Intro CS, Computers, Programming Introduction to Computer Science, Computers, and Programming Barbara Ericson Georgia Tech Aug 2009.

Introduction to Computer Science, Computers, and Programming

Barbara Ericson

Georgia Tech

Aug 2009

Learning Goals

• What is Computer Science?

• What do Computer Scientist do?

• Why study Computer Science?

• What is a computer?

• What can computers do?

What is Computer Science?

• Write down a sentence or two about what you think it is and then share that with your neighbor.– What is different about your definitions?– What is the same about your definitions?

• Look up some definitions of computer science– How do these definitions agree and disagree

with what you thought?

What do Computer Scientists do?• Study theory

– Mostly work in universities and research labs

• Work on the next generation of the Internet– Networking– Cisco

• Work on how to effectively store and process tons of data– Databases– NCR, NASA, Google

• Work on better ways for humans and computers to communicate– Human – Computer Interaction– Cell phone companies

What do Computer Scientists do?• Work on special effects in movies, TV, and games

– Graphics, animation– Pixar, Electronic Arts– See the digital video special effects class movies at Georgia

Tech http://www.cc.gatech.edu/classes/AY2006/cs4480_spring/

• Work on keeping digital data secure– FBI, CIA, Internet Security Systems

• Work on how to use computers to improve education– Educational technology

• Work on making computers smarter– Artificial intelligence, robotics

What is a Computer?

• A device that performs high-speed mathematical and/or logical operations or that assembles, stores, correlates, or otherwise processes information.

• The first computers were people – who did computations– Like the Harvard Computers

shown on the right• astronomical calculations

Parts of a Computer

• User Interface – monitor (screen), mouse,

keyboard, printer

• Brain - Central Processing Unit – can do math and logic

operations

• Memory - Storage– cache – fastest – main - RAM– secondary – Disks, CD-

ROMs, DVDs

CPU – Brain of the Computer

• Arithmetic/Logic Unit (ALU)– Does math and logic

calculations on numbers in registers

• Control Unit– Reads instructions

from memory and decodes and executes them using the ALU

345

263Add register A to register B

608 Store the value in register C into memory location320843202

Fetch, Decode, Execute Cycle

• The control unit reads (fetches) an instruction from memory

• The control unit decodes the instruction and sets up the hardware to do the instruction – like add the values in the A and B registers

and put the result in the C register

• The instruction is executed • The program counter is incremented to

read the next instruction

Processor Speed

• Processors (CPUs) have a clock

• Clock speed is measured gigahertz (GHz)

• Some instructions take just 2-3 clock cycles, some take more

• When the clock speed increases the computer can execute instructions faster

Memory• Computer memory is

used to store data • The smallest unit of

memory is a bit (Binary digIT)

• A bit can be off (no voltage) or on (has voltage) which we interpret to be 0 or 1

• Memory is organized into 8 bit contiguous groups called bytes. A megabyte is 1,048,576 bytes (over 1 million bytes). A gigabyte is over 1 billion bytes.

How does Memory Represent Values?

• The different patterns of the on and off bits in a byte determine the value stored

• Numbers are stored using binary numbers– 101 is 1 * 20 + 0 * 21 + 1 * 22 = 5

• Characters are internally represented as numbers– Different numbers represent different characters– There are several systems for assigning numbers to

characters: • ASCII, EBCDIC, and Unicode

Encodings Make Computer Powerful

• Voltages are interpreted as numbers

• Numbers can be interpreted as characters

• Characters can be interpreted to be part of a link to Sun’s Java Site

0100 0001

off on off off off off off on

a

<a href=http://java.sun.com>Sun’s Java Site </a>

Notepad Exercise

• Open notepad and type a sentence in it

• Save the file• Check the size in bytes

by leaving the cursor over the file name – Or right click and check

properties

• Now count the number of letters and spaces

Binary Numbers

• We usually work with decimal numbers with digits from 0 to 9 and powers of 10

7313 = (7 * 1000 + 3 * 100 + 1 * 10 + 3 * 1)

Or (7 * 103 + 3 * 102 + 1 * 101 + 3 * 100)

• The binary number system uses digits 0 and 1 and powers of 2

0101 = (0 * 23 + 1 * 22 + 0 * 21 + 1 *20)

(0 * 8 + 1 * 4 + 0 * 2 + 1 * 1) = 5

Binary Addition

• To add two decimal numbers you add the digits and if the total is greater than ten you carry the one into the next column

• To add two binary numbers – 0 + 0 = 0– 0 + 1 and 1 + 0 = 1– 1 + 1 = 0 with a carry of 1

into the next column to the left

00 10 111

01 01 001

---- --- ------

01 11 1000

00111001010

01010101101

-------------------

10001110111

2’s Compliment Notation

• Computers actually only know how to add– So, how do they handle subtraction?

• Computers subtract by adding a negative number

• How do you represent a negative number in memory?– Positive numbers in 2’s compliment are just

the same as a binary number– For negative numbers reverse 0s and 1s and

then add 1

2’s Compliment Example

• To subtract 3 from 7

• First represent both as a binary number– 7 is 0000 0111– 3 is 0000 0011

• Reverse the 0s and 1s and then add 1 to get -3– 0000 0011 reversed is 1111 1100– add 1 0000 0001– The result is 1111 1101

Add the Negative Number

• To subtract 3 from 7

• Add -3 to 7 – 7 is 0000 0111– -3 is 1111 1101– The result is 1 0000 0100

• Through away the leftmost 1

• The answer is 0000 0100 which is 4

Patterns Exercise

• How many different patterns of on and off bits are there in 3 bits? How many in 4 bits? How many in 8 bits?

• 000 is one pattern

• 001 is another pattern

• 010 is another pattern

Does the number of patterns matter?

• Some garage door openers in the 70s used 8 bits to set the code to use to open the door– Giving 256 different

patterns– Which is enough that

you won’t open your neighbors door

– But small enough that someone could try each one

Remote Entry Systems• With 8 bits for a code you

have a 1/256 chance of a random code working– You don’t want someone

opening your car in a place with lots of cars (like a mall)

• There are also radio scanners that can capture your code– So you want the code to

change each time • Modern remote entry

systems use a 40 bit rolling code

Types of Memory• Registers

– Very high speed temporary storage areas for use in the CPU

• Cache– High speed temporary storage for use with the CPU

• Main Memory – Random-access Memory (RAM)– High speed temporary storage – Contains programs and data currently being used– Often described in Megabytes (MB)

• Secondary Memory - Disks– Contains programs and data not currently being used– Often described in Gigabytes (GB)

Why are there so many types of memory?

• The faster memory is the more it costs– So we reduce the cost by using small

amounts of expensive memory (registers, cache, and RAM) and large amounts of cheaper memory (disks)

• Why do we need cache?– Processors are very fast and need quick

access to lots of data– Cache provides quick access to data from

RAM

What can Computers Do?

• Add, subtract, multiply, and divide numbers in registers

• Logic operations on numbers in registers– Less than, greater than, equal to– When true jump to a new instruction

• Move data between types of memory and to other input and output devices

Vocabulary Exercise• Look at two computer

advertisements• See if you can figure out what all

the words mean• Use the internet to look up

unfamiliar words – http://computer.howstuffworks.com/– http://en.wikipedia.org/– http://webopedia.com

• Which one is a better for computer games?

• Which one is better for your homework?

What is an algorithm?

• A description of how to solve a problem.– It can be in English.

• The number of squares in a cube of n * n sides is n * n + (n-1) * (n – 1) + (n – 2) + (n – 2) … + 1.

• Can you write an algorithm for how you get to school?

What is a Computer Program?

• Instructions to a computer that allow it to process some data

• You can think of it like a recipe– It contains instructions – Processes ingredients

to produce a result– You can use the same

recipe over and over

Computer Languages

• Each processor has its own language– Machine language – 0001 may be the instruction for adding two registers

• Assembly language is a low-level language– Has the same instructions as machine language but

allows people to use names for the instructions instead of numeric codes (like ADD)

– An assembler translates the names into the numeric machine language

– Executes very fast– Slow for programming

Programming is About Naming

• Computers can associate names with anything– A byte– A group of bytes– A group of letters– A file – A picture– A program, recipe, or function (method) – A type (class)

File Names

• Operating systems associate file names with locations on your hard disk– A hard disk stores values

even after the power is turned off

• When you double click on a file– The operating system

reads the data starting at that location into RAM

Names for Values

• When work with data in main memory we will usually assign a name to it. – Rather than remember the address in memory

• The computer associates the name with the address for us

• Programs are like algebra– Where you use names to make the equations

make sense• PV=nRT or e=Mc2

Programs are for People

• The computer doesn’t care what names we use

• The names are important so that our programs are understandable– To us– To others

• Names should be appropriate– Not too long or confusing

High Level Languages

• Are translated (compiled) into machine language or assembly language

• Contains more instructions than in the original machine language– Which translate into several machine

language instructions

• May have slower execution speed than an assembly language program

• Easier and faster for writing programs

Programming Languages

• A set of names that have encodings– Some names allow us to define new

encodings

• You can assign a name to represent a value (we call this a variable)

• You can assign a name to represent a function or procedure (method)

• You can assign a name to a collection of related variables and functions/procedures (class)

FORTRAN

• Acronym for formula translator

• The first high-level language

• Developed in the 1950s at IBM by John Backus

• Mostly used for scientific applications that require extensive mathematical computations.

• There have been several versions

COBOL

• Acronym for common business oriented language.

• Developed in the late 1950s and early 1960s by the Conference on Data Systems and Languages (CODASYL).

• Popular for business applications that run on large computers

• Meant to be easy to read– Very wordy to program

BASIC

• Acronym for Beginner's All-purpose Symbolic Instruction Code.

• Developed in the mid 1960s at Dartmouth College by John G. Kemeny and Thomas E. Kurtz

• Mostly used in business and education

• VisualBasic is a form of BASIC created by MicroSoft that uses visual programming– Good for graphical user interfaces

C

• Developed by Dennis Ritchie at Bell Labs in the mid 1970s

• Originally a systems programming language– Closer to machine language than other high-

level languages– Used to develop the UNIX operating system

• Popular in business and science in the 80s • Used on personal computers because took less

memory than other computer languages

C++

• Developed by Bjarne Stroustrup at Bell Labs in 1986

• Added object-oriented features to its predecessor, C.

• Very popular in the late 80s to 90s

• Popular for 3-d graphics

Java

• Developed at Sun in the early 1990s– Invented by James Gosling

• Similar to C++ in syntax but easier to use– Java is C++ --

• Cross-platform, object-oriented language

• Used in business, science, and education

• One of the fastest adopted technologies of all time

C#

• Microsoft developed object-oriented language for the .NET platform

• Created by Anders Hejlsberg (author of Turbo Pascal and architect of Delphi), Scot Wiltamuth, and Peter Golde

• Similar to Java– Garbage collection– No explicit pointers– Doesn’t compile to machine language

• Common language runtime (CLR)

Which Language?

• All high-level languages are eventually translated into machine language

• You can write the same program in any language– The computer doesn’t care what high-level

language you use

• The language matters to the programmer– How long does it take to write the program?– How hard is it to change the program?– How long does it take to execute?

Why Don’t We Just Use English?

• English is good for communication between two intelligent humans– Even then we sometimes don’t

understand

• Computers are very stupid– They basically know how to

add, compare, store, and load– Programs are very detailed

instructions• Everything must be precise and

unambiguous

Programming Exercise

• Write down instructions for how to play a game or how to make a sandwich

• Have another group read the directions and do the actions – stop anytime anything

isn’t clear and ask for clarification

Why Learn to Program?• Alan Perlis, first head of Carnegie Mellon's

Computer Science Department, made the claim in 1961 that computer science, and programming explicitly, should be part of a liberal education

• Seymour Papert claimed in the 70’s and 80’s that learning to program is “learning to think, and debug one’s own thoughts.”– If you learned to program, you learned to plan, to

debug, to handle complexity, etc.• Twenty years of research found that that is

simply not true.• Most people don’t learn to program

So, Why Learn to Program?

• The computer is the most amazingly creative device that humans have ever conceived of. If you can imagine it, you can make it “real” on a computer.

• Computers will continue to have a major impact on modern life– Movies, games, business,

healthcare, science, education, etc

Computers Are Commonplace

• Computers, or at least processors, are in many common devices

Programming is Communicating

• Alan Perlis, “You think you know when you can learn, are more sure when you can write, even more when you can teach, but certain when you can program.”

Summary• Computers are fairly simple machines

– Fancy calculators with lots of storage– But incredibly fast

• Computers have changed modern life• An algorithm is a textual description of how to

solve a problem.• Programs are instructions to a computer to

accomplish a task• Programs written in high-level languages are

translated to machine or assembly language by a compiler