A pattern write protect system approximates a memory area for data entry with a multiplicity of rectangles and determines when a point for entry is within the window by establishing a relative position of the address point coordinates to the coordinates of the sides of the rectangles. A less than/greater...http://www.google.com.au/patents/US4764764?utm_source=gb-gplus-sharePatent US4764764 - Write-protect apparatus for bit mapped memory
