CDROM, Floppy and Hard Disk Structure

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CDROM, Floppy and Hard Disk Structure
Plus some basic concepts

• Presenter Kianoosh Mokhtarian 82171002
• Evaluators

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Table of Contents

• CD
• History
• Structure
• Data Recording
• How The CD Drive Works
• CD File Systems
• Multiple Sessions
• CD-ReWritable (CD-RW)
• DVD

• Floppy Disk
• History
• Structure
• Data Recording/Retrieval
• Formatting
• 3½ Inch (2HD) Disks
• Hard Disk
• Some Basic Concepts
• Boot Sector
• Cluster
• FAT
• NTFS

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PART 1

• CD-ROM

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History

• Compact Disc - Digital Audio (CD-DA), the
  original CD specification developed by Philips
  and Sony in 1980
• Specifications were published in Red Book,
  continued to be updated (lastest version in 1999)
• In 1985 a standard for the storage of computer
  data by Sony and Philips, CD-ROM (Compact Disc
  Read Only Memory)
• developments in the technology have been ongoing
  and rapid
• Compact Disc Interactive (CD-I)
• Compact Disc Television (CD-TV)
• Compact Disc Recording (CD-R)
• Digital Video Disc (DVD)

5
Structure

• A CDROM Drive uses a small plastic-encapsulated
  disk that can store data
• This information is retrieved using a Laser Beam
• A CD can store vast amounts of information
  because it uses light to record data in a tightly
  packed form

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CD Layers

• The thickness of a CD can vary between 1.1 and
  1.5mm
• A CD consists of four layers

• The biggest part is clear polycarbonate
  (nominally 1.2mm)
• There is a very thin layer of reflective metal
  (usually aluminum) on top of the polycarbonate
• Then a thin layer of some protective material
  covering the reflective metal
• A label or some screened lettering on top of
  protective material

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CD Layers (contd)

• Different layers of a CD
• (though the reflective metal layer is really
  so thin that it should just be represented by a
  line)

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CD Safety

• The label side of a CD is the most vulnerable
  part of the disk
• the other side is protected by the thick (1.2mm)
  and hard polycarbonate
• It is possible to carefully clean and even to
  polish this surface to remove fingerprints and
  even scratches

• Many flaws on the polycarbonate surface will
  simply go unnoticed

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CD vs. Magnetic Media

• In Magnetic Media (like floppy/hard disk) the
  surface is arranged into concentric circles
  called tracks
• Number of sectors per track is constant for all
  tracks
• the CD has one single track, starts at the center
  of the disk and spirals out to the circumference
  of the disk
• This track is divided into sectors of equal size

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CD Data Recording

• Information is recorded on a CD using a series of
  bumps

• These bumps are called pits because they are
  looking like pits in the polycarbonate layer
  (looking down from the top in the diagram above)
• The disk is read from the bottom, through the
  transparent polycarbonate (the pits appear as
  bumps to the scanning laser)

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Data Recording (contd)

• The unmarked areas between pits are called
  "lands
• Lands are flat surface areas

• The information is stored permanently as pits and
  lands on the CD-ROM. It cannot be changed once
  the CD-ROM is mastered, this is why its called
  CD-ROM

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How The CD Drive Works

• A motor rotates the CD
• the rotational speed varies so as to maintain a
  Constant Linear Velocity (the disk is rotated
  faster when its inner "SPIRALS" are being read)

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How The CD Drive Works (contd)

• A laser beam is shone onto the surface of the
  disk
• The light is scattered by the pits and reflected
  by the lands, these two variations encode the
  binary 0's and 1's
• A light sensitive diode picks up the reflected
  laser light and converts the light to digital
  data

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How The CD Drive Works (contd)
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CD-ROM Drive Speed

• The CD-ROM drives are classified by their
  rotational speed
• Based on the original speed of a CD-Audio (e.g. A
  "2X" CD-ROM drive will run at twice the speed of
  a CD- Audio)

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CD Physical Specifications
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CD File Systems

• 1. ISO-9660
• The base standard defines three levels of
  compliance
• Level 1 limits file names to 83 foramat. Many
  special characters (space, hyphen, equals, and
  plus) are forbidden
• Level 2 and 3 allow longer filenames (up to 31)
  and deeper directory structures (32 levels
  instead of 8)
• Level 2 and 3 are not usable on some systems,
  notably MS-DOS

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CD File Systems (contd)

• 2. Rock Ridge
• Extensions to ISO-9660 file system
• Favored in the Unix world
• Lifts file name restrictions, but also allows
  Unix-style permissions and special files to be
  stored on the CD
• Machines that don't support Rock Ridge can still
  read the files because it's still an ISO-9660
  file system (they won't see the long forms of the
  names)
• UNIX systems and the Mac support Rock Ridge
• DOS and Windows currently don't support it

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CD File Systems (contd)

• 3. Joliet
• Favored in the MS Windows world
• Allows Unicode characters to be used for all text
  fields (including file names and the volume name)
• Disk is readable as ISO-9660, but shows the long
  filenames under MS Windows
• HFS (Hierarchical File System)
• Used by the Macintosh in place of the ISO-9660,
  making the disk unusable on systems that don't
  support HFS

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Multiple Sessions

• Allows CDs to be written more than once (not
  re-written)
• Some CD writers support this feature
• About 640MB of data can be written to the CD, as
  some space is reserved for timing and other
  information
• Each session written has an overhead of
  approximately 20MB per session

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CD-ReWritable (CD-RW)

• It is essentially CD-R
• Allows discs to be written and re-written up to
  1000 times
• The storage capacity is the same as that for CD-R
• Based on phase-change technology
• The recording layer is a mixture of silver,
  indium, antimony and tellurium

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CD-RW Recording Process

• The recording layer is polycrystalline
• The laser heats selected areas of the recording
  track to the recording layer's melting point of
  500 to 700 degrees Celsius

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CD-RW Recording (contd)

• The laser beam melts the crystals and makes them
  non-crystalline (amorphous phase)
• The medium quickly cools, locking in the
  properties of the heated areas
• The amorphous areas have a lower reflectivity
  than the crystalline areas
• This creates a pattern which can be read as pits
  and lands of the traditional CD
• To erase a CD-RW disc, the recording laser turns
  the amorphous areas back into crystalline areas

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DVD

• Digital Versatile Disk (Formerly Digital Video
  Disk)

• same size (120mm) and thickness (1.2mm) as CD
• Improvements in the logarithms used for error
  correction
• Much greater data accuracy using smaller Error
  Correction Codes (ECC)
• More effective use of the track space

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DVD vs. CD

• DVD uses a tighter spiral (track or helix) with
  only 0.74 microns between the tracks (1.6 microns
  on CDs)
• DVD recorders use a laser with a smaller
  wavelength, 635nm or 650 nm (visible red light)
  vs. 780nm (infrared) for CDs
• DVD has smaller "burns" (pits) in the translucent
  dye layer (0.4 microns minimum vs. 0.83 microns
  minimum on CDs)

• These technologies allow DVDs to store large
  amounts of data

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DVD (contd)

• Standard single-sided DVDs store up to 4.7GB of
  data
• Dual-sided discs hold about 8.5GB of data (9.4GB
  for back-to-back layers dual-sided discs)
• In back-to-back layers discs, it must be turned
  over to access the data on the reverse side
• DVD uses MPEG2 compression for high quality
  pictures
• DVD drives have a much faster transfer rate than
  CD drives
• DVD-ROM drives will read and play existing CD-ROM
  and CD-A disks

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PART 2

• Floppy Disks
• (floppies or diskettes)

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History

• The 8-inch disk
• - First attempt by IBM in 1967, the result was a
  diskette storing 80KBytes of data
• 250KB, 800KB and 1MB floppies untill 1975
• The problem was their poor media quality

• 2. The 5¼-inch minifloppy
• First developed in 1976, storing 110KB
• In 1978, double-sided drive doubled the capacity,
  and a new "double density" format increased it to
  360 KB

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History (contd)

• 3. The 3-inch compact floppy disk
• No more capacity than the more popular (and
  cheap) 5¼" floppies
• More reliable thanks to its hard casing
• Their main problems were their high prices

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History (contd)

• 4. The 3½-inch floppy disk
• originally offered in a 360 KB single-sided and
  720 KB double-sided double-density format

• A newer "high-density" format, displayed as "HD"
  on the disks and storing 1440 KB of data, in the
  mid-80s
• Another advance in the oxide coatings allowed a
  new "extended-density" ("ED") format at 2880 KB
  in 1991

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Structure

• Made from circular sheets of plastic which are
  coated with a magnetic material
• A central hole for coupling to the disk drive
• An envelope seals the disk to protect and "clean"
  the disk

• An aperture in the envelope to expose a section
  of the disk to allow magnetic heads to read and
  write
• A button on the corner to switch the disk to
  write-protected mode

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Physical Structure

• A disk is divided into many concentric circles
  (lines of recorded data) called tracks
• The disk is also divided into wedge-shaped
  segments called sectors

• The number of sectors per track is the same in
  all tracks
• So the outer sectors are larger than the inners,
  but has the same capacity

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Data Recording

• Similar to the operation of a domestic tape
  recorder

• An electric current flows through a coil of wire
• A magnetic field is produced
• This field is used to magnetise the coating of
  iron oxide on a floppy disk
• Varying electrical current, the signal is passed
  through the coil and the variations are
  "recorded" on the disk

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Data Retrieval

• The disk is rotated at low speed (200 to 600
  revolutions/min)
• The disk moves under the head
• A very small electric current is induced into the
  head and the stored data is retrieved

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Double Sided Disks

• Are used in a disk drive with two read/write
  heads
• A pressure pad is fitted for each head

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Formatting (IBM)

• During the "formatting" process each sector has
  written in it a
• 51 bytes prologue field containing the track and
  sector number
• 512 bytes data field
• 28 bytes unusable field and delay gap between
  sectors
• The track, sector, and the data field are
  followed by a CRC (cyclic redundancy check)
  checksum
• Whenever data are read from the disk a new
  checksum is calculated and compared to the
  written one
• An error message is generated if the two don't
  agree

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Hard Disk

• Fixed and removable
• Fast (disk rotates at 60 to 200 times per second)

• Currently 20 180 GB (may be limited by the
  version of the operating system)
• Like floppies, uses the magnetic properties of
  the coating material, but the technology is
  different

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Boot Sector (Boot Record)

• A vital sector, disk will be unusable if this
  sector damages
• MBR at CHS 0, 0, 1 in hard disks, contains
  Partition Table
• Each partition has its own boot sector too
• Each operating system has its own boot sector
  format
• For Booting, Bootstrap Loader loads Boot Sector
  data it in a particular address of memory
  (00007C00h) and sets the PC
• In hard disks, the small program in MBR attempts
  to locate an active (bootable) partition in
  partition table
• If found, the boot record of that partition is
  read into memory (location 00007C00) and runs

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DOS/Win Formatted Disk

• A DOS/Win formatted floppy/hard disks Boot
  Sector contains
• A jump and a NOP opcode
• BPB (BIOS Parameter Block)
• Sectors per cluster
• Number of Root directory entries
• Sectors per FAT
• Volume Label
• A program, to load OS if bootable/show error msg
  if not in floppies, to locate the active
  partition in hard disks
• Error messages

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Cluster

• Data units of disk must be addressed, which units
  belong to which file / are free / are damaged
  (bad sectors) /
• On disks having large capacity, purposing one
  sector as a unit makes addressing table so large
  ? Cluster is defined
• Represents the smallest amount of disk space that
  can be allocated
• The smaller the cluster size, the more
  efficiently disk space usage, the more number of
  bits to address one unit
• The number of sectors per cluster is stored in
  the
• Boot Record

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FAT

• FAT-12/FAT-16/FAT-32 are Microsoft favorite File
  Allocation Tables (before NTFS)
• FAT-12 uses 12 bits for addressing, a max. of
  4096 units, considering one sector as a cluster,
  2MB can be addressed
• FAT-16 with max.(128) sectors/cluster (64KB
  cluster size ? wasting large amount of disk
  space) up to 4GB, this is why Win95 cannot
  support more than 4GB partiotions
• FAT-32, the same system,
• 32 bit fields for addressing

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NTFS

• NT File System
• Better performance
• Less wasted space
• More security
• Supports all sizes of clusters (512b - 64 KB)

• The 4 KB cluster is somehow standard
• Practically no partition size limitation
• Very flexible, all the system files can be
  relocated, except the first 16 MFT (Master File
  Table) elements

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NTFS (contd)

• NTFS disk is symbolically divided into two parts
• The first 12 is assigned to MFT area
• The rest 88 represents usual space for files
  storage
• MFT area can simply reduce if needed, clearing
  the space for recording files
• At clearing the usual area, MFT can be extended
  again

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Question

• Why are the bumps on the reflective layer of the
  CD called pits?
• Do you know what a Boot Loader program is? And
  how it works?