Slide 0-1 Copyright © 2003 Pearson Education, Inc. Figure: Overzicht Informatica Docent: Frank Seinstra Computer Science an overview EDITION 7 J. Glenn.

Slide 0-1 Copyright © 2003 Pearson Education, Inc.

Figure:

Overzicht Informatica

Docent: Frank Seinstra

Computer Sciencean overview

EDITION 7J. Glenn Brookshear


Overzicht Informatica:Huishoudelijke Mededelingen (1)

• Web:– http://www.science.uva.nl/~fjseins/– click op ‘Teaching’

• UvA Blackboard:– http://blackboard.ic.uva.nl/

• Email:– [email protected]


Overzicht Informatica:Huishoudelijke Mededelingen (2)

• College 13 september:– vervanging Jan-Mark Geusebroek (Chap. 10)

• College 20 september:– vervalt

• Let op (!) :– college-zalen + tijden steeds verschillend

• Vrijblijvend:– schrijf-opdracht (max. 1.5 pnt):

• “informatica in medische praktijk / onderzoek”


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’



• Science of algorithms:– algorithm (informally):

• set of steps that defines how a task is performed

• compare: ‘recipe’

– machine-compatible representation = ‘program’

– central issues:• (1) algorithm discovery

• (2) algorithm representation

• (3) handling complex collections of algorithms

• (4) hardware implications, ...


Opdracht:

• Schrijf in een reeks stappen op hoe je een CD in een CD speler afspeelt


The central role of algorithms in computer science



• Science of ‘abstraction’:– obtaining external properties of an entity, by

hiding its internal details.



• Abstraction... on abstraction... on...


Computer Science in relation to desktop PC...

2. Software (Operating Systems, Programming, ...)

1. Machine Architecture (Data Storage, Data Manipulation, ...)

3. Data Organization (Data Structures, File Structures, Databases, …)

4. Potential of Computers (A.I., Theory of Computation, …)


Central issues identical in the past...

- Abacus (ca. 50 BC)

- Difference Engine (Babbage, ca. 1822)

- ENIAC (Univ. of Pennsylvania, 1945)


… 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)


… and in the future!

• World Wide Computing


C H A P T E R 1

Data Storage (& Representation)


1.1 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


1.1 The Boolean Operations AND, OR, and XOR

• Note: AND and OR exist in natural language!


1.1 AND and OR Gates


1.1 XOR and NOT Gates


1.1 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


1.1 Setting the Output of a Flip-flop to 1


1.1 Setting the Output of a Flip-flop to 1 (cont’d)


1.1 Setting the Output of a Flip-flop to 1 (cont’d)


Opdracht:

• Bedenk hoe deze flip-flop de output-waarde 1 kan bewaren, en weer kan wissen:


1.2 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)


1.2 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)


1.4 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)


1.4 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.


1.4 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


1.4 The Decimal and Binary Systems

• Many number-systems can be created this way!


1.4 Decoding the Binary Representation 100101

0

– 1×25 + 0×24 + 0×23 + 1×22 + 0×21 + 1×20 = 37


1.4 Obtaining the binary representation of 13


1.5 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)


1.5 Fractions in the Binary System

• Radix point has same role as in decimal system


1.6 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


1.6 Addition in two’s complement notation

• Note: no circuitry for subtraction needed!

• Note: overflow errors: 0101 + 0100 = 1001 (5 + 4 = -7)


1.1 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)


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



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’…



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


1.7 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


1.7 Truncation Errors: Coding the value 2 5/8


1.7 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!


Opdracht:

• Wat klopt hier niet?


1.3 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


1.3 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, ...


1.3 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


1.3 Old, but still commonly used: Magnetic Tape

• Offers little or no random access (slooooooooooooooow!)• Good choice for off-line data storage (archives)


1.3 Conclusion

• Main memory, magnetic disks, compact disks, and magnetic tape exhibit decreasing degrees of random access to individual bytes of data!


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...


Tot slot

• Lees ook Hoofdstuk 0

• Leuk om te lezen: ‘Social Issues’

• Denk na over de schrijf-opdracht!

Slide 0-1 Copyright © 2003 Pearson Education, Inc. Figure: Overzicht Informatica Docent: Frank Seinstra Computer Science an overview EDITION 7 J. Glenn.

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