Reading Guide, Spring '95

This page summarizes the goals and important ideas from each week of Computer Science 100, including lists of important terms and ideas from the text, The Most Complex Machine. Information is available for

First Week

Chapter 1. Introduction: What Computers Do

Your primary goal in reading the first chapter should be to understand "structured complexity", the idea that complex systems can be built up level-by-level from simple components. I claim that complexity, and this approach to dealing with complexity, are central ideas in computers science.

It is important to understand what "level-by-level" means here. A person can handle only so much complexity in one gulp; this seems to be a basic fact about the way our brains work. But we do have this neat ability to "chunk" together a manageable portion of complexity and then to deal with it as a unit. That unit can then be used as a building block in the construction of another, higher level of complexity. The step from one level to the next is easy; the step from the bottom level to the top level can be breathtaking. It is only through multiple levels of chunking that real complexity is achieved.

Three main examples of structured complexity are introduced in Chapter 1: Complex data structures are built level-by-level starting from single bits; complex circuits, including entire computers, are build level-by-levle starting from transistors, which are simple switches; and complex programs are build level-by-level from simple machine-language instructions.

Important terms and ideas from Chapter 1:

   Mechanical manipulation of symbols    Fetch-and-execute cycle
   Bits                                  Program counter
   Binary numbers                        Machine language
   ASCII code                            High-level language
   Pixels                                Loops and decisions
   Transistor                            Jump instructions
   AND, OR and NOT gates                 Subroutines
   Main memory                           Structured complexity
   CPU (Central Processing Unit)         Chunking

Second Week

Chapter 2. Teaching Silicon to Think

In Chapter 2, I have asked you to read only: from the beginning of the chapter through Subsection 2.2.2, the Chapter Summary, and Questions 1 through 4 (along with their answers in the back of the book). You should also be familiar in general way, from lecture and lab, with some of the material in the rest of chapter.

The goal of this material is simply to understand how circuits can be built that compute. A circuit has inputs and outputs. If a circuit does not have any feedback loops, then it associates some output to each possible combination of inputs. One could make a table showing the output for each set of inputs. This table describes the computation performed by the circuit. In this chapter, you learn how to go in the other direction: Given any input/output table describing a desired computation, it is possible to build a circuit to perform that computation. An intermediary in this process is Boolean algebra, since there is a neat relationship between Boolean algebra and circuits without feedback loops.

In practice, in designing large circuits, we also make use of "structured complexity" by designing components to perform small computations and combining them to perform the complex computations that computers do. We can build a circuit for adding 16-bit binary numbers, for example. And we can build an ALU that can do several different arithmetic and logical operations.

A small but important side issue in this chapter is what to do with circuits that DO have feedback. These are used primarily to build memory circuits. In a memory circuit, the output doesn't just depend on the current input: The output can depend on what happened in the past.

Important ideas in Chapter 2:

    Rules for AND, OR, and NOT gates
    The correspondences among true/false, 1/0, and ON/OFF
    Boolean algebra
    Input/Output tables
    Adding binary numbers
    The basic idea and operation of an ALU
    The basic idea and operation of a multibit memory

Third Week

Chapter 3. Building a Computer

In Chapter 3, I have asked you to read from the beginning of the chapter, Section 3.1 (omitting the rather technical second half of Subsection 3.1.1), Section 3.3, the Chapter Summary, and Questions 5, 6, and 7. From the lab, you should also be familiar with the basic idea of assembly language and with some of the most basic assembly language instructions.

The goal is to understand how a circuit can be constructed that can automatically and mechanically perform computations under the control of a program that is stored in memory. Chapter 2 showed how circuits can compute and how they can "remember". The question now is, how can a computer execute a program? This is done by the "fetch-and-execute" cycle. Each cycle represents the execution of one machine language instruction. It takes the CPU several steps to perform each fetch-and-execute cycle. Here are the key ideas:

It is important for you to understand that this is a mechanical process made up of simple steps. Nevertheless, it is the basis for running programs of any degree of complexity.

Important terms and ideas in Chapter 3:

    Main memory
    Address of a location in memory
    Fetch-and-execute cycle implemented as a sequence of simple steps
    Control wires
    Control circuit
    Basic idea of assembly language
    Basic idea of the operation of xComputer

Fourth Week

Chapter 4. Theoretical Computers

The goal of Chapter 4 is to provide some basic understanding of what might be called the "nature of computation". What can we say, in general terms, about what it means to "compute" and what can be done by computing?

In a sense, this question can be answered by looking at what any computer does. It turns out that it doesn't depend on which computer you look at. A fundamental and surprising result in computer science is that, ignoring limitations of memory and running time, all computers are equivalent in the problems they can solve. This property of a computer--that any computer can do anything that any other computer can do--is called "computational universality." This can be rigorously proved. The "Church-Turing Thesis" is the claim that even if someone comes up with some "new and improved" type of computation, it will still be something that computers could have done.

The second section of Chapter 4 introduces Turing Machines to show just how simple a machine can be and still do computation. You'll also find another example of constructing a complex system, in this case a Turing Machine, from simpler components. Look for the analogy between building Turing Machines and writing programs.

The last section shows that there are questions about computation that cannot be answered, at least not by computation. The Halting Problem asks: Write a program that will be able to analyze any program and tell, in advance, whether or not it will ever halt when it is run. We can prove that this is unsolvable, that is, that no such program can ever be written.

Important terms and Ideas from Chapter 4:

    Computational universality
    Simulation, including:  Translation
                            Low-level simulation
    The Church-Turing Thesis
    Turing Machines, including:  the tape
                                 the rules
                                 the method of operation
    Computable functions
    Universal Turing Machine
    Unsolvable problems
    The halting problem

Fifth Week

Section 5.2. Usable Computers

A test this week limited the amount of material that could be covered. The reading from Chapter 5 is just one section, which deals with how real computers work. Real computer systems have more than just a CPU and main memory. They include many devices, such as disk drives, keyboards, monitors, printers, scanners, sound cards, and network connections. The purpose of Section 5.2 is to explain something about how such systems can be organized and how they work. The important ideas that you should be familiar with include:

     Operating systems, including:  device drivers
                                    user interface
     Interrupts and interrupts handlers
     Busses for communicatin among devices

The lab this week was unusual in that it introduced a lot of material that was not previously covered in class. The lab was an introduction to a "high-level programming language", xTurtle. The idea of the lab was that having seen similar ideas in other contexts, you should be able to absorb the basics of the xTurtle language. The ideas that you should take away from the lab include:

     built-in xTurtle subroutines (forward, turn, PenUp, ...)
     assignment statements
     loop statements
     if statements

These ideas are also covered, along with some more theoritical stuff, in Sections 6.1 and 6.2 of the text.

Sixth Week

Chapter 6. Programming

Chapter 6 is the first of three chapters on programming. (We will only cover the first two of these in this course, but I would urge you to read Chapter 8 some time, maybe even after the course is over.) Chapter 6 introduces a made-up high-level programming language called "xTurtle" and uses it to illustrate some of the fundamental ideas in computer programming. Besides the specifics of xTurtle that were covered in the previous week's lab, the important ideas in this chapter include:

     software engineering
     syntax and semantics
     subroutine call statements
     flow of control in a program
     BNF (Backus-Naur Form)
     processes and states
     preconditions and postconditions

The syntax of a language is the set of rules that determines what strings of symbols are legally part of that language; the syntax of a program or statement refers to its "structure," that is, to the way this one particular string of symbols conforms to the grammar of the language. Syntax is mechanical, and can be checked by a computer. BNF is a formal method for describing the syntax of a language. A BNF "template" is a picture or recipee for construting legal strings of symbols.

The semantics of a language is the set of rules that determines the meaning of syntactically correct strings of symbols in that language; the semantics of a program or a statement in a programming language can be thought of as the effect that that program or statement will have when it is executed. As we know from the unsolvability of the Halting Problem, the only completely general way to determine the meaning of a program is to run it. To deal with semantics is a somewhat formal way, we can use states and processes. The meaning of a statement in a program can be thought of as the change that it causes in the state.

Preconditions and postconditions are especially important. They are related to the idea that programs should be designed, not simply thrown together. Programming is difficult, but there are ways of thinking about and approaching programming problems that can make them more manageable. Preconditions and postconditions are one of the tools of software engineering.

Seventh Week

Chapter 7. Subroutines and Recursion

The reading for this week is Chapter 7, not including Section 7.4. The first section covers the intricacies of defining and using new subroutines in the xTurtle language. The second section concentrates more on the design of programs and on the software life cycle. The third introduces recursion and recursive subroutines in xTurtle. Important ideas include:

     subroutine declarations
     dummy parameters and actual parameters
     local variables and global variables
     black boxes
     interface and implementation
     modules and modularity
     the software life cycle, including specification,
           analysis and design, coding, testing, and maintenance
     top-down design and bottom-up design
     recursion and recursive subroutines.

The ideas of black boxes, the interface/implementation distinction, and modularity are closely connnected. You should understand why they are important in all phases of the software life cycle.

Eigth Week

Chapter 9. Applications

This chapter has three sections. The first discusses very briefly some of the major applications of desktop (or "personal") computers. Most people are familiar with at least some of these. The intent here is not to cover the applications in detail but to survey what is available and to give some idea of what is involved in implementing the applications on a computer. The applications that are surveyed include:

          word processing and desktop publishing
          painting and drawing
          networks and communication.

The second section of the chapter deals with applications that require too much computing power to be done comfortably on desktop computers (although with their ever-increasing power, that might not be true for long). Two major classes of application are discussed:

           computer modeling, such as weather forecasting
           optimization, such as linear programming

In computer modeling, a set of equations that represent some system are programmed into the computer. A "system" here might mean a phsical system such as the atmosphere or a galaxy full of stars. It could also mean an economic system, an ecosystem, etc. By applying these equations to data representing the current situation, the computer can make predictions on what will happen in the system in the future. The quality of the prediction can depend on the accuracy of the model, the quality of the initial data, and the amount of computer processing power that is available to apply to the task. One example of computer modeling is weather forecasting.

Optimization means finding the best option among a range of possible options. Usually, the range of posible options is limited by some constraints, such as the amount of raw materials available. In this case, optimization would mean determining how to allocate the resourses to maximize, for example, profit. One type of optimization is linear programming, which is an example of resource allocation in which the constraints are given by linear functions.

The final section of the chapter is a complete departure from the first two parts. It covers the analysis of algorithms, which is the theoretical study of practical limits on computing. Much of the section is devoted to the question of measuring exactly how fast or efficient an algorithm is. The main ideas include:

       O(n^2) vs. O(n*log(n))
       sorting algorithms
       the P=NP problem

Ninth Week

Chapter 11. Computer Graphics

Most of the computer graphics you see are created in a two step process: "modeling" followed by "rendering". In geometric modeling, an abstract wire-frame representation of a scene made up of polygons is constructed. The objects in the scene can then be assigned attributes such as color, reflectivity, and surface texture; this gives a more complete model of the scene. Rendering is the process of producing, pixel-by-pixel, an actual two-dimensional image of the scene. The major ideas in the chapter include:

           geometric modeling
           wire-frame models
           transformations: scaling, rotation, translation
           hidden line removal
           lighting, shadows, etc.
           ray tracing
           scene description language

Tenth Week

Chapter 12. Artificial Intelligence

In the tenth and final week of the course, we come to a philosophical question: Can machines think. This question was raised in 1950 by Alan Turing in a paper in which he proposed a test which could be used to judge whether computers are intelligent. (This is now called the "Turing test".) For a while, it was supposed that computers could be made to think, but there were from the beginning philosophical objections, and the debate continues today.

Besides the philosophical questions, we can talk about the many practical applications of AI, such as Expert Systems and Neural Nets. Even if true general intelligence is never achieved, the field has made valuable contributions.

The major ideas in Chapter 12 include:

        The Turing Test
        The Physical Symbol System Hypothesis
        Expert Systems
        Knowledge Representation
        Arguments against AI:
             Limits of Logic, and limations of rule-following
             The Chines Room argument
             The problem of consciousness
        Neural Nets
        Artificial Life
        The Genetic Algorithm

(by David Eck)