Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [169]
HTML is similar to the Rich Text Format that I described earlier, in that it contains ASCII text with formatting information. HTML also allows referencing pictures in the form of GIF files, PNG (Portable Network Graphics) files, and JFIF (JPEG File Interchange Format) files. Most World Wide Web browsers allow you to look at the HTML files, which is an advantage of their text format. Another advantage of defining HTML as a text file is that it's more easily searchable. Despite its name, HTML isn't really a programming language such as we've explored in Chapters Chapter 19 and Chapter 24. The Web browser reads the HTML file and formats the text and graphics accordingly.
It's sometimes helpful if some special program code runs while you are viewing and working with particular Web pages. Such code can run on either the server (which is the computer on which the original Web pages are stored) or the client, which is your computer. On the server side, usually all necessary work (such as interpreting online forms that a client fills out) can be handled with Common Gateway Interface (CGI) scripts. On the client side, HTML files can contain a simple programming language known as JavaScript. Your Web browser interprets the JavaScript statements just as it interprets HTML text.
Why can't a Web site simply provide an executable program that can run on your computer? Well, for one thing, what is your computer? If it's a Macintosh, it needs an executable that contains PowerPC machine code and uses the Mac OS API. A PC-compatible needs an executable that contains Intel Pentium machine code and probably uses the Windows API. But there are other computers and other graphical operating systems as well. Moreover, you don't want to be indiscriminately downloading executable files. They could originate from an untrustworthy source and might be malicious in some way.
An answer to these problems was provided by Sun Microsystems in the language Java (not to be confused with JavaScript). Java is a full-fledged object-oriented programming language much like C++. In the preceding chapter, I explained the difference between compiled languages (which result in an executable that contains machine code), and interpreted languages (which don't). Java is somewhere in between. Java programs must be compiled, but the result of the compilation usually isn't machine code. It's instead Java byte codes. These are similar in structure to machine code, but they're for an imaginary computer called the Java virtual machine (JVM). A computer running the compiled Java program emulates the JVM by interpreting the Java byte codes. The Java program uses whatever graphical operating system is on the machine, thus allowing platform-independent programming.
While much of this book has focused on using electricity to send signals and information through a wire, a more efficient medium is light transmitted through optical fiber—thin tubes made of glass or polymer that guide the light around corners. Light passing through such optical fibers can achieve data transmission rates in the gigahertz region—some billion of bits per second.
So it seems that photons, not electrons, will be responsible for delivering much of the information of the future into our homes and offices; they'll be like faster dots and dashes of Morse code and those careful pulses of blinking light we once used to communicate late-night wisdom to our best friend across the way.
Appendix A. Acknowledgments
Code was conceived in 1987. It rattled around in my head for nearly a decade and was finally committed to a Microsoft Word file between January 1996 and July 1999. I offer many thanks:
to the readers of early drafts of Code who contributed comments, criticisms, and suggestions: Sheryl Canter, Jan Eastlund, Peter Goldeman, Lynn Magalska,