Nearly all UNIX systems base their user interface on the X Window System (often called X),
developed ar MIT as part of project Athena in the 1980s. It is very portable and runs entirely
in user space. It was originally intended for connecting a large number of remote user terminals
with a central computer server, so it is logically split into client software and host software,
which can potentially run on different computers. On modern personal computers, both parts can
run on the same machine. On Linux systems, the popular Gnome and KDE desktop environments
run on top of X. When X is running on a machine, the software that collects input from the keyboard
and mouse and writes output to the screen is called the X server. It has to keep track of which
window is currently selected (where the mouse pointer is), so it knows which client to send any
new keyboard input to. It communicates with running programs (possible over a network) called
X clients. It may seem odd that the X server is always inside the user's computer while the X
client may be off on a remote computer server, but just think of the X server's main job: displaying
bits on the screen, so it makes sense to be near the user. From the program's point of view, it
is a client telling the server to do things, like display text and geometric figures. The server (in the
local PC) just does what it is told, as do all servers. The arrangement of client and server is shown
in Fig 5-37 (Please refer to the textbook to see the figure), for the case where the X client
and X server are on different machines. But when running Gnome and KDE on a single machine,
the client is just some application program using the X library talking to the X server on the same
machine (but using a TCP connection over sockets, the same as it would do in the remote case).
The reason it is possible to run the X window system on top of UNIX (or another operating system)
on a single machine or over a network is that what X really defines is the X protocal between X client
and X server. It does not matter whether the client and server are on the same machine; separated
by 100 meters over a local area network, or are thousands of kilometers apart and connected by the
internet. The protocal and operation of the system is identical in all cases.