CS 111A Lecture Notes - Computer Basics - Chapter 1

Hardware and Software

• Hardware - physical machines
• Program - set of instructions to run on those machines
• Software - the collection of such programs

5 Main Components of Computer Hardware

1. Central Processing Unit (CPU) - Fetches and follows instructions, and does simple arithmetic calculations. (Following simple instructions given to it.)
2. Main Memory (RAM) - A silicon-wired card inside the computer box containing huge numbers of numbered memory locations (bits organized into bytes). High speed, low capacity. Volatile (memory is lost when power shuts down).
   • bit - a binary number; either 0 or 1. Stored electronically with 0 meaning off (no power flowing through a circuit) and 1 meaning on (power flowing).
   • Because computer memory is based upon this principle of 2 possible states, it's useful to work with powers of 2 for computer memory.
   • byte - 8 bits, giving 2⁸ (256) possible values - a more useful unit of measure
   • memory address - The location of the first byte of memory used for a variable. Stored numerically, often in hexidecimal (base 16) format.
3. Secondary Memory - Hard drive, flash memory, CD-ROM, etc. Low speed, high capacity. Non- volatile (nothing lost when power is cut.)
4. Input Device - Keyboard Mouse, etc. (input stream)
5. Output Device - Screen, Printer, etc. (output stream)

Making Software Work

• Operating System - Windows, Linux, Mac OS X, Android, iOS, etc. intermediary between hardware and software. Coordinates operation of tasks, allocates resources. The OS is a program itself.
• High-level language - Java, C, C++, PHP, JavaScript, Python, etc. Platform-independent (commands are the same for all kinds of computers). Designed to be understandable by people, and usually compiled to work on computers.
• Low-level language - Very simple computer instructions given one small step at a time. True Machine Language is made up of binary numeric codes for these instructions. We generally deal with its equivalent Assembly Language, which uses short abbreviations for commands, such as "add" for an addition calculation, or "sub" for subtraction. Platform-dependent (commands are different for different kinds of computers)
• Compiler - Converts a high level language to assembly language, saving it in a new file. Source code to Object code. A compiler is a program, generally written in a high level language itself. Compilers are generally platform-dependent. They produce assembly language that works only on the machine they are running on.
• Interpreter - a program which inputs a source code program (such as Perl or PHP) and executes it directly, converting each line of code to machine language as it goes. This is like compiling and running a program all in one step.

Here is an historical Glossary of Programming Languages

Java Code

• Java programs are compiled and run in an unsual way - they are both compiled and interpreted:
  1. The Java source code you write is compiled into Java Byte Code, which is like assembly language, but it's machine-independent. (Java Byte Code is the assembly language for an imaginary machine called the "Java Virtual Machine".) The javac program does this for you. The compiler saves the java byte code in a file with the class's name followed by the extension .class
  2. This Java Byte Code is interpreted and run - it is converted to your specific machine's language as it is run. The java program ("run time environment") does this for you.
• When you're running a program called Prog1, the file you're actually executing (through the java interpreter) is Prog1.class, but you type simply "java Prog1" to execute it.
• If you do type "java Prog1.class", the period will be perceived as a directory symbol, meaning the java interpreter will attempt to run a file called "class" in a directory called "Prog1".
• This use of the period to specify a directory can also be seen in java code such as this:
      import javax.swing.*;
  in which we're saying that our program will need to import all the classes in the javax/swing library directory. (We'll learn more about this soon)
• Here is a simple Java program:

  ---

      /* Craig Persiko
         HelloWorld.java
  
         Simple Hello World program
      */
  
      class HelloWorld
      {
        public static void main(String[] args)
        {
          System.out.println("Hello World!");   //Display the string.
        }
      }
    

  ---

• The above program will compile correctly, but if you make one small change, it probably will produce a compiler error.
• The formatting of the above program is largely for human readability. Programs are written for 2 audiences: they must make sense to both the computer and to other programmers.
• For example, the semicolon is used to mark the end of a line of code. Hitting return after each line is just us to be able to read it better.
• The first line, which begins with "/*" is the beginning of a comment. Everything from /* to */ is ignored by the compiler; it's just there for us to read. Another way to specify a comment is to begin it with "//", then the comment ends at the end of the line (like "//Display the string." above)
• All java code must be inside a class definition, and the class must be given a name which matches the filename.
• Every stand-alone java program must have a main function, which begins with the header:
  public static void main(String[] args)
• Key Words such as "class", "public", "static", and "void" are words that have a special meaning in Java. Also called Reserved Words
• Operators are symbols that have special meaning in Java (common examples are: + - * / )
• Punctuation such as the semi-colon ";" the curly-braces "{" and "}" and parentheses "(" and ")" is needed in Java to mark boundaries between statements, etc.
• Literals are explicit values in a program, such as the string "Hello, World!"
• Oracle's Java Tutorial website has some excellent instructions for getting started with Java. We'll follow their instructions to compile and run the above program.

Software Development

• Software Development Life Cycle - Analysis, Design, Implementation, Testing, Maintenance/Evolution, Obsolescence. Implementation (writing code in Java or other languages) occupies only 20% of the total development time for a project.
• Testing and Debugging - 3 kinds of errors: Syntax/Compile-time, Run-time, and Logic. Syntax errors often confuse the compiler, so attack the first error, then try compiling again.
• Algorithm - a sequence of precise instructions. May be conveyed using flowchart, logic diagram, pseudo-code, or such. Planning saves time! Think in terms of what you want, how you can do it, before you dive in.
• Here's an example of a pseudo-code algorithm to buy the textbooks for a class, using parameters for the course number and section number:

  To buy the books for CS 111A Section 001, you would write:

  BuyTextBooks(CourseNumber="CS 111A", SectionNumber="001");

  To buy the books for English 1A Section 581, you would write:

  BuyTextBooks(CourseNumber="ENGL 1A", SectionNumber="581");

  Here is the generic algorithm to buy textbooks:

  BuyTextBooks(CourseNumber, SectionNumber):

  1. Go to the college bookstore.
  2. Pick up a shopping basket.
  3. Find the section for textbooks.
  4. Locate the shelf labeled for CourseNumber and SectionNumber.
  5. Repeat for each required book for this course section:
     1. Determine the required book's title and author.
     2. Pick up a copy of the required book and put it in your shopping basket.
  6. Repeat for each optional book for this course section:
     1. Determine the optional book's title and author.
     2. Pick up a copy of the optional book.
     3. Look at optional book to decide if you want to buy it:
     4. If you want to buy the optional book:
        1. Put the optional book in your shopping basket.
     5. If you do not want to buy the optional book:
        1. Put the optional book back on the shelf where you found it.
  7. Go to the cash register.
  8. Pay for the book(s) in your shopping basket.
  9. Exit the bookstore with your books.

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