A windowing system 26 (and method) for displaying windows generated by application programs 24 comprises an update manager 30 that can received instructions and update application window buffers 32n that store window display data; a compositor 36 that utilises stored display data to generate composite image data; and a frame buffer 38 to store generated image data. The update manager prevents an application program from writing display data to an application window buffer if the stored display data has been updated since the compositor last generated composite image data. The compositor may also be prevented from generating composite image data if the display data in application window buffers has not been updated since the composite image data was last generated. Each application window buffer may have state data indicative of whether writing display data to the application is permitted. The application programs may be permitted to write image data directly to the frame buffer when their associated windows do not overlap.
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