david evans [email protected] cs.virginia/evans

David [email protected]://www.cs.virginia.edu/evans

101 Things Every Computer Scientist

Should Know

University of VirginiaDepartment of Computer Science

2

24 August 2002 101 Things 2

101 Questions0. What is Computer Science?1. What problem did the first electronic

programmable computer solve?2. Why was the first “personal computer” built?3. Is Computer Science a science, engineering

or other?4. What are the world’s most complex

programs?


0. What is Computer Science?


Let AB and CD be the two given numbers not relatively prime. It is required to find the greatest common measure of AB and CD.

If now CD measures AB, since it also measures itself, then CD is a common measure of CD and AB. And it is manifest that it is also the greatest, for no greater number than CD measures CD. But, if CD does not measure AB, then, when the less of the numbers AB and CD being continually subtracted from the greater, some number is left which measures the one before it.

Euclid’s Elements, Book VII, Proposition 2 (300BC)


By the word operation, we mean any process which alters the mutual relation of two or more things, be this relation of what kind it may. This is the most general definition, and would include all subjects in the universe. Again, it might act upon other things besides number, were objects found whose mutual fundamental relations could be expressed by those of the abstract science of operations, and which should be also susceptible of adaptations to the action of the operating notation and mechanism of the engine... Supposing, for instance, that the fundamental relations of pitched sounds in the science of harmony and of musical composition were susceptible of such expression and adaptations, the engine might compose elaborate and scientific pieces of music of any degree of complexity or extent.

Ada, Countess of Lovelace, around 1830


What is the difference between

Euclid and Ada?

“It depends on what your definition of ‘is’ is.” Bill Gates (at Microsoft’s anti- trust trial)


Geometry vs. Computer Science

• Geometry (mathematics) is about declarative knowledge: “what is”

If now CD measures AB, since it also measures itself, then CD is a common measure of CD and AB

• Computer Science is about imperative knowledge: “how to”Computer Science has nothing to do with beige (or translucent blue) boxes called “computers” and is not a science.


Computer Science

“How to” knowledge:• Ways of describing and carrying out

imperative processes (computations)• Ways of reasoning about (predicting)

what imperative processes will do


1. What problem did the first electronic programmable

computer solve?


ColossusFirst Programmable Computer

• Bletchley Park, 1943• Designed by Tommy

Flowers• 10 Colossi in operation at

end of WWII• Destroyed in 1960, kept

secret until 1970s• (ENIAC: 1946 – calculating

artillery tables)


Colossus’ Problem• Decode Nazi high

command messages from Lorenz Machine

• XOR encoding:Ci = Mi Ki

– Perfect cipher, if K is random and secret


For any given ciphertext, all plaintexts are equally possible.Ciphertext: 0100111110101Key: 1100000100110Plaintext: 1000111010011 = “CS”

Why perfectly secure?

1

0 B


Breaking Lorenz• Operator and receiver need same

keys• Generate key bits using rotor

machine, start with same configuration

• One operator retransmitted a message (but abbreviated message header the second time!)

• Enough for Bletchley Park to figure out key – and structure of machine that generated it!

• But still had to try all configurations


Colossus

• Read ciphertext and Lorenz wheel patterns from tapes

• Tried each alignment, calculated correlation with German

• Decoded messages (63M letters by 10 Colossus machines) that enabled Allies to know German troop locations to plan D-Day


2. Why was the first personal computer built?


Apollo Guidance Computer, 1961-69

1 cubic foot, 70 pounds

Why did they need to fit the guidance computer in the rocket?

4KB of read/write magnetic core memory64KB of read-only memory


AGC History

• Needed all guidance to be on board in case Soviets jammed signals for Earth

• Design began in 1961• Risky decision to use Integrated Circuits

(invented in 1958)– Building 4 prototypes used 60% of all ICs

produced in the US in the early 60s!– Spurred industry growth


3. Science, Engineering or Other?


Science?• Understanding Nature through

Observation– About real things like bowling balls, black

holes, antimatter, electrons, comets, etc.• Math and Computer Science are about

fake things like numbers, graphs, functions, lists, etc.– Computer Science is a useful tool for doing

real science, but not a real science


Engineering?“Engineering is design under constraint… Engineering is synthetic - it strives to create what can be, but it is constrained by nature, by cost, by concerns of safety, reliability, environmental impact, manufacturability, maintainability and many other such 'ilities.' ...”

William Wulf


Computing Power 1969-2002(in Apollo Control Computer Units)

0

500000

1000000

1500000

2000000

2500000

3000000

3500000

4000000

450000019

6919

7119

7219

7419

7519

7719

7819

8019

8119

8319

8419

8619

8719

8919

9019

9219

9319

9519

9619

9819

9920

0120

02

Moore’s Law: computing power doubles every 18 months!

If Apollo Guidance Computer power is 1 inch, you have 5 miles!(1GB/4KB = 262144)


Constraints Computer Scientists Face• Not like those for engineers:

– Cost, weight, physics, etc.– If 4 Million times what people had in 1969

isn’t enough for you, wait until 2006 and you will have 32 Million times…

• More like those for Musicians and Poets:– Imagination and Creativity– Complexity of what we can understand

• Cost of human effort


So, what is computer science?

• Science– No: its about fake things like numbers, not

about observing and understanding nature • Engineering

– No: we don’t have to deal with engineering-type constraints

Must be a Liberal Art!


The Liberal Arts

Trivium (3 roads)language

Quadrivium (4 roads)

numbers

Grammar Rhetoric Logic Arithmetic

Geometry

Music

Astronomy


Liberal Arts• Grammar: study of meaning in written

expression• Rhetoric: comprehension of verbal and

written discourse• Logic: argumentative discourse for

discovering truth• Arithmetic: understanding numbers• Geometry: quantification of space• Music: number in time• Astronomy: laws of the planets and

stars

Yes, we need to understandmeaning to describe

computations

Interfaces between components, discourse

between programs and usersLogic for controlling and reasoning about

computations

Yes

Yes (graphics)

Yes (read Gödel, Escher, Bach)

Yes, read Neil DeGrasse Tyson’s essay

Triv

ium

Qua

driv

ium


4. What are the world’s most complex programs?


Complex Programs• Apollo Guidance Software

– ~36K instructions• F-22 Steath Fighter Avionics Software

– 1.5M lines of code (Ada)• 5EEE (phone switching software)

– 18M lines• Windows XP

– ~50M lines (1 error per kloc ~ 50,000 bugs)• Anything more complex?


Human Genome

Produces60 Trillion Cells (6 * 1013)50 Million die every second!


How Big is the Make-a-Human Program?

• 6 Billion Base Pairs– Each nucleotide is 2 bits (4 possibilities)– 6B bases * 1 byte/4 pairs = 1.5GB

1 CD ~ 650 MB


Encoding is Redundant

• DNA encodes proteins• Every sequence of 3 base pairs one of

20 amino acids (or stop codon)– 21 possible codons, but 43 = 64 possible

values– So, really only 1.5GB * (21/64) ~ 500 MB

• Most of genome is inactive (perhaps only 3% = 45MB matters)


Expressiveness of DNA

• Genetic code for 2 humans differs in only 2.1 million bases– 4 million bits = 0.5 MB

• Trillions of creatures, over millions of years, had to die to create this program!

1/3 of a floppy disk<1% of Windows 2000


Summary

• Computer Science is a real intellectual discipline: not like “Automotive Engineering”

• Lots of interesting computer science happened before 2001

• All you have left to do to get a PhD, is convince 5 PhDs you deserve one!

1950 1M years ago


Any Questions?


4. Who Invented the Internet?


4a. Who Invented Networking?


What is a Network?

A group of three or more connected communicating entities


Beacon Chain NetworkingThus, from some far-away beleaguered island, where all day long the men have fought a desperate battle from their city walls, the smoke goes up to heaven; but no sooner has the sun gone down than the light from the line of beacons blazes up and shoots into the sky to warn the neighbouring islanders and bring them to the rescue in their ships. Iliad, Homer, 700 BC

Chain of beacon’s signaled Agammemnon’s return (~1200BC), spread on Greek peaks over 600km.


Chappe’s Semaphore Network

Mobile Semaphore Telegraph Used in the Crimean War 1853-1856

First Line (Paris to Lille), 1794


Latency and Bandwidth• Napoleon’s Network (Paris to Toulon, 475 miles)• Latency: 13 minutes (1.6s per mile)

– What is the delay at each signaling station, how many stations to reach destination

– At this rate, it would take ~1 hour to get a bit from California

• Bandwidth: 2 symbols per minute (98 possible symbols, so that is ~13 bits per minute– How fast can signalers make symbols– At this rate, it would take you 122 hours to download

department homepage (without pictures)


internetwork

A collection of multiple networks connected together, so messages can be transmitted between nodes on different networks.


Okay, so who invented the Internet?


The First Internet• 1800: Sweden and Denmark worried about

Britain invading• Edelcrantz proposes link across strait

separating Sweden and Denmark to connect their (signaling) telegraph networks

• 1801: British attack Copenhagen, network transmit message to Sweden, but they don’t help.

• Denmark signs treaty with Britain, and stops communications with Sweden


First Use of Internet• October 1969: First packets on the

ARPANet from UCLA to Stanford. Starts to send "LOGIN", but it crashes on the G.

• 20 July 1969:

Live video (b/w) and audio transmitted from moon to Earth, and to several hundred televisions worldwide.


The Modern Internet• Packet Switching: Leonard Kleinrock (UCLA)

thinks he did, Donald Davies and Paul Baran, Edelcrantz’s signalling network (1809) sort of did it

• Internet Protocol: Vint Cerf, Bob Kahn• Vision, Funding: J.C.R. Licklider, Bob Taylor • Government: Al Gore (first politician to promote

Internet, 1986; act to connect government networks to form “Interagency Network”)


Licklider and Taylor’s VisionAvailable within the network will be functions and services to which you subscribe on a regular basis and others that you call for when you need them. In the former group will be investment guidance, tax counseling, selective dissemination of information in your field of specialization, announcement of cultural, sport, and entertainment events that fit your interests, etc. In the latter group will be dictionaries, encyclopedias, indexes, catalogues, editing programs, teaching programs, testing programs, programming systems, data bases, and – most important – communication, display, and modeling programs. All these will be – at some late date in the history of networking - systematized and coherent; you will be able to get along in one basic language up to the point at which you choose a specialized language for its power or terseness.

J. C. R. Licklider and Robert W. Taylor, The Computer as a Communication Device, April 1968

david evans [email protected] cs.virginia/evans

Documents