1. Chapter 3
Data Storage
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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
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3. Additional Reading Essential Reading Further 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.
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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
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5. Introduction (2) Breakthrough: Different type of media storage
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
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6. Media Storage Main memory (Electronic Memory): Secondary Memory
Stores data currently being used
Is made of semiconductor chips.
Secondary Memory
magnetic (floppy discs, hard disc )
Optical (CD-ROM, DVD)
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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
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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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9. Logic Gates (2) AND OR NOT ¬ a. a b a.b 0 0 0 0 1 0 1 0 0 1 1 1 a 0 1 a
¬ a.
0 1
1 0
a b
a.b
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10. Logic Gates (3) NAND NOR XOR For more details see
a b
a.b
a b
a.b
a b
a.b
For more details see
Schneider and Gersting (2004: )
Burrell (2004: 43-62)
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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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12. A Simple Flip-flop Circuit
As long as both inputs remain 0: output does not change
Temporarily placing 1 on upper input => output = 1
Temporarily placing 1 on lower input => output = 0
So: output flip-flops between 2 values under external control
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13. Setting the Output of a Flip-flop to 1
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14. Setting the Output of a Flip-flop to 1 (cont’d)
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15. Setting the Output of a Flip-flop to 1 (cont’d)
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16. Controlled Flip-Flop
If control = 0 the the flip-flop does not change the state
If control = 1, then if D=0 then Q =1 else Q = 0
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17. Clocked SR flip-flop If CP = 0 the output of both AND gates is 0.
Regardless of the values of S and R.
If S=R=CP=1, then both outputs are set to 0
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18. 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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19. Arrangement of Memory Cells
Each cell has a unique address
Longer strings stored by using consecutive cells
value =
RAM (random access memory)
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20. Q One-bit Memory CP 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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21. Main memory = linking many flip-flops
See Burrell (2004: ) and Tanenbaum (1990: ) t
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22. Memory cells n-bit cell Can hold m*n bits m cells
In reality, most electronic memories have 8-bit cells.
Can hold
m*n bits
m cells
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23. 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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24. 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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25. Main
memory
CPU
Add. bus
Data bus
Control bus
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26. To identify each memory
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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27. Address Bus Address Address Of the cell Of the cell To activated Main
memory
CPU
Address bus
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28. Binary Address Representation
Each cell has a unique address.
I.e. using 4 digit binary representation we have:
cell 0
cell 1
cell 2
cell 3
How many bits are needed to represent an address?
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29. Address Decoder Unique cell Has a unique Address. Address Of the cell
To activated
Main
memory
CPU
Decoder
Address bus
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30. A Simple Address Decoder
Q C0
2 ad-lines A1
Q C1
22 = 4 address
cells A0
Q C2
Q C3
Decoder is a device between the Main Memory and
the address lines.
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31. Decoder with N Address Lines
Main Memory
0000…0000 a0 a1
0000…0001
0000…0010 2n
add
cell
n add.
lines
1111…1111
an-1
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32. Main Memory with 4 Chips a0 a1. . aN-1 decoder Main memory Chip 1
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33. 0 0 0………….. 0 0 0 1………….. 1 Chip 1 0 1 0………….. 0 Chip 1 0 1 1………….. 1
The higher 2 bits of
Address line to select
The chip.
a n a n-2 …..………..a0
………….. 0
………….. 1
………….. 0
………….. 1
………….. 0
………….. 1
………….. 0
………….. 1
Chip 1
Chip 1
Chip 2
Chip 3
Chip 4
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34. 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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35. 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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36. Decoder with 2 Address Lines (multiplexed addresses)00 01 10 11
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111 00 01 10 11
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37. 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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38. Example 1
Suppose computer’s 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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39. Answer
Suppose computer’s 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 28=256
22*8 = 216
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40. Example 2 Suppose that a computer’s 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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41. Answer
Suppose that a computer’s 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 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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42. 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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43. 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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44. Features of the Main Memory
Memory Capacity.
Access of information
Access time
Transfer rate
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45. Memory Capacity Most computer’s 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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46. Capacity Units 1kB = 210 = 1024 Byte.
1MB =1024 KB = 220 Bytes= 1, 048,576 B.
1GB =1024 MB = 230 kB=1, 073,741,824 Bytes.
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47. 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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48. 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 n*m (bytes/s)
Main memory
electronic signals
Implies fast transfer rate in the scale about 100MB/sec
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49. 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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50. 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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51. 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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52. 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, ...
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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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54. 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) pages:
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55. Disk Data Layout
spots
sector
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56. Magnetic Disks
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57. 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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58. Read/write Head Coil of wire 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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59. 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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60. Read and Write Mechanism (2)
Add. bus
Data bus
Control bus
CPU 1
01010
01010 1 1
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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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62. 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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63. Seek time and average 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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64. Example
A disk has 5 tracks and the read/write head takes 1ms to move from a track to an adjacent one
1-2:1ms, 1-3:2ms, 1-4:3ms, 1-5:4ms, :1ms, 2-4:2ms, 2-5:3ms, 3-4:1ms, :2ms, 4-5:1ms
Average seek time: 20/10= 2ms.
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65. Average latency Average latency : Example:
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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66. 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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67. Constant Angulair Velocity (CAV)
Variable
density
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68. 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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69. Multiple Platters (2)
Disk platters speed (3600 to 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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70. Cylinders
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71. Magnetic Tape (1) Serial access Slow Very cheap High capacity Backup
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72. Magnetic tape (2) Serial access (slow)
Good choice for off-line data storage (archives)
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73. 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) pages:
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74. 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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75. 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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76. 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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77. 22/02/2019
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78. 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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79. CD-ROM Operation
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80. Laser 22/02/2019 CIS110 Monochromatic light (single wave length)
Coherent (photons move in the same direction)
Directional (very tight beam, strong concentrated)
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81. 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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82. 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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83. Constant linear velocity
sector
Constante
density
edges
rev/m
centre
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84. 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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85. Read and Write Mechanism (1)
head
Read and Write Mechanism (1) 1
Add. bus
Data bus
Control bus
CPU
Head
01010
01010 1 1
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86. CD-ROM for & against Large capacity Easy to mass produce Removable
Expensive for small runs
Slow
Read only
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87. Other Optical Storage CD-Recordable (CD-R) CD-RW
WORM(write once read many)
Compatible with CD-ROM drives
CD-RW
Erasable
Mostly CD-ROM drive compatible
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88. DVD - what’s in a name? Digital Video Disk Digital Versatile 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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89. 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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90. Summary Main memory Magnetic memory Optical memory RAM
Low storage capacity
Fast (electrical signals)
Volatile.
Magnetic memory
Floppy disk
Hard disk
Magnetic tape
Optical memory
CD_ROM disk
DVD
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