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					Lecture No 11

Storage Devices
    Storage Involves Two Processes

Writing data

Reading data
                 Storage Media
 The materials on which data is stored.

 The physical material on which data is stored is called
  a storage medium.

 E.g. Diskette is a storage medium.
               Storage Devices
 The hardware that writes data to or reads data from
  a storage medium is called a storage device.

 E.g. A floppy disk drive is a storage device.
            Two Categories of
           Storage Technology

Magnetic storage

Optical storage
          Magnetic Storage

Hard disks

High-capacity floppy disks

Magnetic tape
             Optical Storage
 Compact Disk Read-Only Memory (CD-ROM)

 CD-Recordable (CD-R)
  CD-Rewritable (CD-RW)

 Digital Video Disk Read-Only Memory (DVD-

 DVD Recordable (DVD-R/DVD), DVD Rewritable

 Photo CD
   Magnetism Allows Data Storage

Hard disks, diskettes, high-capacity floppy disks and
tapes have a magnetic coating on their surface that
enables each medium to store data.
        Magnetic Storage Devices
       How Magnetic Storage Works
 A magnetic disk's medium contains iron particles,
  which can be polarized when given a magnetic
  charge in one of two directions.

 Each particle's direction represents a 1 (on) or 0 (off),
  representing each bit of data that the CPU can

 A disk drive uses read/write heads containing
  electromagnets to create magnetic charges on the
                    Write head


Random particles         Current flow
 (no data stored)      (write operation)

                          Organized particles
                           (represent data)
As the medium
rotates, the head
writes the data
       Preparing Disks for Storage
 Before a magnetic disk can be used, it must be
  formatted—a process that maps the disk's surface
  and determines how data will be stored.

 During formatting, the drive creates circular tracks
  around the disk's surface, then divides each track into
      Formatted Disks Have Three

 Main sections, called Tracks

 Track subsections, called Sectors

 Groups of sectors, called Clusters
                Logical Format

A logical format is the labeling of tracks and
 The Logical Format has Four Disk
 Master boot record
 File allocation table
 Root folder
 Data area
       Master Boot Record (MBR)
 This program determines whether the disk contains
  the basic components of an operating system
  necessary to run successfully.

Boot sector:

 It stores the master boot record (MBR), a small
  program that runs when you first start (boot) the
      File Allocation Table (FAT)

A log created during the logical formatting process
that records the location of each file and status of
each sector on the disk.
                  Root Folder
 The top folder or directory in the disk’s folder

 Enables the user to store data on the disk in a logical
                   Data Area

Place where data or program files are actually
      3.5-inch Diskettes (Floppy Disks)
 Diskettes are used to transfer files between computers
  as a backup medium.

 Spin rate: 300 revolutions per minute (rpm)

 Disk Density=Total no of Sectors * Total no of bytes
  that each sector can hold.

 Storage Capacity of HD disks is 1.44 MB
                  Hard Disks
 Spin rate: from 3,600 to
  15,000 rpm
 Storage capacity ranges
  from several hundred
  MB to more than 200
 High rotational speed
  allows more data to be
  recorded. Why????
                Tape Drives

 Commonly used for
  hard disk backup.

 Sequential Access
               PC Cards

Used to connect new components like
 memory and expanding storage capacity to a

Up to 512MB memory.
   Two Ways to Measure Storage
       Device Performance

Average Access Time

Data Transfer Rate
          Average Access Time
 Amount of time the device takes to move its
  read/write heads to any spot on the medium.

 Measurement should be an Average Time.

 Important measure of performance for storage
 devices and memory.
Access Time Measured in Two Ways

 Storage devices: milliseconds (ms)

 Memory devices: nanoseconds (ns)
  Examples of Average Access Time
 Floppy Disk: 100 ms

 Hard disk: 6 to 12 ms

 High-performance hard disk: 4 or 5 ms

 CD-ROM: 80 to 800 ms

 Tape drives: slowest at a few seconds to a few
           Data Transfer Rate
 Amount of time it takes for one device to transfer
  data to another device.
 Measured in units of data per second
 Note: MBps= Megabytes/sec, Mbps =megabits/sec
             8bits = 1 byte

              Hard drives are the fastest
                  CD-ROM’s, slow
                Floppies, the slowest
       Optimizing Disk

When the PC’s performance slows down,
disk maintenance, or disk optimization,
       can speed things up again
      File or Data Compression
 Technology for making files smaller to free up
  storage space.

 File compression enables to store more data on a disk
  effectively increasing the disk’s capacity.

 File Compression is performed by a software.

 Utilities such as Windows' DriveSpace enable you
  to compress the entire contents of your hard disk.
    File Compression Software

PKZIP and WinZip: Windows-based PC’s

StuffIT: Macintosh
Ways to Optimize Disk Performance
Delete temporary files

Uninstall little-used software programs

Run a disk scanner to look for and fix errors

Defragment the disk.
   Drive-Interface Standards
 Another factor to determine how quickly a drive can
  read and write data is the Type Of Controller that
  driver uses.
 Storage Devices need a controller to act as an
  interface between the Drive and CPU.
 Personal Computers use one of two drive-interface
  standards for built-in disk drives: EIDE or SCSI.
 Other type of interface (USB & FireWire) to attach
  additional disk drives and other devices to a
   Drive-Interface Standards

 EIDE -- Enhanced Integrated Drive Electronics

 SCSI – Small Computer System Interface

 USB – Universal Serial Bus

 FireWire – IEEE 1394

 Most new computer systems use EIDE drive-
  interface standard.

 Latest version supports data transfer rates of

 Variants: Fast IDE, ATA, Fast ATA, Ultra-ATA,
  ATA 66 etc.
 Earlier developed to connect third-party peripheral
  devices to mainframe computers.

 Allows high data transfer rates than EIDE.

 Supports 80MBps.

 Normally found in Servers, Workstations etc.
           USB & FireWire

 All sort of peripheral devices can be attached to a
  computer through USB or FireWire port.

 Support High Data transfer rates.

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