TRANSPARENT BLOCKING OF CRT REFRESH FETCHES DURING VIDEO OVERLAY USING DUMMY FETCHES

A graphics controller overlays a movie window over the graphics pixel data. Movie pixels are from a movie source and are muxed into the pixel path by a pixel mux near the end of the graphics pipeline. A comparison of the current pixel count to the pixel address of the start and ending boundaries of the movie window controls the pixel mux, which selects either graphics pixels or movie pixels for display to a screen. Since the graphics controller is pipelined, the pixel compare near the end of the pipeline does not restart the graphics pipeline early enough for it to pre-process the graphics pixels. The graphics pipeline does not stop during the movie window but instead performs dummy fetches from the graphics memory to a CRT FIFO in the graphics pipeline. Dummy fetches are fetches of graphics pixels that are not displayed. Since these fetches contain non-displayed pixels, they are not needed except to keep the fetch count counting even during the movie window. Other requests are given priority during dummy fetches. Thus fetches to fill the CRT FIFO with pixels underneath the movie overlay which are not displayed are transparently blocked by activating dummy fetches, allowing other requestors to have full access to the graphics memory. Dummy fetches are activated by a compare of the fetch counts to the width of the graphics and movie portions of the screen, expressed in memory fetches rather than in pixels. A comparison of the total fetches in a horizontal line to the current fetch count prevents fetching un-displayed pixels past the end of the line.