CS100: Introduction to Computer Science

CS100: Introduction to

Computer Science

Lecture 3: Data Storage -- Mass storage & representing information

In-class Exercise:

What is a flip-flop?

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What are the properties of flip-flops?

Draw a simple flip-flop circuit?

Review:

bits, their storage and main memory

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Bits

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Boolean operations

Gates

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Flip-flops (store a single bit)

Main memory (RAM)

q Cell, Byte, Address

Mass Storage or Secondary Storage

Magnetic disks

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CDs

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DVDs

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Magnetic tapes

Flash drives

Mass Storage or Secondary Storage

On-line versus off-line

q Online - connected and readily available to the

machine

q Offline - human intervention required

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Typically larger than main memory

Typically less volatile than main memory

Typically slower than main memory

Mass Storage Systems

Magnetic Systems

Optical Systems

Flash Drives

Figure 1.9 A magnetic disk storage

system

Magnetic Disks

n Floppy disk q Low capacity

n 3.5 inch diskettes 1.44MB q A single plastic disk n Hard Disk system

q High capacity systems

q Multiple disks mounted on a spindle, multiple read/write heads move in unison

n Cylinder: a set of tracks

n Platter : a flat circular disk

q Heads do not tough the surface of disks

Measuring the Performance of Hard Disk

Systems

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(1) seek time

q The time to move heads from one track to another

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(2) rotation delay

q Half the time required for the disk to make a

complete rotation n

(3) access time

q Seek time + rotation delay

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(4) transfer rate

q The rate at which data can be transferred to or

from the disk

Capacity of Hard Disk Systems

5MB (1956 by IBM)

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20MB (1980s)

1 GB (1990s)

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20 GB – 768 GB (3/4) (2006)

q the lowest-capacity - the highest-capacity desktop q On 4 platters

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1 TB (2007)

q 5 platters

Figure 1.10 Magnetic tape storage

Magnetic Tapes

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High capacity

q Many GBs

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A big disadvantage

q Very time-consuming, much longer data access

times

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Good for archival storage

q High capacity q Reliability q Cost efficiency

Figure 1.11 CD storage- Optical Systems

Compact Disks

A spiral approach:

q one long track that spirals around a CD from

inside out.

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Capacity in the ranges of 600 - 700 MB

Good for long continuous strings of data

q music

DVDs (Digital Versatile Disks) or (Digital

Video Disks)

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Same size as CDs (5 inches in diameter)

Encoded in in a different format at a much

higher density

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Multiple layers

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High capacity of several GBs

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Good for lengthy multimedia presentations,

movies with high video and sound quality

Flash Drives

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Flash memory technology

q Bits are stored by sending electronic signals

directly to the storage medium where they cause electrons to be trapped in tiny chambers of silicon dioxide.

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Capacity of up to a few GB

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Portable, small size, easy to connect to a

computer

Flash Drives

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Storing and retrieving data faster than optical

and magnetic systems

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Digital cameras, cellular telephones,

hand-held PDAs

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Vulnerable, repeated erasing slowly damages

the chambers

q Not suitable for general main memory applications q Not good for long term applications

Questions:

n 1. When recording data on a multiple-disk storage

system, should we fill a complete disk surface before starting on another surface, or should we first fill an entire cylinder before starting on another cylinder?

n Why should the data in a reservation system that is

constantly being updated be stored o a magnetic disk instead of a CD or DVD?

n What advantage do flash drives have over the other

Files

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File: A unit of data stored in mass storage

system

q A complete text documents q A photograph

q A program q A music recording

q A collection of data about the students in a

college

Files

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Logical records

q Correspond to natural divisions with data

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Physical Records

q Correspond to the size of a sector

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Buffer: A memory area used for the

temporary storage of data (usually as a step

in transferring the data)

Figure 1.12 Logical records versus

physical records on a disk

Representing Information as bit Patterns

Representing text

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Representing numeric values

Representing Images

Representing sounds

Representing Text

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Each character (letter, punctuation, etc.) is

assigned a unique bit pattern.

q ASCII: Uses patterns of 7-bits to represent most

symbols used in written English text

q Unicode: Uses patterns of 16-bits to represent

the major symbols used in languages world side

q ISO standard: Uses patterns of 32-bits to

represent most symbols used in languages world wide

Figure 1.13 The message “Hello.” in

ASCII

Find the meaning of the following text which is encoded in ASCII:

01000011 01101111 01101101 01110000 01110101 01110100 01100101 01110010

Representing Numeric Values

Binary notation: Uses bits to represent a

number in base two

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Hexadecimal notation: Uses bits to represent

a number in base 16

Hexadecimal Notation

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Hexadecimal notation: A shorthand notation

for long bit patterns

q Divides a pattern into groups of four bits each q Represents each group by a single symbol

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Example: 10100011 becomes A3

Figure 1.6 The hexadecimal

coding system

Representing Numeric Values

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Binary notation: Uses bits to represent a

number in base two

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Hexadecimal notation: Uses bits to represent

a number in base 16

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Limitations of computer representations of

numeric values

q Overflow – happens when a value is too big to be

represented

q Truncation – happens when a value is between

two representable values

Question:

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1. What is the largest numeric value that

could be represented with three bytes if each

digit were encoded using one ASCII pattern

per byte?

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What if binary notation were used?

What if hexadecimal notation were used?

Covert binary representations to its

equivalent base ten form

q 0101 q 10011

Representing Images

n Bit map techniques

n Pixel: short for “picture element” q 1 bit for 1 pixel

n A black and white image is encoded as a long string of bits representing rows of pixels in the image.

n The bit is 1 if the corresponding pixel is black, 0 otherwise.

q 8 bit for 1 pixel

n For white and black photos, allows a variety of shades of grayness to be represented.

q 3 bytes for 1 pixel

n For color images. RGB encoding, 1 byte for the intensity of each color

Representing Images

Bit map techniques

Vector techniques

q Scalable

q Word processing systems use vector techniques

to provide flexibility in character size.

q PostScript

q Also popular in Computer-aided design systems

Representing Sound

Sampling techniques

q Sample the amplitude of the sound waves at

regular intervals and record the series of values obtains.

q 8000 samples per second

n used in long-distance telephone communication

q 44,100 samples per second for high quality

recordings (each sample represented in 16 or 32 bits) a million bits for a second of music

Figure 1.14 The sound wave represented by

the sequence 0, 1.5, 2.0, 1.5, 2.0, 3.0, 4.0, 3.0, 0

Representing Sound

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Sampling techniques

MIDI

q Used in music synthesizers in electronic

keyboards

q Contains individual instructions for playing each

individual note of each individual instrument.

q Encoding directions for producing music on a

synthesizer rather than encoding the sound itself.

Homework Assignment1:

(Due in-class next Monday)

Page 71, 1b, 2b, 3b

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Page 72, 5, 6, 9,11, 12, 19

Next Lecture:

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The binary system, storing integers and

fractions