Data Storage

1
Chapter 3

• Data Storage

2
Learning outcomes

• By the end of this Chapter you will know
• The difference between electronic, magnetic and
  optical memory
• How data are stored in these types memories
• The main memory is made up of logic gates
• The main memory is organised in terms of cells
  and addresses
• memory terms
• Memory capacity, access time, transfer rate, etc
  
• How the address decoder works

3
Additional Reading

• Essential Reading
• Stalling (2003) Chapters 5 and 6
• Further Reading
• Burrell (2004) Chapters 3 and 7
• Schneider and Gersting (2004) Chapters 4 and 5
• Tanenbaum (1990) Chapter 3
• White (2002) Parts 3 and 4.

4
Introduction (1)

• Information can be stored in different ways
• Books,
• Films
• Paintings,
• It is not information if it could not used
• Information in computers must be able to able to
  be processed by computers
• Information must be represented in appropriate
  format
• Information must be stored in appropriate places

5
Introduction (2)

• Breakthrough
• The use of the binary system (Base 2)
• In the binary system
• There is only two types of values, 1s and 0s.
• It is easy to store binary information/data in
  physical media
• It is also easy to process binary information
• Different type of media storage
• Electronic memory (main memory)
• Magnetic memory
• optical memory

6
Media Storage

• Main memory (Electronic Memory)
• Stores data currently being used
• Is made of semiconductor chips.
• Secondary Memory
• magnetic (floppy discs, hard disc )
• Optical (CD-ROM, DVD)

7
Main Memory (Electronic Memory)

• Main memory stores data which are currently used
  by the CPU.
• To run a program, it is first loaded in the main
  memory
• Main Memory is volatile
• Its content changes frequently
• Data is lost when the power is off
• It is also called electronic memory
• Based on electronic principles.
• Formed with logic gates
• Group of transistors
• Cells
• Sequence of one-bit memories
• Addresses
• Each cell has a unique address

8
The physical principles of electronic memory

• Transistor
• The smallest unit of an electronic memory
• Logic Gates
• Groups of transistors
• Flip-Flops
• Special type of circuit

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Logic Gates (2)
OR
AND
NOT
10
Logic Gates (3)
NAND
XOR
NOR

• For more details see
• Schneider and Gersting (2004 155-177)
• Burrell (2004 43-62)

11
Flip-Flop circuits

• Up to now the output of combinational circuits
  depends solely up the input
• Combinational circuits has no memory
• To build a sophisticated digital signal circuits,
  memory, we need
• We need circuits whose output depends upon both
  the input of the circuit and its previous states.
• In other words, we need circuit that have memory.

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A Simple Flip-flop Circuit

• As long as both inputs remain 0 output does
  not change
• Temporarily placing 1 on upper input gt output
  1
• Temporarily placing 1 on lower input gt output
  0
• So output flip-flops between 2 values under
  external control

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Setting the Output of a Flip-flop to 1
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Setting the Output of a Flip-flop to 1 (contd)
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Setting the Output of a Flip-flop to 1 (contd)
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Controlled Flip-Flop

• If control 0 the the flip-flop does not change
  the state
• If control 1, then if D0 then Q 1 else Q 0

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Clocked SR flip-flop

• If CP 0 the output of both AND gates is 0.
• Regardless of the values of S and R.
• If SRCP1, then both outputs are set to 0

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Main Memory

• Large collection of circuits, each capable of
  storing a single bit
• Arranged in small cells, typically of 8 bits each
  (a.k.a. byte)

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Arrangement of Memory Cells

• Each cell has a unique address
• Longer strings stored by using consecutive cells

value 01101101

• RAM (random access memory)

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One-bit Memory

• To write a datum (0 or 1) to this memory
• send data to D, and at the same time
• send a WRITE signal to CP
• To read a datum from this memory
• connect to Q by sending a READ signal

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Main memory linking many flip-flops
See Burrell (2004 111-112) and Tanenbaum (1990
105-109) t
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Memory cells
n-bit cell

• In reality, most electronic memories have 8-bit
  cells.

Can hold mn bits
m cells
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Accessing Data in the Main Memory

• Instructions and data are stored in the main
  memory in a serial order.
• CPU executes instructions one by one top down.
• An instruction may tell the CPU
• to jump to particular cell and execute the
  instruction held in it,
• or fetch the data stored is that cell.
• How is this done?

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System Bus

• Main memory and CPU are linked using a set of
  wire
• Three wires
• address lines,
• data lines and
• control lines.
• Known as
• address bus,
• data bus and
• control bus.

System bus
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Main memory
CPU
Add. bus
Data bus
Control bus
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To read data from each cell
To issue read or write signal
To identify each memory cell
Main memory
CPU
Add. bus
Data bus
Control bus
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Address Bus
Address Of the cell To activated
Address Of the cell To activated
Main memory
CPU
Address bus
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Binary Address Representation

• Each cell has a unique address.
• I.e. using 4 digit binary representation we
  have
• 0000 cell 0
• 0001 cell 1
• 0010 cell 2
• 0100 cell 3
• How many bits are needed to represent an address?

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Address Decoder
Unique cell Has a unique Address.
Address Of the cell To activated
Main memory
CPU
Decoder
Address bus
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A Simple Address Decoder
Q0 00 C0
2 ad-lines
A1
Q1 01 C1
22 4 address cells
A0
Q2 10 C2
Q3 11 C3
Decoder is a device between the Main Memory and
the address lines.
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Decoder with N Address Lines
Main Memory
00000000
a0
00000001
a1
00000010
2n add cell
n add. lines
11111111
an-1
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Main Memory with 4 Chips
decoder
Main memory
a0 a1. . . . . . . aN-1
Chip 1
Chip 2
Chip 3
Chip 4
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The higher 2 bits of Address line to select The
chip.
a n-1 a n-2 ....a0 0 0 0.. 0
0 0 1.. 1 0 1 0.. 0 0
1 1.. 1 1 0 0.. 0 1
0 1.. 1 1 1 0.. 0 1
1 1.. 1
Chip 1
Chip 1
Chip 2
Chip 3
Chip 4
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Multiplexer

• Cells form rows and columns.
• Each cell can be identified by a row address and
  column address.
• Each cells address uses only n/2 address lines.
• This can be done using a multiplixed addresses.

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Decoder with 4 Address Lines (non-multiplexed
addresses)
0000 0001 0010 0011
0100 0101 0110 0111
1000 1001 1010 1011
1100 1101 1110 1111
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Decoder with 2 Address Lines (multiplexed
addresses)
00
01
11
10
0000 0001 0010 0011
0100 0101 0110 0111
1000 1001 1010 1011
1100 1101 1110 1111
00
01
10
11
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Two-Input Multiplexer

• A multiplexer is an electronic device that allows
  multiple logical signals to be transmitted
  simultaneously across a single physical channel
  (address line).

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Example 1

• Suppose computers Main Memory is linked to a
  decoder with 8 address lines.
• Can 1000 memory cells be used?
• If no what is the maximum number of addresses
  that can generated?
• What is the maximum number of addresses that can
  be generated is multiplexed addresses are used?

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Answer

• Suppose computers Main Memory is linked to a
  decoder with 8 address lines.
• Can 1000 memory cells be used?
• If no what is the maximum number of addresses
  that can generated?
• Answer
• NO
• With 8 address lines, the maximum number of
  addresses is 28256
• 228 216

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Example 2

• Suppose that a computers Main Memory has 1013
  cells.
• How many address lines are needed in order for
  all the cells to be useable? Explain your answer.

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Answer

• Suppose that a computers Main Memory has 1013
  cells. How many address lines are needed in order
  for all the cells to be useable? Explain your
  answer.
• Answer
• With N address lines a computer can have a
  maximum 2N usable cells. 29 512, 210 1024.
• 9 address lines would not generate enough
  addresses for 1013 cells to be used. 10 address
  lines would.
• Having more than 10 address lines would lead to
  too many addresses wasted. So the desired number
  of address lines is 10.
• N ?log2(1050)? can be used to find the number
  of address lines.
• If multiplexed addresses is used, then 5 address
  lines would be sufficient for 1013 cells to be
  useable.

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What does a word mean?

• A word is the length of instructions the CPU can
  execute at one time.
• Some processor can handle 8-bit words others
  16-bit, 32-bit, 64-bit.
• A cell does not necessarily store one word.
• A word can occupy more than one cell.

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Address Space

• The address space of a computer is the maximum
  number of cells a computer can hold.
• The address space is determined by the number of
  address lines used in a computer.
• If each cell in a memory is 8-bit, then the
  memory is called byte addressable 1 byte long
  has a unique address

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Features of the Main Memory

• Memory Capacity.
• Access of information
• Access time
• Transfer rate

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Memory Capacity

• Most computers memory have 8-bit (1-byte) cells.
• In this case we have
• 32KB, 256MB and 20GB are used to describe the
  memory capacity.

Address lines No of cells Capacity (byte)
n 2n 2n x 1
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Capacity Units
1kB 210 1024 Byte. 1MB 1024 KB 220 Bytes 1, 048,576 B. 1GB 1024 MB 230 kB1, 073,741,824 Bytes.
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Access Time

• Access time is taken between the moment when the
  CPU wants the read/write from/into a cell and
  the moment when the cell is activated.
• It is the moment that the CPU takes to activate a
  cell.
• 60ns (10-9 sec)

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Transfer Rate

• Is the amount of information per second exchanged
  between the CPU and main memory.
• If the CPU can read n cells in a second and each
  cell has m bytes then transfer rate is nm
  (bytes/s)
• Main memory
• electronic signals
• Implies fast transfer rate in the scale about
  100MB/sec

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Random Access

• If the CPU wants to activate particular cell.
• It does not search for the target cell from top
  to bottom.
• It does put the address of the target cell in
  the address line, then the cell will be
  activated.
• This type of accessing information is called
  Random Access

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The need for other type of memories.

• Main memory
• Fast as all the exchange between CPU and Main
  memory is done electronically.
• However, it is volatile.
• Information lost when the machine is turned off.
• The need for non-volatile memory
• Hold information when the machine is off.
• i.e. Magnetic disk, optical disk, magnetic tape

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Magnetic Memory

• Another way of storing information in the binary
  framework.
• Magnetic memory contains a number of spots.
• The information is stored by magnetising and
  demagnetising these spots.
• Magnetised spot 1
• unmagnetised spot 0
• i.e. floppy disk

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A Magnetic Disk Storage System

• Each track contains same number of sectors
• Location of tracks and sectors not permanent
  (formatting)
• Examples hard disks, floppy disks, ...

53
Magnetic Disk Terminology

• Platter
• rigid metal or glass platter Coated with magnetic
  material.
• rotating at constant angular velocity
• Arm
• With movable magnetic read/write heads
• Track
• A complete ring of data
• The disk surface is divided into tracks
• Sectors
• Each track is subdivided into sectors
• Cylinder (see slides 71-72)
• A vertical collection of tracks at the same
  radial position

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Data Organization and Formatting

• Concentric rings or tracks
• Gaps between tracks
• Reduce gap to increase capacity
• Same number of bits per track (variable packing
  density)
• Constant angular velocity
• Tracks divided into sectors
• Minimum block size is one sector
• May have more than one sector per block
• See Stalling (2003) pages167-168

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Disk Data Layout
spots
sector
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Magnetic Disks
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Magnetic Disks
Thus as the platter rotates under the head, a
stream of bits can be written and later read
back.
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Read/write Head

• A coil of wire wound onto an iron former.
• gap.
• If a spot on the magnetic memory passes under the
  gap then an electrical current is induced in the
  coil. And the read/write head will know that
  there is a 1 stored on that spot. Otherwise it
  is 0.
• By passing an electric current on the wire we
  can magnetise and demagnetise spots.

Coil of wire
Iron former
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Read and Write Mechanism (1)

• Recording and retrieval via conductive coil
  called a head
• May be single read/write head or separate ones
• During read/write, head is stationary, platter
  rotates
• Write
• Electric Current through coil of wire produces
  magnetic field
• Magnetic Pulses sent to the head
• Magnetic pattern recorded on surface below
• Read
• Magnetised bit pattern
• Magnetic field induces an electrical current in
  the coil
• The bit pattern contains 1
• Demagnetised bit pattern
• No Magnetic field induced, hence, no electrical
  current in the coil
• The bit pattern contains 0

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Read and Write Mechanism (2)
1
01010
01010
1
1
61
Fixed/Movable Head Disk

• Fixed head
• One read/write head per track
• Heads mounted on fixed ridged arm
• Movable head
• One read/write head per side
• Mounted on a movable arm

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Access Information on a Floppy disk

• To access information on a floppy
• Track number, and
• Sector number.
• Head moves to the target track.
• waits for the desired sector to spin underneath
  it
• read/write begins.

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Seek time and average seek time

• Seek time
• is the time it takes, the read/write head to
  move from one track to a particular track on a
  disk
• Average seek time
• is the average of seek time between every pair
  of tracks.

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Example

• A disk has 5 tracks and the read/write head takes
  1ms to move from a track to an adjacent one
• 1-21ms, 1-32ms, 1-43ms, 1-54ms,
  2-31ms, 2-42ms, 2-53ms, 3-41ms,
  3-52ms, 4-51ms
• Average seek time 20/10 2ms.

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Average latency

• Average latency
• is the time taken to make half a revolution.
• Example
• Disk rotates at a speed of 100 rev/sec
• Average latency is 1 / 200 sec.

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Maximum data transfer rate

• It is the rate at which data passes under the
  read/write head (bytes/sec).
• Number of bytes / track Number of rev / sec

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Constant Angulair Velocity (CAV)
Variable density
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Multiple Platter (hard disk)

• Permanent storage that is inside of the computer,
  and NOT portable.
• Consists of several platters which spin very fast
• Heads are joined and aligned
• Aligned tracks on each platter form cylinders
• Data is striped by cylinder
• reduces head movement
• Increases speed (transfer rate)

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Multiple Platters (2)

• Disk platters speed (3600 to 10 000 rpm
  (rev/min).
• floppy (360rpm).
• The read data we need to specify cylinder, head,
  and sector numbers.
• Each cylinder represents a track number.

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Cylinders
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Magnetic Tape (1)

• Serial access
• Slow
• Very cheap
• High capacity
• Backup

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Magnetic tape (2)

• Serial access (slow)
• Good choice for off-line data storage (archives)

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Magnetic Tape
column
R/W head
Blocks of data
Track 1
Track 2
Track 9
A magnetic tape is a series of columns. Each
column can store a word or two. Tapes offer a
large storage capacity for backup. See Stalling
(2003) pages189-190.
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Features of Magnetic Memory

• Memory capacity
• Floppy can hold 700KB 120MB.
• Hard disk can hold dozen of GB, 10, 20,..
• Tapes can hold 100MB- 80GB.
• Access method
• Floppy and hard disks is random as the main
  memory
• Tape is serial
• Access time
• It is the average time taken to position the R/W
  head over the data to be read
• For disk drives is about 10-3 sec when in MM 10-9
  sec.
• Transfer rate is slower. It is the transfer of
  data between MM and Mag/M. Floppy (500kB-2MB)
  and hard disc (4-12MB).

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Optical Storage CD-ROM

• Originally for audio
• 650 Mbytes giving over 70 minutes audio
• Polycarbonate coated with highly reflective coat,
  usually aluminium
• Data stored as pits
• Read by reflecting laser
• Constant packing density (data/surface constant)
• More data in outer edges
• Less data towards the centre of the disc
• Constant linear velocity
• The drive must adjust the disc speed (495 to 212
  rev/m) ?edges
• Faster when reading data closer to the centre
• Slower when reading data in outer edges

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CD-ROMs
In recent years, optical (as opposed to magnetic)
disks have become available. They have much
higher recording densities than conventional
magnetic disks. Optical disks were originally
developed for recording television programs, but
they can be put to more esthetic use as computer
storage devices. Due to their potentially
enormous capacity, optical disks have been the
subject of a great deal of research and have gone
through an incredibly rapid evolution.
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Optical Storage CD-ROM

• Is a disc with highly reflective surface.
• Tiny areas flat and depressed
• Flat (land) ? strong reflection.
• Depressed (pits) ? low reflection.
• Laser ? land?strong reflection?photo-sensor
  generates electrical voltage?store 1s.
• Laser (light Amplification stimulated emission
  of radiation).
• Light?pits?low reflection? no electrical voltage
  ? stores 0s.

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CD-ROM Operation
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Laser

• Monochromatic light (single wave length)
• Coherent (photons move in the same direction)
• Directional (very tight beam, strong
  concentrated)

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CD/DVD Storage Format

• Data stored by creating variations in the
  reflective surface
• Data retrieved by means of a laser beam
• Data stored uniformly (so CD rotation speed
  varies)
• Random access much slower than for magnetic
  disks

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The pits and lands are written in a single
continuous spiral starting near the hole and
working out a distance of 32 mm toward the edge.
The spiral makes 22,188 revolutions around the
disk (about 600 per mm). If unwound, it would be
5.6 km long.  
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Constant linear velocity
sector
Constante density
centre
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Random Access on CD-ROM

• Difficult
• Move head to the right position
• Set correct speed
• Read address
• Adjust to required location

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Read and Write Mechanism (1)
1
01010
01010
1
1
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CD-ROM for against

• Large capacity
• Easy to mass produce
• Removable
• Expensive for small runs
• Slow
• Read only

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Other Optical Storage

• CD-Recordable (CD-R)
• WORM(write once read many)
• Compatible with CD-ROM drives
• CD-RW
• Erasable
• Mostly CD-ROM drive compatible

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DVD - whats in a name?

• Digital Video Disk
• Used to indicate a player for movies
• Only plays video disks
• Digital Versatile Disk
• Used to indicate a computer drive
• Will read computer disks and play video disks

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DVD - technology

• Multi-layer
• Very high capacity (4.7GB per layer)
• Full length movie on a single disk
• Using MPEG (Moving Picture Expert Group)
  compression
• Digital Compression of audio and video signals.
• MPEG achieves high compression rate by storing
  only changes from one frame to another, instead
  of each entire frame.

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Summary

• Main memory
• RAM
• Low storage capacity
• Fast (electrical signals)
• Volatile.
• Magnetic memory
• Floppy disk
• Hard disk
• Magnetic tape
• Optical memory
• CD_ROM disk
• DVD