Computer Science Fundamentals

1. Computer Science Fundamentals A topics course in Computer Science

2. Topics we hope to cover • Data representation (especially binary and hex), arithmetic, base change algorithms • Operating systems (including an introduction to Threads of execution) • Boolean algebra and switching functions • Combinational and flip-flop circuitry (an introduction to architecture) • Computer communication (an introduction to sockets and/or datagrams) • Theory of computation (an introduction to automata and computability) • Assembly language programming • Program translation • We’ll also learn some java programming!

3. Data representation • Information in the computer is represented in base 2 (binary), although, for convenience, base 16 is also sometimes used. • In radix positional notation, a digit d, located in position p, in a number base b, represents d*bp. So, the decimal value 8352 represents 8*(1000)+3*(100)+5*(10)+2*1. Similarly, the binary value 1011101 represents the value 1*64+0*32+1*16+1*8+1*4+0*2+1*1. • Note that the powers base 2 are 1,2,4,8,16,32,… 20=1, 21=2, etc. Make sure you are familiar with the two-powers.

4. You will need to memorize the values 0..15 in binary and hex

5. Kinds of data • There are many types of data stored in computer memory. Some memory cells hold instructions, some hold integers, some hold float values, some hold char data, other store composite data types.

6. Char data is stored internally using the ASCII (or UNICODE) collating sequence • You can find a complete ASCII table in most computer texts. • Exercise: In C++ or Java, print your own ASCII table: print the int value x = 32…255 in the left column of a table and print (char)(x) in the right column. • You will need to know the collating sequence values for some characters like ‘0’, ‘1’, … also ‘a’,’b’, … and ‘A’, ‘B’, … • Exercise: write these values in binary.

7. Int data • Integers may be stored internally in one of several formats: • BCD (binary coded decimal) often used in spreadsheet programs • Unsigned int: a positive binary value • Signed int: positive or negative value

8. BCD • BCD allocates 4 bits (a nibble) of binary storage for each decimal digit to be represented. • Example: 598 in decimal will require 3digits*4bits each =12 bits. In BCD it looks like this: 010110011000 • Practice converting a few decimals to BCD

9. Unsigned ints • In unsigned (positive) representations, all bits are used to indicate the value. • In 4 bits of binary, you can represent decimal values 0,…,15. (see table a few slides back.) What values could be represented in 5 bits? In 8 bits? In 16 bits? • What is the general rule? • Represent values 0, 48, 65, 97, and 127 in 8 bits. Example: 48 decimal is 00110000 in 8 binary bits. • 8 bits is called a byte.

10. Quick conversion between bases: From base b to decimal • To convert from any base to decimal, use Horner’s algorithm for polynomial evaluation. A number in base b can be regarded as a polynomial p in the variable b. Let P(b)= anbn + a n-1 b n-1+…+a1b1+a0b0 • This would be written as the series of (coefficient) digits. For example, the base 6 value 32501 represents 3*(64)+2*(63)+5*(62)+0*(61)+1*(60) • Using nested multiplication we can rewrite this without needing to compute exponentials of the base: ((((3*6)+2)*6+5)*6+0)*6+1

11. Base b to decimal via Horner • If a value in a base b is entered as a String in a TextField named input, and the base is entered in a TextField called basefield, we could use the following Java code to do the conversion: • String value; • int b, answer; • answer=0; • b=Integer.parseInt(basefield.getText()); • value=input.getText(); • for(int j=0;j<value.length();j++) • answer = answer*base+ (int) (value.charAt(j)-48);

12. Remarks on this code & exercises • Note that the length of a String in Java can be obtained using the class method length(). The String method charAt(p) returns the char in position of a String. The TextField method getText() returns the String entered into a TextField. • For which bases would the java code fragment not provide the correct answer? Why? • Fix this problem. • Complete the JApplet. Write a JApplet which provides labels and textfields for a base, a value and an answer and performs the conversion from base b to base 10.

13. Converting from decimal to other bases • In determining what a decimal value would look like if represented in another base (b), we need to figure out what digit in base b goes in each position of the representation. • Suppose n=295 and b=2, ie., we are trying to convert to binary. The 1s place will have a 1 in it since 295 is odd. • 295 (10) == ???..??1(2) • But how many other bits are there, and what are each of their values? Since positions in the binary representation indicate powers of two, having figured out the ones place means we have only half of n (295/2= 147) left to represent.

14. Example: convert 295 decimal to binary • 295 is odd…least significant bit is a 1: 295(10) = ???..?1(2) • 295/2= 147. Since 147 is odd, 2s bit is also a 1: 295(10) = ???..?11(2) • 147/2 =73. Since this is odd, next bit is also a 1: 295(10) = ???..111(2) • 73/2=36. Even, so next bit is a zero: 295(10) = ???..0111(2) • 36/2=18, which is even.295(10) = ???…00111(2) • 18/2=9, odd. 295(10) = ???100111(2) • 9/2=4 even. ??0100111(2) • 4/2=2 even. ?00100111(2) • 2/2=1 0dd. 100100111(2) • ½=0 stop.

15. Decimal to any other base • Convert from decimal to any base using the remainder/division algorithm described previously. Each remainder operation generates one digit of the answer. Then divide the number by the base and continue. • String answer=””; • int base =Integer.parseInt(basefield.getText()); • int value = Integer.parseInt(input.getText()); • while (value!=0){ • answer= value%base+answer; • value/=base;}

16. Remarks & Exercises • Note, we use the static Integer method parseInt() to convert a String to an int value.  • For what input values would this Java code not work properly? Why? • Can you fix this problem to convert decimal values into values in any other base? • Write the JApplet which would provide labels, textfields and buttons enabling the user to enter a decimal int and a new base value and convert the decimal int to the new base. • Practice doing these conversions by hand.

17. Base conversions: base b to base bn • If you are converting between bases where one base is a power of the other, like binary to hex, or binary to octal, or base 3 to base 9 then the conversion may be done very easily. n digits of base b will be needed for each 1 digit of base bn . • Consider converting A6E9 in base 16 to binary. Each hex digit will generate 4 binary bits. If you have not memorized your hex digits yet, this is a good time to start. In decimal A is 10, B is 11, C is 12, …F is 15. So A= 1010(2), etc. • A6E9 (16) = 1010011011101001 (2) • Convert this number (right hand side above) to base 8 and base 4. • Convert the base 9 value 783361 to base 3.

18. An introduction to Java • Java syntax is the same as C++. In other words, an assignment statement, an if, a while, or a for statement in Java looks the same as in C++. • Java class methods are called, like in C++, using the dot ‘.’ operator: • ClassInstance.methodName(parameters); • To get a String which a user has entered into a TextField named input we might use the code: • String value= input.getText();//getText is a TextField method • Java methods (functions) have return types, parameters, and their definitions are enclosed in a block {}, like in C++ programming. Prototypes are not used in Java.

19. An introduction to Java • Java provides components (classes) buttons, textfields, and labels, among many others. These three are called JTextField, JLabel, and JButton in Java 1.2 and higher. These classes must be instantiated with “new”. Example: • JButton b=new JButton(“press me”); • In a small program, declarations are typically global (not inside a method) and data allocation occurs in the JApplet method init(). The init method is executed when a browser loads an applet. • All java applets and applications must consist of an outer class: • //sample Java JApplet  • import javax.swing.*; //contains Java2 classes  • public class OuterClass extends JApplet • {//open block   • //methods, declarations go in here • }//close outer block

20. JApplet with JLabel, JTextField and JButton • import javax.swing.*;   • public class OuterClass extends JApplet • {//global applet declarations typically go here: • public JLabel label; • public JTextField text; • public JButton button; • public void init(){//first method browser will execute • Container c=getContentPane();//get the panel for this applet • c.setLayout(new FlowLayout());//one sort of layout • button=new JButton(“press me”); • label =new JLabel (“enter value”); • text=new JTextField(20); • c.add(label);//components appear in the order they were added to the panel • c.add(text); • c.add(button); • }//end init • }//end applet

21. JApplet structure • Note that imports appear the top of a java Applet. These are like includes in C++. • Applets may have many user defined methods. • They may also override the Applet methods init(), paint(), start(), stop(), and many others. • A Java applet or an application must consist of an outer class (with a block) and any data fields, and methods (with their blocks) defined inside the outer block.

22. Applet deployment • We must perform a number of steps in order to deploy an applet to our webspace so a visitor there could run it. • Type the code into an editor or IDE • Compile the .java file. This process will check syntax. If it is successful, a .class file will be created which has the same name as the original java source file. Ie. Prog.java when compiled with generate a file called prog.class. Create an appropriate html file to point a browser to your class file. • Check things out on your desktop by running the applet in appletviewer. • Use FTP to send appropriate html and class files to your webspace

23. Using an editor Type the java source code into an editor or IDE (integrated development environment like JBuilder) and save it as a .java file. The filename must be the same as the outerclass name. We will use TextPad, a shareware program, as our editing environment, although you can also use the Borland C++ editor or Word. (If you use MS Word, make sure the file is saved as ASCII text format and has the extension .java) TextPad can be downloaded on your own computer. It is produced by Helios.com TextPad has some features of an IDE: you can compile and run java applets and applications from TextPad. TextPad is available in the campus labs.

24. Compile the .java file. • This process will check syntax. If it is successful, a .class file will be created which has the same name as the original java source file. Ie. Prog.java when compiled with generate a file called Prog.class.Java is case sensitive. • Compile in TextPad by selecting compile from the tools menu. • Alternative. If you’ve downloaded JDK1.3.X onto your own computer, you have java, the java compiler as part of the kit. Open a DOS window (run command.com)and navigate to your jdk bin directory where you should have saved your .java file. At the prompt, type: java Prog.java • This will create a file called Prog.class. A class file is not an executable, it contains portable byte-code. It requires a java interpreter (named java) to be executed.

25. Create an appropriate html file to point a browser to your class file • Sample html: • <html> • <applet code =”Prog.class” height=300 width=500> • </applet> • </html> • This must be typed into a file and saved with the .html extension. You could call this file Prog.html to make it easier to remember which applet it pointed to.

26. A possible problem with the html and current browser versions (IE & Netscape) • Depending on what browser you use, and its version, it may require a plugin to run java2. The html file will then need to be modified to tell the browser to go load the plug in. Campus labs have the plugin already downloaded. • There should also be a java program called HTMLConverter you can use to convert the html file we created into a more complicated one that tells the browser it needs to load a plugin to run the applet. • My class page has examples of this converted html file format – you can cut and paste from the examples there to create a new html. • See class notes about this topic.

27. Check things out on your desktop by running the applet in appletviewer • If you can get into DOS (on your own computer) and have downloaded the jdk you can use the simple html file format I presented earlier to check whether your applet works now. • Assuming you have an applet called Prog.java, a class file named Prog.class and an html file named Prog.html, navigate to the bin directory and type: appletviewer Prog.html • appletviewer is a program in the jdk which will run your applet as a browser would, so you can check if it works ok. • Textpad is also configured to run appletviewer in the labs. If you’ve created an html file for your applet, select appletviewer from the textpad tool menu. You may need to have saved these files in particular directories, like the java/bin directory, for this to work properly in the labs. See class notes on this.

28. Use FTP to send appropriate html and class files to your webspace • I have a screen shot of how to configure your WinFTP session to ship files to your webspace on our class pages. • Make sure to send .class files as type binary. Later, you’ll use binary type for jpg and wav files also. • HTML can be sent as ascii. • Type your applet url into the browser to go visit your applet. It will be something like • http://www.webserver2/students/smitxy87/Prog.html

29. An applet with a listener • Applets are event-driven programs. This means things happen when the user click buttons, moves the mouse, or enters values into fields. • It is up to the programmer to insure the applet listens for button clicks (or whatever) and does what needs to be done if a button is clicked. • This is done by means of listeners in Java. There are many (!) listeners. One of the simplest is the one which checks if values have been entered into textfields or if a button is clicked. It is called ActionListener. • Listeners are types of interfaces. A java applet or application program may use many interfaces.

30. Implementing the ActionListener • import javax.swing.*;   • public class OuterClass extends Japplet implements ActionListener • {//global applet declarations typically go here: • public JLabel label; • public JTextField text1, text2; • public JButton button; • public void init(){//first method browser will execute • Container c=getContentPane();//get the panel for this applet • c.setLayout(new FlowLayout());//one sort of layout • button=new JButton(“press me”); • label =new JLabel (“enter value”); • text1=new JTextField(20); • Text2=new JTextField(20); • c.add(label);//components appear in the order they were added to the panel • c.add(text1); c.add(text2); • button.addActionListener(this);//button is listening now for user to press it • c.add(button); • }//end init • //see next slide!

31. ActionListener interface requires the programmer to define the method actionPerformed(…) • The above applet is not complete. It is missing the outer close block, but even so, will not compile without the definition of the method actionPerformed which tells the aplet what to do if the button is pressed. • public void actionPerformed(ActionEvent x){ • int z= Integer.parseInt(text1.getText());//get int entered by user • int p =2*z;// do elaborate computations • text2.setText(“”+p); • //when button is clicked, 2*input value is calculated and displayed • }//end actionPerformed • }//end outer class