Computer Science 109:
Computers in Our World

Fall 2006

Princeton University

Brian Kernighan

Computer Science Dept.

Mon Jan 29 07:20:04 EST 2007

final exam answers

What we covered     2005 final exam     answers

Lecture notes:    9/18    9/20    9/25-27    10/2-4    10/9    10/11    10/16-18    10/23-25    10/25    11/6    11/8    11/13    11/15    11/20-22    11/28-30    12/4    12/6    12/11    12/13   

Problem sets:    1     2     3     4     5     6     7     8    

Solutions:    1     2     3     4     5     6     7     8    

Labs:    1     2     3     4     5     6     7     8    

Old stuff:    playlist    survey results    TA office hours    Slashdot    Princeton supercomputers    Toy simulator    binary converter    spelling correction    Ig Nobel Peace Prize    Web pages from lab 2    Javascript examples    midterm practice problems    2005 midterm    2005 midterm answers    2006 midterm answers    Benkler on open source    GPL case    Spring courses    Flamholz on net neutrality    online search privacy    social networking    Benkler on IP rights    how MP3 works    MP3 benchmark song    RSA demo    RSA factoring challenge    Touretzky's Gallery of CSS Descramblers    ATM/ISBN checker    Frequency allocation chart    crypto in the comics

Problem sets, labs and announcements will be posted only on the web page.

You are responsible for monitoring the web page frequently.

Course summary, schedule and syllabus     What was covered in 2005
Comparison of COS 109, 116 and 126   Office hours   Labs   Problem Sets   Exams   Lateness Policy   Collaboration Policy   Textbook   Bibliography

Course Summary

Computers, computing, and many things enabled by them are all around us. Some of this is highly visible, like personal computers and the Internet; much is invisible, like the computers in gadgets and appliances and cars, or the programs that fly our planes and keep our telephones and power systems and medical equipment working, or the myriad systems that quietly collect and share personal data about us.

Even though most people will not be directly involved with creating such systems, everyone is strongly affected by them. COS 109 is intended to provide a broad, if rather high level, understanding of how computer hardware, software, networks, and systems operate. Topics will be motivated by current issues and events, and will include discussion of how computers work; what programming is and why it is hard; how the Internet and the Web operate; and how all of these affect security, privacy, property and other issues. We will also touch on fundamental ideas from computer science, and some of the inherent limitations of computers.

This course is meant for humanities and social sciences students who want to understand how computing works and how it affects the world they live in. No prior experience with computers is assumed. There are no prerequisites. COS 109 satisfies the QR requirement.

The labs are complementary to the classroom work, though intended to reinforce the basic ideas. They will cover a spectrum of practical applications; two of the labs are a gentle introduction to programming in Javascript.

The course will have fundamentally the same structure as in previous years, but lectures, case studies and examples change every year according to what's happening. Privacy and security are of growing importance in an era of viruses, spyware, phishing, identity theft, and corporate and government snooping. Search engines have moved from curiosities to highly competitive and profitable businesses with vast archives of information about all of us. Digital rights management restricts our rights, and trivial patents impede our progress. Stay tuned -- this will be different by the time the course starts.


	 S  M Tu  W Th  F  S
Sep	10 11 12 13 14 15 16
	17 18 19 20 21 22 23	first class
	24 25 26 27 28 29 30	problem set 1 due; lab 1 due
Oct	 1  2  3  4  5  6  7	problem set 2 due; lab 2 due
	 8  9 10 11 12 13 14	problem set 3 due; lab 3 due
	15 16 17 18 19 20 21	problem set 4 due; lab 4 due
	22 23 24 25 26 27 28	takehome midterm due (no lab or problem set)
	29 30 31
Nov	          1  2  3  4	fall break
	 5  6  7  8  9 10 11	problem set 5 due; lab 5 due
	12 13 14 15 16 17 18	problem set 6 due; lab 6 due
	19 20 21 22 23 24 25	Thanksgiving (starts Thu, not Wed)
	26 27 28 29 30
Dec	                1  2	problem set 7 due; lab 7 due
	 3  4  5  6  7  8  9	problem set 8 due; lab 8 due
	10 11 12 13 14 15 16	last class
	17 18 19 20 21 22 23	winter break
	24 25 26 27 28 29 30
Jan	    1  2  3  4  5  6
	 7  8  9 10 11 12 13	Q/A session
	14 15 16 17 18 19 20
	21 22 23 24 25 26 27	final exam 1:30 Friend 101


This will evolve over the semester, so check it out from time to time.

Sep 18, 20:    Introduction. What's in a computer

  • Reading: (1) A famous 1945 article by Vannevar Bush, As we may think, is often cited as predicting the Internet and the web. Think about how Bush's "Memex" relates to Google and your computer. (2) The Hive is a recent informative and entertaining description of Wikipedia, the collaborative online encyclopedia. This is something definitely not foreseen by Bush, or by other much more recent prognosticators. (Both of these articles are included in a retrospective in The Atlantic Monthly, which has an admirable history of publishing outstanding technology articles.) Wikipedia is an excellent source for many of the topics of this course; for the first few weeks, check out its articles on bits, bytes, and binary.
  • Problem set 1, due Sep 27

    Sep 25, 27:    How does it work. Representation of information

  • Reading: Preliminary discussion of the logical design of an electronic computing instrument. Original article by Burks, Goldstine and von Neumann. The first page is a remarkably clear description of how computers are organized, though in archaic terminology. No need to read much beyond that, though section 3 is also quite germane.
  • Lab 1: Operating systems, SSH, mail, file transfer, a first web page
  • Problem set 2, due Oct 4

    Oct 2, 4:    What the components are and how they are made

  • Reading: Intel web pages on How microprocessors work, how chips are made, and Moore's Law.
  • Lab 2: HTML and web page design
  • Problem set 3, due Oct 11

    Oct 9, 11:    Software and algorithms

  • Reading: a Wikipedia article on algorithms. None of the details matter for the course, but skim it to see the different ways the basic ideas are expressed.
  • Lab 3: Advanced HTML
  • Problem set 4, due Oct 18

    Oct 16, 18:    Languages, programming; Javascript

  • Reading: some Javascript tutorials, out of many: one, two, three.
  • Lab 4: Graphics
  • Problem set 5, due Nov 8

    Oct 23, 25:    Javascript. Operating systems

  • Reading: Professor Michael Mahoney's 1998 Freshman seminar on The World of the Computer points to a good collection of interesting reading, including an oral history of Unix.
  • Take-home midterm, due Oct 27 at 5 PM. To be handed out in class on Oct 23. A review session will be scheduled.   
  • No lab, no problem set due this week.

    [fall break]

    Nov 6, 8:    File systems, information storage. Applications.

  • Reading: These articles describe some of the realities of software development: Patriot missile defense system failure, a GAO report; A Bug and a Crash, by James Gleick (who has written numerous other fine technical articles); the Therac-25 accidents (long and scary); a Mars Rover software problem (more technical, recent). Skim these and think about the many computer systems that we rely on.
  • To prepare for the upcoming elections, browse these items on electronic voting machines, a troubling place to encounter the problems of software development: EFF page, Summary judgement, security analysis of a voting machine, the Diebold variations.
  • Lab 5: Introduction to Javascript / Programming fundamentals
  • Problem set 6, due Nov 15

    Nov 13, 15:    Networks & communications, Internet

  • Reading: Skim some of the Internet history papers
  • Lab 6: More Javascript / user interfaces
  • Problem set 7, due Nov 29:   

    Nov 20, 22:    World Wide Web

  • Reading: browse around in some ancient Web history
  • Lab 7: Spreadsheets
  • Problem set 8, due Dec 6

    Nov 27, 29:    Threats; security and privacy

  • No problem set or lab this week, but there is class on Nov 22.
  • Reading: what do they know about you? -- general information on privacy; the Electronic Privacy Information Center is also informative. Here's an entertaining view of the future.

    Dec 4, 6:    Cryptography; Compression & error detection.

  • Reading: FAQ on cryptography. The Heavenly Jukebox, by Charles Mann, an excellent article from the middle of the Napster era, circa 2000. Think about how much has changed since, and how much is the same, but amplified. DRM, spyware and security by Ed Felten and Alex Halderman.
  • Lab 8: Privacy

    Dec 11, 13:    Intellectual property. Case studies. Wrapup

  • Reading: The Chilling Effects Clearinghouse is an excellent, authoritative site dealing with the legal issues around free speech, intellectual property, and online activities. The Electronic Frontier Foundation and Greplaw also have good coverage of these issues. Finally, Your Rights Online at Slashdot points to numerous stories about online rights.
  • No more labs or problem sets!

    [winter break]

    Jan 10:    Q/A session. (Depending on interest, this date might move around, and we might arrange for two sessions.)

    Jan 23:    Final exam, 1:30 pm, Friend 101   

  • Note that this date is set by the registrar and I am not allowed to make any individual arrangements. Take this into account when making travel plans.
  • Registrar's Official Exam Schedule page.

    Administrative Information

    Lectures: Monday and Wednesday 11:00-12:20, Peyton 145.
    Lab sessions: Friend 009.

    Professor: Brian Kernighan, 311 CS Building, 609-258-2089, bwk at cs dot princeton dot edu. Office hours Monday and Tuesday 2:30-4:00, or by appointment, or just drop in if my door is open, which it usually is.

    Teaching Assistants and office hours:
        Sonya Nikolova, nikolova@, CS 416, Wednesday 3:00-4:00
        Ji Min Song, jisong@, CS 103A, Tuesday 11:00-12:00
        Qian Xi, qxi@, CS 216, Monday 4:00-5:00
        Yaping Zhu, yapingz@, CS 103C, Wednesday 2:00-3:00

    Send mail to with any questions any time.


    There will be eight labs to give hands-on practice in important aspects of computing. The labs are designed to be easily completed within three hours, if you have read through the instructions beforehand, which should take at most an hour. Assistants will be available to help out during scheduled lab sections. Labs are held in the Friend Center; they can be done in dorm rooms or campus clusters, but there will be lab assistants in Friend, and no help elsewhere.

    Labs are together worth about 20 percent of the course grade. To receive credit, students must complete labs by 5:00 PM Friday of the week they are assigned, unless there are extraordinary circumstances.

    Labs start the week of Monday, September 25. There will be no labs in the week before fall break, Thanksgiving week, or the last week of classes. Lab sessions are Tuesday and Thursday at 1:30 and Monday through Thursday at 7:30 in Friend 009.

    Problem sets:

    Eight weekly problem sets, together worth about 20 percent of the course grade, will be assigned. Problems are intended to be straightforward, reinforcing material covered in class and providing practice in quantitative reasoning, and should take 1-2 hours to complete.

    Problem set solutions will be due by 5:00 PM Wednesday, one week after they are assigned. Turn in solutions in the box outside room 311 on the third floor of the CS building, or at the beginning of class. There will be no problem set due in the week before Fall break (midterm instead) or the last week of the term. No credit can be given for late submissions unless there are extraordinary circumstances, and in no case after solutions have been discussed in class.

    Lateness Policy

    For both labs and problem sets, extracurricular activities and heavy workloads in other classes don't count as "extraordinary", no matter how unexpected or important or time-consuming.

    Nevertheless, everyone gets truly behind from time to time. In recognition of this, you are allowed two late submissions (no more than 4 days late in each case). Please let us know ahead of time that you will be submitting late so we can keep track of things more easily.

    Students must complete all labs and assignments to pass the course.

    Collaboration Policy

    You are encouraged to collaborate on problem sets, but must turn in separate solutions; the names of your collaborators must appear on each submission.

    (This elaboration of the policy on collaboration is paraphrased from COS 126:) You must reach your own understanding of the problem and discover a path to its solution. During this time, discussions with friends are encouraged. However, when the time comes to write down the solution to the problem, such discussions are no longer appropriate -- the solution must be your own work, so you must work on the written assignment on your own. If you have a question, you can certainly ask friends or teaching assistants, but do not, under any circumstances, copy another person's work or present it as your own. This is a violation of academic regulations.


    There may well be short, unannounced, in-class quizzes to verify your existence and test your understanding. These will be worth 5-10 percent of the course grade. Regular class attendance is expected; frequent absences are grounds for a failing grade regardless of other performance.

    A take-home, open-book midterm examination will be given during the week before fall break. It will cover material presented and discussed in class and any relevant reading through Wednesday, October 18. It will be worth 20 percent of the course grade.

    An open-book final examination will be given during the fall-term exam period. It will cover all of the relevant readings and material presented and discussed in class. It will be worth 35-40 percent of the course grade.

    Sorry: no collaboration on take-home exams or the final.

    Textbook and Readings

    There is no assigned text for this course; I have never found anything that seems right. Fluency with Information Technology by Larry Snyder (Addison-Wesley, 2005) is the closest so far, but it's expensive. Notes and readings will be posted online. The weekly readings are for background, context, general education, and/or entertainment. You are not expected to know any of the detailed content, but you should at least understand the general ideas.

    I have half a dozen books that purport to cover the same kinds of areas as the course. None feels right to me, but among them there is coverage of many class topics, so you might find it helpful to browse in them a bit. Available for short term loans; drop in to take a look. Meanwhile, check out the bibliography for other suggestions.