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  • Modern Operating System --- The X Window System

    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. 

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  • 原文地址:https://www.cnblogs.com/miaoyong/p/4865646.html
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