1. David Evans evans@cs.virginia.edu http://www.cs.virginia.edu/evans/talks/cs390-s04/ 1001 Things Every Self-Respecting Computer Scientist Should Know 2 CS390 14 April 2004

2. http://www.cs.virginia.edu/evans/talks/cs390-s04/ 2 1001 Questions 0000What is Computer Science? 0001What problem did the first electronic programmable computer solve? 0010Why was the first “personal computer” built? 0011Is Computer Science a science, engineering or other? 0100What are the world’s most complex programs? 0101How do Computer Scientists manage complexity? 0110 What is and is not computable? 0111 Who was the first object-oriented programmer? 1000 Who invented the Internet?

3. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 3 0. What is Computer Science?

4. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 4 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. Euclid’s Elements, Book VII, Proposition 2 (300BC)

5. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 5 The note on the inflected line is only difficult to you, because it is so easy. There is in fact nothing in it, but you think there must be some grand mystery hidden under that word inflected! Whenever from any point without a given line, you draw a long to any point in the given line, you have inflected a line upon a given line. Ada Byron (age 19), letter to Annabella Acheson (explaining Euclid), 1834

6. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 6 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)

7. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 7 Geometry vs. Computer Science Geometry (mathematics) is about declarative knowledge: “what is” Computer Science is about imperative knowledge: “how to” –Ways of describing imperative processes (computations) –Ways of reasoning about (predicting) what imperative processes will do Language Logic

8. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 8 1.What problem did the first electronic programmable computer solve?

9. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 9 Colossus First Programmable Computer Bletchley Park, 1943 Designed by Tommy Flowers 10 Colossi in operation at end of WWII Destroyed in 1960, kept secret until 1970s 2 years before ENIAC – calculating artillery tables

10. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 10 Colossus’ Problem Decode Nazi high command messages from Lorenz Machine XOR encoding: C i = M i  K i –Perfect cipher, if K is random and secret

11. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 11 For any given ciphertext, all plaintexts are equally possible. Ciphertext: 0100111110101 Key: 1100000100110 Plaintext: 1000111010011 = “CS” Why perfectly secure? 1 0 B

12. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 12 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

13. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 13 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

14. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 14 2.Why was the first personal computer built?

15. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 15 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 memory 64KB of read-only memory

16. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 16 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

17. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 17 3. Science, Engineering or Other?

18. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 18 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

19. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 19 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

20. Computing Power 1969-2004 (in Apollo Control Computer Units) Moore’s Law: computing power doubles every 18 months! If Apollo Guidance Computer power is 1 inch, you have 5 miles! 1GB/4KB = 262 144 but why not 18M x?

21. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 21 Constraints Computer Scientists Face Not like those for engineers: –Cost, weight, physics, etc. –If 1 Million times what NASA had in 1969 isn’t enough for you, wait until 2007 and you will have 4 Million times… More like those for Musicians and Poets: –Imagination and Creativity –Complexity of what we can understand –Cost of human effort

22. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 22 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!

23. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 23 The Liberal Arts Trivium (3 roads) language Quadrivium (4 roads) numbers GrammarRhetoricLogicArithmetic Geometry Music Astronomy

24. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 24 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 understand meaning to describe computations Interfaces between components, discourse between programs and users Logic for controlling and reasoning about computations Yes Yes (graphics) Yes (read Gödel, Escher, Bach) Sort of, read Neil DeGrasse Tyson’s essay Trivium Quadrivium

25. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 25 4. What are the world’s most complex programs?

26. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 26 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?

27. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 27 Human Genome Produces 60 Trillion Cells (6 * 10 13 ) 50 Million die every second!

28. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 28 How Big is the Make-a-Human Program? 3 Billion Base Pairs –Each nucleotide is 2 bits (4 possibilities) –3B bases * 1 byte/4 pairs = 750 MB –Highly redundant encoding (21/64) ~ 250 MB –About 95% of it is junk ~ 12 MB 1 CD ~ 650 MB Wal-Mart’s database is >280 Terabytes

29. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 29 Expressiveness of DNA Genetic sequence for 2 humans differs in only 2 million bases –4 million bits = 0.5 MB 1/3 of a floppy disk <1% of Windows 2000

30. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 30 5. How do Computer Scientists manage complexity?

31. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 31 Abstraction Adapted from Gerard Holzmann’s FSE Slides

32. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 32 Abstraction in Computer Science Procedural Abstraction (CS101) –Abstract what to do from specific values to do it to Data Abstraction (CS201) –Abstract away representation details by specifying what you can do with something Abstraction by Specification (CS340) –Abstract away how details by saying what a procedure does

33. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 33 6. What is and is not computable?

34. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 34 Halting Problem Input: a procedure P Output: true if P halts (finishes execution), false otherwise. Is it possible it implement a procedure that correctly implements halts and always terminates?

35. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 35 Halts is not Computable boolean contradictHalts (Program P) if (halts “contradictHalts (P);”) while (true) ; else return true; If contradictHalts halts, the if test is true if enters the while loop - it doesn’t halt! If contradictHalts doesn’t halt, the if test if false, and it evaluates to true. It halts!

36. Learned Discussion on Computability (Video)

37. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 37 Ali G Multiplication Problem Input: two d digit numbers (mostly 9s) Output: the product of all the numbers Is it computable? Yes – a straightforward algorithm solves it. Is it tractable? Yes – elementary multiplication solves it in O( d 2 ) Can real computers solve it?

39. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 39 Ali G was Right! Theory assumes ideal computers: –Unlimited, perfect memory –Unlimited (finite) time Real computers have: –Limited memory, time, power outages, flaky programming languages, etc. –There are many decidable problems we cannot solve with real computer: the numbers do matter

40. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 40 6. Who was the first Object-Oriented Programmer? “I invented the term Object-Oriented and I can tell you I did not have C++ in mind.” Alan Kay

41. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 41 What is an Object? Packaging state and procedures –State: the representation What a thing is –Procedures: methods and constructors What you can do with it

42. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 42 Object-Oriented Programming State of mind where you program by thinking about objects It is difficult to reach that state of mind if your language doesn’t have: –Mechanisms for packaging state and procedures (Java has class) –Subtyping (Java has extends and implements ) Other things can help: dynamic dispatch, implementation inheritance, automatic memory management, Krispy Kremes, etc.

43. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 43 Who was the first object-oriented programmer?

44. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 44 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…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 Augusta Byron, 1842

45. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 45 7. Who Invented the Internet?

46. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 46 What is a Network? A group of three or more connected entities communicating indirectly Ancient Greeks had beacon chain networks on Greek island mountaintops

47. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 47 Chappe’s Semaphore Network Mobile Semaphore Telegraph Used in the Crimean War 1853-1856 First Line (Paris to Lille), 1794

48. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 48 internetwork A collection of multiple networks connected together, so messages can be transmitted between nodes on different networks.

49. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 49 The First Internetwork 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, transmit message to Sweden, but they don’t help. Denmark signs treaty with Britain, and stops communications with Sweden

50. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 50 First Use of The 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 million televisions worldwide.

51. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 51 Licklider and Taylor’s Vision Available 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

52. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 52 The Modern Internet Packet Switching: Leonard Kleinrock (UCLA), Donald Davies and Paul Baran, Edelcrantz’s signaling network (1809) 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”)

53. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 53 Summary Computer Science is a real intellectual discipline: not like “Automotive Engineering” or “Toaster Science” Seek out and work on problems that are at the intellectual core of computer science Most of Computer Science was not invented yesterday (or even since you were born) For slides and links to more information: http://www.cs.virginia.edu/evans/talks/cs390-s04/

54. 14 April 2004 http://www.cs.virginia.edu/evans/talks/cs390-s04/ 54 Any Questions?