Frame buffer architecture capable of accessing a pixel aligned M by N array of pixels on the screen of an attached monitor

A frame buffer memory organization for use with a raster scan video monitor which organization is capable of accessing a pixel aligned M by N array of contiguous pixels on the screen of the monitor from a frame buffer memory constructed of an M by N array of memory chips, memory addressing and control means for driving a common address bus to all the memory chips, and for energizing selectively N row address strobe (RAS) wires across the memory chip array and M column address strobe (CAS) wires down the memory chip array,

• means to effect the writing of individual pixels in the array being enabled by energizing the write enable pins to each memory chip directly,
• means connecting the data wires in the memory organization together such that M pixels in a single row can be read or written simultaneously,
• means for supplying the same row and column address to all of the memory sections when address decoding means determine that the accessed M by N array lies along a physical word address boundary on said screen and in said memory whereby the pixel accessed from each section of the memory is at the same address in each section, or alternatively,
• means for sequentially supplying within a memory access cycle, two consecutive sets of row and column addresses to said memory and means including clock means for selectively energizing a first set of RAS and CAS wires during a first phase and a second set of RAS and CAS wires during a second phase of the memory access cycle wherein a total of N rows and M columns of address strobe wires are energized during a given memory access cycle, when said address decoding means determine that the accessed M by N array does not lie along a physical word address boundary, and
• means for generating an M by N direct mask for controlling the access to specified pixels of an accessed M by N array during a particular frame buffer cycle said mask comprising a configuration of "1's" and "0's" depending on which pixel(s) is to be accessed, said generating means including,
• a register for storing said direct mask,
• first rotation means operable in response to an X-offset signal cooperable with the direct mask register for rotating the mask in the X direction an amount equal to the offset of the array origin address (P0) from a physical word boundary along the X axis and second rotation means operable in response to a Y-offset signal cooperable with said direct mask register for rotating the mask in the Y direction an amount equal to the offset of the array origin address from a physical word boundary along the Y axis, and
• wherein means for determining the X and Y offset values comprises an address decoder which decodes the low order bits of the X and Y addresses defining the array origin (P0) to determine the pixel offset of the array origin, if any, from a physical word boundary.