X Window System Architecture Overview HOWTO
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# 2. Introduction

Back when UNIX was a new thing, around 1970, graphical user interfaces were only a weird thing being played with in a laboratory (Xerox's PARC to be precise). Nowadays, however, any operating system in hopes of being competitive needs to have a GUI subsystem. GUIs are supposed to be easier to use. This is not much of a concern under UNIX, which has traditionally been, to some extent, pretty user-hostile, preferring versatility over ease of use. However, there are several reasons why a GUI is desirable even on a UNIX system. For instance, given UNIX's multitasking nature, it's natural to have a lot of programs running at any given time. A GUI gives more control over how things are displayed on-screen, thus providing with better facilities for having a lot of programs on-screen at the same time. Also, some kinds of information are better displayed in graphical form (some, even, can only be displayed in graphical form; like pr0n and other inherently graphical data).

Historically, UNIX has had a lot of improvements done by academic types. A good example is the BSD networking code added to it in the late 1970's, which was, of course, the product of work at the University of California at Berkeley. As it turns out, the X Window System (also called X, but never X Windows), which is the foundation for most GUI subsystems found in modern UNIX (unices?), Linux and the BSD's included, was also the result of an academic project, namely the Athena project at the Massachusetts Institute of Technology (MIT).

Unix has been a multiuser, multitasking, timesharing operating system since its beginnings. Also, since the incorporation of networking technologies, it's had the ability to allow a user to connect remotely and perform work on the system. Previously this was accomplished either via dumb serial terminals, or network connections (the legendary telnet).

When the time came to develop a GUI system that could run primarily under Unix, these concepts were kept in mind and incorporated into the design. Actually, X has a pretty complex design, which has often been mentioned as a disadvantage. However, because of its design, it's also a really versatile system, and this will become quite clear as we explain how all the parts comprising a GUI under Unix fit together.

Before taking a look at X's architecture, a really brief tour of its history, and how it ended up on your Linux system, is in order.

X was developed by the Athena project, and released in 1984. In 1988 an entity called the "X Consortium" took over X, and to this day handles its development and distribution. The X specification is freely available, this was a smart move as it has made X almost ubiquitous. This is how XFree86 came to be. XFree86 is the implementation of X we use on our Linux computers. XFree86 also works on other operating systems, like the *BSD lineage, OS/2 and maybe others. Also, despite its name, XFree86 is also available for other CPU architectures.

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