media, etc.) and the possibility to mount filesystems residing on physically distinct filesystems into a hierarchical namespace, but adds the possibility to mount a connection to a server program that speaks a standardized protocol and treat its services as part of the namespace. For example, the original window system, called 8½, exploited these possibilities as follows. Plan 9 represents the user interface on a terminal by means of three pseudo-files: , which can be read by a program to get notification of mouse movements and button clicks, , which can be used to perform textual input/output, and , writing to which enacts graphics operations (see bit blit). The window system multiplexes these devices: when creating a new window to run some program in, it first sets up a new namespace in which , and are connected to itself, hiding the actual device files to which it itself has access. The window system thus receives all input and output commands from the program and handles these appropriately, by sending output to the actual screen device and giving the currently focused program the keyboard and mouse input. The program does not need to know if it is communicating directly with the operating system's device drivers, or with the window system; it only has to assume that its namespace is set up so that these special files provide the kind of input and accept the kind of messages that it expects. Plan 9's distributed operation relies on the per-process namespaces as well, allowing client and server processes to communicate across machines in the way just outlined. For example, the command starts a remote session on a computation server. The command exports part of its local namespace, including the user's terminal's devices (, , ), to the server, so that remote programs can perform input/output using the terminal's mouse, keyboard and display, combining the effects of remote login and a shared network filesystem. 9P protocol All programs that wish to provide services-as-files to other programs speak a unified protocol, called 9P. Compared to other systems, this reduces the number of custom programming interfaces. 9P is a generic, medium-agnostic, byte-oriented protocol that provides for messages delivered between a server and a client. The protocol is used to refer to and communicate with processes, programs, and data, including both the user interface and the network. With the release of the 4th edition, it was modified and