Essentially all Linux systems use the X Windows system (X11) as the graphical user interface (GUI)—generally XFree86, which is the most used port of X11 on Intel. This is the underlying code of Linux graphical user interface systems. Exceptions include some servers that may not need a GUI and run in character mode, and some embedded systems that use other GUI systems not based on X11 to get better performance, such as Qtopia, which is used on the Sharp Zaurus.