david evans evans@cs.virginia cs.virginia/evans
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David Evansevans@cs.virginia.eduhttp://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?
24 August 2002 101 Things 3
0. What is Computer Science?
24 August 2002 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)
24 August 2002 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
24 August 2002 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)
24 August 2002 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 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.
24 August 2002 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
24 August 2002 101 Things 9
1. What problem did the first electronic programmable
computer solve?
24 August 2002 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)
24 August 2002 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
24 August 2002 101 Things 12
For any given ciphertext, all plaintexts are equally possible.Ciphertext: 0100111110101Key: 1100000100110Plaintext: 1000111010011 = “CS”
Why perfectly secure?
1
0 B
24 August 2002 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
24 August 2002 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
24 August 2002 101 Things 15
2. Why was the first personal computer built?
24 August 2002 101 Things 16
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
24 August 2002 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
24 August 2002 101 Things 18
3. Science, Engineering or Other?
24 August 2002 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
24 August 2002 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
24 August 2002 101 Things 21
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)
24 August 2002 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
24 August 2002 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!
24 August 2002 101 Things 24
The Liberal Arts
Trivium (3 roads)language
Quadrivium (4 roads)
numbers
Grammar Rhetoric Logic Arithmetic
Geometry
Music
Astronomy
24 August 2002 101 Things 25
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
24 August 2002 101 Things 26
4. What are the world’s most complex programs?
24 August 2002 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?
24 August 2002 101 Things 28
Human Genome
Produces60 Trillion Cells (6 * 1013)50 Million die every second!
24 August 2002 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
24 August 2002 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)
24 August 2002 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
24 August 2002 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
24 August 2002 101 Things 33
Any Questions?
24 August 2002 101 Things 34
24 August 2002 101 Things 35
4. Who Invented the Internet?
24 August 2002 101 Things 36
4a. Who Invented Networking?
24 August 2002 101 Things 37
What is a Network?
A group of three or more connected communicating entities
24 August 2002 101 Things 38
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.
24 August 2002 101 Things 39
Chappe’s Semaphore Network
Mobile Semaphore Telegraph Used in the Crimean War 1853-1856
First Line (Paris to Lille), 1794
24 August 2002 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)
24 August 2002 101 Things 41
internetwork
A collection of multiple networks connected together, so messages can be transmitted between nodes on different networks.
24 August 2002 101 Things 42
Okay, so who invented the Internet?
24 August 2002 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
24 August 2002 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.
24 August 2002 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”)
24 August 2002 101 Things 46
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
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