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Slide 0-1 Copyright © 2003 Pearson Education, Inc.
Figure:
Overzicht Informatica
College 1 - September 5 (Docent: Frank Seinstra)
Computer Sciencean overview
EDITION 7 / 8J. Glenn Brookshear
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Overzicht Informatica:Huishoudelijke Mededelingen (1)
• Web:– http://www.science.uva.nl/~fjseins/– click op ‘Teaching’
• UvA Blackboard:– http://blackboard.ic.uva.nl/
• Email:– fjseins@science.uva.nl
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Overzicht Informatica:Huishoudelijke Mededelingen (2)
• College woensdag 7 september a.s.:– vervalt!
• College maandag 19 september:– vervanging Jan-Mark Geusebroek (o.a. AI, etc.)
• Let op (!) :– college-zalen steeds verschillend
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Overzicht Informatica:Huishoudelijke Mededelingen (3)
• Vrijblijvend: schrijf-opdracht (max. 1.5 pnt):• “informatica in medische praktijk / onderzoek”
• Voorbeelden:– Surveillance-camera’s in ziekenhuis
• Wat is typisch voor medische sector?
• Automatische herkenningstechnieken?
– NOS Journaal, 2 September j.l.• Patientgegevens toegankelijk voor hackers…
– Tsunami December 2004• Computergestuurde technieken RIT?
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Introduction:Computer Science - What is it? (1)
• A combination of many things...– includes a.o.:
• (1) hardware design, (2) programming, (3) human computer interaction, (4) artificial intelligence, etc...
– in other words:• mathematics, engineering, psychology, linguistics,
biology, business administration, ethics, sociology, …
• Certainly not:– ‘science’ of computer applications– ‘science’ of programming in language ‘X’
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Introduction:Computer Science - What is it? (2)
• Science of algorithms:– algorithm (informally):
• set of steps that defines how a task is performed
• compare: ‘recipe’
– machine-compatible representation = ‘program’
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Algorithmen zijn overal:
• Denk aan de reeks stappen die je uitvoert als je een CD in een CD speler afspeelt
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The central role of algorithms in computer science
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Introduction:Computer Science - What is it? (3)
• Science of ‘abstraction’:– obtaining external properties of an entity, by
hiding its internal details.
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Introduction:Computer Science - What is it? (4)
• Abstraction... on abstraction... on...
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Central issues identical in the past...
- Abacus (ca. 50 BC)
- Difference Engine (Babbage, ca. 1822)
- ENIAC (Univ. of Pennsylvania, 1945)
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… today ...
- Distributed ASCI Supercomputer 2 (Vrije Universiteit, Amsterdam, 2002) (contains 72 1-Ghz Dual Pentium-IIIs)
- Earth Simulator (ES Center, Yokohama, Japan, 2001) (contains 5120 0.5-Ghz NEC CPUs)
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… and in the future!
• World Wide Computing
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C H A P T E R 1
Data Storage & Representation
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Bits and Their Storage
• Information represented as patterns of bits (binary digits)
• A bit is either 0 or 1 (true or false)
• Meaning of bit(-stream)s varies• numeric values, characters, images, sounds…
• Requires a device that can be in one of two states (& remain in that state as long as needed)
• Flip-flop circuits
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The Boolean Operations AND, OR, and XOR
• Note: AND and OR exist in natural language!
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AND and OR Gates
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XOR and NOT Gates
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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 => output = 1• Temporarily placing 1 on lower input => 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 (cont’d)
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Setting the Output of a Flip-flop to 1 (cont’d)
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Opdracht:
• Bedenk hoe deze flip-flop de output-waarde 1 kan bewaren, en weer kan wissen:
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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
address = 2 = 0x02
value = 01101101
• Longer strings stored by using consecutive cells
• RAM (random access memory)
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Representing Information as Bit Patterns
• Now that we know how to store single bits, we can consider how information can be encoded as bit patterns
• Different encoding systems exist for different types of information– numbers, text, images, sound, …
• Encoding systems more and more standardized– American National Standards Institute (ANSI)– International Organization for Standardization (ISO)
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Representing Text
• Each symbol represented by a unique bit pattern
• Text represented by long stream of patterns
• Today’s standard coding system:– ASCII (American Standard Code for Information Interchange)– Bit patterns of length 7 (generally extended by 1 bit)– See ASCII-table in Appendix A.
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Representing Numbers
• ASCII-encoding inefficient for numeric values• Consider storing the value 25:
– In ASCII: 00110010 00110101 (16 bits)– Worse: largest 16-bit number would be 99
• More efficient approach is to use binary system– uses digits 0 and 1, incl. factor 2 for all bit-positions
• Compare decimal system– uses digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9, incl. factor
10 for each decimal position
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The Decimal and Binary Systems
• Many number-systems can be created this way!
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Decoding the Binary Representation 100101
0
– 1×25 + 0×24 + 0×23 + 1×22 + 0×21 + 1×20 = 37
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Obtaining the binary representation of 13
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The Binary System: Addition
• Knowing how numeric values are encoded, we can consider how to do calculations
• Binary addition:
• Example: 0 0 1 1 1 0 1 0+ 0 0 0 1 1 0 1 1
10101010(58 + 27 = 85)
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Fractions in the Binary System
• Radix point has same role as in decimal system
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Storing Integers: Two’s Complement Notation
• In general: values of 32 bits
• Includes negative numbers
• Leftmost bit indicates the sign– sign bit
• Note:– Positive and negative numbers are
identical from right to left up to & including first ‘1’; from there on are complements of one another
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Addition in two’s complement notation
• Note: no circuitry for subtraction needed!
• Note: overflow errors: 0101 + 0100 = 1001 (5 + 4 = -7)
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The Hexadecimal Coding System
• Bit-streams often very long
• For simplicity of notation:– Hexadecimal system
• Reduces 4 bits to 1 symbol• Especially important in
assembly language programming (next chapter)
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Opdracht - Chapter 1: Problem 6
How many cells can be in a computer’s main memory if each cell’s address can be represented by 3 hexadecimal digits?
• Three digits:– 3 positions, each of which can be one of 16 values
(from the range: 0, 1, …, 9, A, B, C, D, E, F)
– smallest: 000 = 0×162 + 0×161 + 0×160 = 0– largest: FFF = 15×162 + 15×161 + 15×160 = 4095
– So, total number of unique addresses = 163 = 4096
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Opdracht - Chapter 1: Problem 23
Here's a message in ASCII. What does it say?
01010111 01101000 01100001 01110100 00100000 01100100 01101111 01100101 01110011 00100000 01101001 01110100 00100000 01110011 00110001 01111001 00111111
• Each block of 8 bits represents one character: – See ASCII table in Appendix A– Example: 01010111 = ‘W’– Message says: ‘What does it s1y?’– Note: 00110001 = ‘1’, while 01100001 = ‘a’…
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Opdracht - Chapter 1: Problem 28
a. Write the number 14 by representing the 1 and 4 in ASCII.
b. Write the number 14 in binary representation.
• a. See ASCII Table in Appendix A:– 14 = 00110001 00110100
• b. In binary system each ‘1’ represents a power of 2:– 14 = 8 + 4 + 2 = 1×23 + 1×22 + 1×21 + 0×20 => binary: 1110
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Storing Fractions: Floating-point Notation
• In contrast to integers, fractions require storage of the radix point– Floating-point notation
• Example: 1 110 1011 = -10.11 = -2.75
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Truncation Errors: Coding the value 2 5/8
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Truncation Errors (cont’d)
• Significance of truncation errors reduced by using larger mantissa & exponent fields (32bits)
• Problem of nonterminating expansion (e.g. 1/3)– worse in binary than in decimal system (e.g. 1/10)
• Interesting:– 2 1/2 + 1/8 + 1/8 = 2 1/2
– 1/8 + 1/8 + 2 1/2 = 2 3/4
• When adding numbers, order may be important– rule: add smaller values first!
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Opdracht:
• Wat klopt hier niet?
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Mass Storage
• Main memory is volatile and limited in size
• Additional memory devices for mass storage:– a.o.: magnetic disks, optical disks, magnetic tapes
• Advantages over main memory:– less volatile, large capacity, capability of removal,
generally much cheaper
• Disadvantages over main memory– mechanical motion for data access/retrieval (slow!)– in general: lesser degree of random access
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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, Zip disks, ...
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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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Old, but still commonly used: Magnetic Tape
• Offers little or no random access (slooooooooooooooow!)• Good choice for off-line data storage (archives)
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Conclusion
• Main memory, magnetic disks, compact disks, and magnetic tape exhibit decreasing degrees of random access to individual bytes of data!
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Chapter 1: Conclusions
• Information stored as streams of bits
• Bit streams stored in main memory or on mass storage devices - each with different degree of random access (and thus: speed)
• Meaning of bit streams application dependent
• Standardized representations exist for (a.o):– text, numeric values, images, sounds, …
• For numeric values: overflow and truncation errors may make life difficult sometimes...
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Tot slot
• Lees ook Hoofdstuk 0
• Leuk om te lezen: ‘Social Issues’
• Denk na over de schrijf-opdracht!
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