@node software note, Topic Guide, Editors Note, Top
@cindex A Note on Software 
@unnumbered A Note on Software

This section is intended for people who have little or no knowledge of
the technical aspects of computer science. It is not necessary to read
this section to understand the essays and speeches presented in this
book; however, it may be helpful to those readers not familar with
some of the jargon that comes with programming and computer science.

A computer @emph{programmer} writes software, or computer programs.  A
program is more or less a recipe with @emph{commands} to tell the
computer what to do in order to carry out certain tasks.  You are more
than likely familiar with many different programs: your Web browser,
your word processor, your email client, and the like.  

A program usually starts out as @emph{source code}. This higher-level
set of commands is written in a @emph{programming language} such as C
or Java. After that, a tool known as a @emph{compiler} translates this
to a lower-level language known as @emph{assembly language}. Another
tool known as an @emph{assembler} breaks the assembly code down to the
final stage of @emph{machine language}---the lowest level---which the
computer understands @emph{natively}.

@image{images/code}

For example, consider the
``hello world'' program, a common first program for people learning C,
which (when compiled and executed) prints ``Hello World!'' on the screen.
@footnote{In other programming languages, such as
Scheme, the @emph{Hello World} program is usually not your first program. 
In Scheme you often start with a program like this:
@smallexample
(define (factorial n)
  (if (= n 0) 
      1 
      (* n (factorial (- n 1)))))
@end smallexample 
This  computes the factorial of a number; that is, running
@code{(factorial 5)}would output 120, which is computed by doing 
5 * 4 * 3 * 2 * 1 * 1.}
@smallexample
@group
int main()@{ 
    printf(''Hello World!''); 
    return 0; 
@}
@end group
@end smallexample

In the Java programming language the same program would
be written like this:

@smallexample
@group
public class hello @{ 
    public static void main(String args[]) @{
        System.out.println(''Hello World!''); 
    @} 
@}
@end group
@end smallexample

@*@*
However, in machine language, a small section of it may look similar to
this:

@smallexample
@group
1100011110111010100101001001001010101110
0110101010011000001111001011010101111101
0100111111111110010110110000000010100100
0100100001100101011011000110110001101111
0010000001010111011011110111001001101100
0110010000100001010000100110111101101111
@end group
@end smallexample

The above form of machine language is the most basic representation
known as binary. All data in computers is made up of a series of
0-or-1 values, but a person would have much difficulty understanding
the data.  To make a simple change to the binary, one would have to
have an intimate knowledge of how a particular computer interprets the
machine language. This could be feasible for small programs like the
above examples, but any interesting program would involve an
exhausting effort to make simple changes.

As an example, imagine that we wanted to make a change to our ``Hello
World'' program written in C so that instead of printing ``Hello World''
in English it prints it in French. The change would be simple; here is
the new program:

@smallexample
@group
int main() @{ 
    printf(''Bonjour, monde!''); 
    return 0; 
@}
@end group
@end smallexample

It is safe to say that one can easily infer how to change the program
written in the Java programming language in the same way.  However,
even many programmers would not know where to begin if they wanted to
change the binary representation.  When we say ``source code,'' we do
not mean machine language that only computers can understand---we are
speaking of higher-level languages such as C and Java. A few other
popular programming languages are C++, Perl, and Python. Some are
harder than others to understand and program in, but they are all much
easier to work with compared to the intricate machine language 
they get turned into after the programs are compiled and assembled.

Another important concept is understanding what an @emph{operating
system} is. An operating system is the software that handles input and
output, memory allocation, and task scheduling. Generally one
considers common or useful programs such as the @emph{Graphical User
Interface} (GUI) to be a part of the operating system. The GNU/Linux
operating system contains a both GNU and non-GNU software, and a
@emph{kernel} called @emph{Linux}. The kernel handles low-level tasks
that applications depend upon such as input/output and task
scheduling. The GNU software comprises much of the rest of the
operating system, including GCC, a general-purpose compiler for many
languages; GNU Emacs, an extensible text editor with many, many
features; GNOME, the GNU desktop; GNU libc, a library that all
programs other than the kernel must use in order to communicate with
the kernel; and Bash, the GNU command interpreter that reads your
command lines.  Many of these programs were pioneered by Richard
Stallman early on in the GNU Project and come with any modern
GNU/Linux operating system.

It is important to understand that even if @emph{you} cannot 
change the source code for a given program, or directly use all these
tools, it is relatively easy to find someone who can. Therefore, by
having the source code to a program you are usually given the power to
change, fix, customize, and learn about a program---this is a power that
you do not have if you are not given the source code. Source
code is one of the requirements that makes a piece of software
@emph{free}. The other requirements will be found along with the
philosophy and ideas behind them in this collection. Enjoy!
@*
@*@noindent @emph{Richard E. Buckman}
@*@noindent @emph{Joshua Gay}


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