assembler Machine Code Object Files Executable File

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1 CSCE A211 Programming Intro What is a Programming Language Assemblers, Compilers, Interpreters A compiler translates programs in high level languages into machine language that can be executed by the computer. C++, C, Pascal, Ada, etc. are all examples of high level languages. Here is a big picture of what is happening when you write and compile a C or C++ program. Human Brain English Algorithm High Level Language (C++, C, Pascal ) compiler Low Level Language - Assembly assembler Machine Code Object Files linker Executable File Note that the compiler takes the source program and typically produces an object program the compiled or machine code version of the source program. If there are multiple source files that make up a final program, these source programs must then be linked to produce a final executable. The executable is generally unique to a particular CPU and operating system. For example, a Macintosh cannot understand machine code intended for an Intel processor. However, if there is a standard version of C++ (which there is, the latest is C++11), then one could write a program and have it compile on the two different architectures. This is the case for standard programs, but any programs that take part of a machine s unique architecture or OS features will typically not compile on another system. PC Compiler PC Machine Code C++ source Mac Compiler Mac Machine Code

2 Under this model, compilation and execution are two different processes. During compilation, the compiler program runs and translates source code into machine code and finally into an executable program. The compiler then exits. During execution, the compiled program is loaded from disk into primary memory and then executed. C++ falls under the compilation/execution model. However, note that some programming languages fall under the model of interpretation. In this mode, compilation and execution are combined into the same step, interpretation. The interpreter reads a single chunk of the source code (usually one statement), compiles the one statement, executes it, then goes back to the source code and fetches the next statement. Source Code X=3 X=X+1 Machine Language Statement Interpreter Next statement Execute Examples of some interpreted programming languages include JavaScript, VB Script, some forms of BASIC (not Visual Basic), Lisp, and Prolog. What are the pro s and con s of interpreted vs. compiled? Compiled: Runs faster Typically has more capabilities o Optimize o More instructions available Best choice for complex, large programs that need to be fast Interpreted: Slower, often easier to develop Allows runtime flexibility (e.g. detect fatal errors, portability) Some are designed for the web In the midst of compiled vs. interpreted programming languages is Java. Java is unique in that it is both a compiled and an interpreted language (this is a bit of a simplification with Just In Time compilers, but we will ignore that distinction for now). A Java compiler translates source code into machine independent byte code that can be executed

3 by the Java virtual machine. This machine doesn t actually exist it is simply a specification of how a machine would operate if it did exist in terms of what machine code it understands. However, the byte code is fairly generic to most computers, making it fairly easy to translate this byte code to actual native machine code. This translation is done by an interpreter that must be written on different architectures that can understand the virtual machine. This interpreter is called the Java Virtual Machine (JVM) or Java Runtime Environment. Public class Foo { if (e.target=xyz) then this.hide(); Java compiler Mac Interpreter PC Interpreter PalmPilot Interpreter The great benefit of Java is that if someone (e.g. Oracle, the makers of Java) can write interpreters of java byte code for different platforms, then code can be compiled once and then run on any other type of machine. Unfortunately there is still a bit of variability among Java interpreters, so some programs will operate a bit differently on different platforms. However, the goal is to have a single uniform byte code that can run on any arbitrary type of machine architecture. Another benefit is we can also control runaway code that does things like execute illegal instructions, and better manage memory. We will discuss these later in the course. Hello, World in C++ The first program that many people write is one that outputs a line of text. In keeping with this vein, we will start with a program that prints, Hello, world. First, here is the program in its entirety: // A first program in C++. The double slashes are comments for one line. /* We could add more comments if we like using slash and an asterisk. The commends end with an asterisk and then a slash. */ #include <iostream> using namespace std; int main() { cout << Hello, world << endl; return 0; // return code // Not iostream.h like in C // include all members of the standard namespace

4 Before we try to compile and run this program, let s go through a description of what is in here. First, the // and /* */ statements are for comments. Descriptive text about what the program does should go into the comment fields. We ll use these to describe the input, output, and dependency behavior of the program. The line #include <iostream> is a preprocessor directive. This is a message to the compiler to perform some directive before compiling the program. In this case, we are including the contents of the iostream.h file (the.h is implicit and can be left off on most compilers, but not all). The iostream.h file includes input and output library routines essentially you will be using software written by someone else. We will talk more about these later. The next line, using namespace std; will identify the scope of names we will be using for the program. The default is the standard namespace. Most compilers will assume the standard namespace if this line is left out. Some programmers consider it bad form to use this exact line, since it includes everything from the standard namespace, and we might only want to use a little bit from the standard namespace. The line cout << Hello, world << endl; will output the line of text Hello, world. If we did not use the line using namespace std; above, then we would have had to write this line as std::cout << Hello, world << endl;. It becomes cumbersome to add the std:: in front of all the statements, which is why in this class we will use the using statement. No matter how the function is invoked, cout is a function that belongs to the namespace std. cout is the standard output stream, which is where we will see our output text. The << operator is the stream insertion operator. When the program runs, items to the right of the operator, i.e. the operands, are fed into the output stream to the left. Finally, the last line return 0; returns the number 0 to the caller. We could return any number we want here, but perhaps we have defined the program in such a way that 0 indicates that the program has terminated successfully. Compilation The process of entering and compiling your program is different depending upon what development environment you are using. We ll have a few assignments in both Unix and Microsoft Visual C++, and then you will be able to choose which environment you prefer for the rest of the class. Visual Studio is more graphical than the pure Unix environment and in general offers more features. A convenient benefit of the Unix environment is that your code and programs are all in one place, so if you work from home and also from school you won t have to shuttle your files back and forth on floppies to work on your program from different locations. Also, if you don t have a Windows machine at home, you would only be able to use Visual Studio on the campus labs. Finally, be warned

5 that there are some subtle differences between Unix s g++ and Visual C++, so a program that works in one environment might not work in the other! Compiling under Unix See the lab assignment for details about logging in and editing files. To compile the program, invoke the C++ compiler using g++ : g++ hello.cpp If there were any errors, the compiler will complain and tell you it encountered problems. If all goes well, you ll be returned to the prompt and you should now have a file named a.out in your current directory. To run it, simply enter a.out and it should run. You can rename the file to something else if you wish (the mv command) or if you wanted to name the output file something other than a.out, you could compile with the flag: -o outname. For example: g++ -o hello hello.cpp Would produce a file named hello instead of a.out. Hello, World io Java Here is Hello World in Java: File: HelloWorld.java /* * Normally you would put your name and assignment info here * This program prints out "Hello, World". */ public class HelloWorld { public static void main(string[] args) { System.out.println("Hello, world!"); The structure is pretty similar to C++. A Java program can be composed of many files. In this case, our program consists of one file. Generally, each file corresponds to something called a class. For now a class is basically a container that holds our program. A Java program should have only one file that contains the special method called main which tells Java where to start running our program.

6 Next is the line public class HelloWorld. This defines the name of the class (i.e. the object). The class name should match the name of the file! Java is case-sensitive, so Java will complain about a file named helloworld.java but the contents of this file contains HelloWorld instead. We will place each class object into its own separate file. If you are on a Windows machine, note that by default file extensions are hidden so if you rename a file, it might really be named HelloWorld.java.txt instead of HelloWorld.java. To show all file extensions, go to this site: The line public static void main(string[] args) { defines a method called main in the HelloWorld object. For C and C++ people, a method is basically the same thing as a function. Just like in C and C++, this is where your program begins execution. As you might guess, System.out.println will output to the screen whatever is contained in double quotes. The dotted notation of using periods starts at a high-level Java object, and each dot indicates a more specific Java object. Objects form a hierarchy. For example, here is a small object hierarchy for System: Sample Object Hierarchy System out in err println() BufferedInputStream() read() OutputStream() System.out.println() invokes the method to print a string of data to the screen. Similarly, System.in.read() would invoke a function to read input from the keyboard. Each dot moves us down the hierarchy to a more specific function or object. To print a string, note that we used double-quotes. Any double-quoted block of characters is considered a literal string, and is created by concatenating together the appropriate ASCII characters. Once again, you might not understand everything that is going on here, but it should make more sense as we go along. Compilation To compile under Unix we would use: javac Hello.java

7 and this produces a file called Hello.class. This contains the Java Byte Code for the compiled code that would run on the hypothetical Java virtual machine. To run this code use: java Hello Make sure your case matches or there will be error messages.