Graphics system with shadow ram update to the color map

Abstract

A graphics system uses a programmable tile size and shape supported by a frame buffer memory organization wherein (X, Y) pixel addresses map into regularly offset permutations on groups of RAM address and data line assignments. Changing the mapping of (X, Y) pixel addresses to RAM addresses for the groups changes the size and shape of the tiles. A pixel data/partial address multiplexing method based on programmable tile size reduces the number of interconnections between a pixel interpolator and the frame buffer. A programmable pipelined shifter allows the dynamic alteration of the mapping between bits of the RGB intensity values and the planes of the frame buffer into which those bits are stored, as well as allowing those values to be truncated to specified lengths. Tiles are cached. Tiles for RGB pixel values are cached in an RGB cache, while Z values are cached in a separate cache. The Z buffer for hidden surface removal need not be a full size frame buffer, as a lesser portion of frame buffer is, if need be, used repeatedly. Updates to the color map are performed from a separate shadow RAM during vertical retrace. The shadow RAM is large enough to accommodate two copies of the color map, and can load them in automatic alternation, producing a blinking effect without the use of an additional plane of frame buffer memory.