KDrive
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KDrive (or freedesktop.org Xserver or just Xserver) was a small X Window System server implementation created by Keith Packard. Unlike the X.Org Server, Xserver was not based on XFree86 code. It was used by X.org as a testing ground for new ideas, such as EXA.
It practically superseded SmallX, also known as TinyX and is commonly used in embedded systems with the need for a very thin X server.
A specific feature of KDrive was that it was configured at compile-time, not at run-time. It was often compiled to use the native Linux framebuffer device as output.
As of X.Org Server version 7.1, the KDrive framework was integrated into the reference implementation and is now part of the generic source code release of the server.