8-1/2, the Plan 9 Window System

The Plan 9 window system, 81⁄2, is a modest-sized program of novel design. It provides textual I/O and bitmap graphic services to both local and remote client programs by offering a multiplexed file service to those clients. It serves traditional UNIX files like /dev/tty as well as more unusual ones that provide access to the mouse and the raw screen. Bit­ map graphics operations are provided by serving a file called /dev/bitblt that interprets client messages to perform raster opera­ tions. The file service that 81⁄2 offers its clients is identical to that it uses for its own implementation, so it is fundamentally no more than a multi­ plexer. This architecture has some rewarding symmetries and can be implemented compactly.