Graphics control planes for windowing and other display operations

A system for controlling the drawing of a window in a system for displaying multiple windows, the system comprising a write enable test, and having a data structure comprising a single write-enable plane and a window data structure for storing information regarding the relative positions of the displayed windows including relative window priorities, window intersection regions and window clip boundaries information, the system comprising:

• means for checking the window data structure and determining whether the relative position of the window has changed since a previous operation;
• means for updating the single write-enable plane, using window intersection regions in the window data structure, with a complementary bit pattern so that all obscured regions of the single write-enable plane which correspond with the window are set to a first logic level and so that all unobscured regions of the single write-enable plane are set to a second logic level, wherein the single write-enable plane is updated only if the relative position of the window has changed since said previous operation;
• means for setting the write enable test true for unobscured regions of the window to be displayed;
• and
• means for determining if a pixel is to be written to a frame buffer, based on said write enable test and on the window clip boundary.

An apparatus for controlling pixel writes in a system for displaying multiple windows, comprising:

• frame buffer means for storing a pixmap of a window for display, said frame buffer means comprising a single write-enable plane configured to indicate whether a pixel is within a window boundary of said window and whether said pixel is obscured by another window,
• wherein said single write-enable plane indicate whether said pixel is to be written to said frame buffer means by using a complementary bit pattern and said single write-enable plane is updated by said frame buffer means only when the relative position of said window has changed since a previous display operation;
• and.
• graphics controller means, electronically coupled to said frame buffer means, for determining whether a pixel is to be written to said frame buffer means,
• wherein said determination is made based on said single write-enable plane and window clip boundary data.