sequentially read until taret sector found.

    		All forms of information (data or movie) stored as file.
    Track skip-read until close to target sector.
    		 See DVD.
    Read/Write properties Infinite read/write capabilities.
    Written/re-written at sector level.     		Audio/Comerrcial CD. Stamped Disc. Read-only.
    Data stored by creating actual pits and lands on disc.

    WORM - write once read memory - dye is used to create dark spot that simulates pit. Home units capable of writing to disc. Compatible with CDROM
    Can be written in stages, but written sector not reusable.

    CD-RW - can be erased and re-written. Uses a reflective material whose reflectivity can be altered to create "lands" and "pits".
    Whole disc has to be re-written if user wishes to recover used sectors.

    		Similar to CD. Similar to CD.
    Disc Speeds Hard drives use a constant rotational speed.

    Current speeds
    3600 RPM
    7200 RPM
    10000 RPM

    Faster drives available but data interface often bottle neck at RPMs greater than 10K.

    		Access for Audio - constant data stream - 160 KiB/s
    Constant linear(angular) read speed.
    Rotational speed ~ 500 RPM at hub, ~ 200 near edge.

    Access for data - 72x ~ 10,800 KiB/s
    ~ 10K RPM, constant rotational speed.

    		Access for Video - constant data stream ~ 2.77 MiB/s (2x)
    Constant linear(angular) read speed.

    Access for data - up 24x
    constant rotationa speed.