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David Evans

David Evans

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David Evans

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  1. 101 Things Every Computer Scientist Should Know 2 David Evans University of Virginia Department of Computer Science

  2. 101 Questions 0. 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? 101 Things

  3. 0. What is Computer Science? 101 Things

  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. 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) 101 Things

  5. 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 101 Things

  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) 101 Things

  7. Geometry vs. Computer Science • Geometry (mathematics) is about declarative knowledge: “what is” If now CD measures AB, since it also measures itself, then CDis 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. 101 Things

  8. Computer Science “How to” knowledge: • Ways of describing and carrying out imperative processes (computations) • Ways of reasoning about (predicting) what imperative processes will do 101 Things

  9. 1. What problem did the first electronic programmable computer solve? 101 Things

  10. 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) 101 Things

  11. Colossus’ Problem • Decode Nazi high command messages from Lorenz Machine • XOR encoding: Ci = Mi Ki • Perfect cipher, if K is random and secret 101 Things

  12. Why perfectly secure? For any given ciphertext, all plaintexts are equally possible. Ciphertext: 0100111110101 Key: 1100000100110 Plaintext: 1000111010011 = “CS” 1 B 0 101 Things

  13. 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 101 Things

  14. 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 101 Things

  15. 2. Why was the first personal computer built? 101 Things

  16. Apollo Guidance Computer, 1961-69 1 cubic foot, 70 pounds 4KB of read/write magnetic core memory 64KB of read-only memory Why did they need to fit the guidance computer in the rocket? 101 Things

  17. 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 101 Things

  18. 3. Science, Engineering or Other? 101 Things

  19. 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 101 Things

  20. 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 101 Things

  21. Computing Power 1969-2002(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 = 262144) 101 Things

  22. 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 101 Things

  23. 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! 101 Things

  24. The Liberal Arts language numbers Quadrivium (4 roads) Trivium (3 roads) Grammar Rhetoric Logic Arithmetic Music Geometry Astronomy 101 Things

  25. Liberal Arts Yes, we need to understand meaning to describe computations • 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 Trivium Interfaces between components, discourse between programs and users Logic for controlling and reasoning about computations Yes Yes (graphics) Quadrivium Yes (read Gödel, Escher, Bach) Yes, read Neil DeGrasse Tyson’s essay 101 Things

  26. 4. What are the world’s most complex programs? 101 Things

  27. 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? 101 Things

  28. Human Genome Produces 60 Trillion Cells (6 * 1013) 50 Million die every second! 101 Things

  29. 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 101 Things

  30. 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) 101 Things

  31. 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 101 Things

  32. 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 101 Things

  33. Any Questions? 101 Things

  34. 101 Things

  35. 4. Who Invented the Internet? 101 Things

  36. 4a. Who Invented Networking? 101 Things

  37. What is a Network? A group of three or more connected communicating entities 101 Things

  38. Beacon Chain Networking Thus, 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. 101 Things

  39. Chappe’s Semaphore Network First Line (Paris to Lille), 1794 Mobile Semaphore TelegraphUsed in the Crimean War 1853-1856 101 Things

  40. 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) 101 Things

  41. internetwork A collection of multiple networks connected together, so messages can be transmitted between nodes on different networks. 101 Things

  42. Okay, so who invented the Internet? 101 Things

  43. 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 101 Things

  44. 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. 101 Things

  45. 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”) 101 Things

  46. 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 101 Things