Hard-wired circuit for handling screen windows

Claim 1

A hard-wired screen window handler circuit coupled between an image memory and a screen controller, the image memory being organized in as many pages as there are possible windows, with each page corresponding to a particular window and storing aspect words relating to point or character positions of a document to be displayed in full or in part in the corresponding window on the screen, and the screen controller generating a signal for refreshing a screen display by scanning lines of successive point or character positions while simultaneous reading from any one of the pages in the image memory by means of an address word including an X-axis group of bits having a value which is a function of a column number of a current point or character position along a scan line and a Y-axis group of bits having a value which is a function of a line number of a current line in the scan, wherein the handler comprises: two auxiliary memories, one being a window X-axis projection memory which is addressed by at least a more significant portion of an X-axis group of bits and which stores for each of the resulting possible X-axis addressing values a window X-axis occupation word indicating which of the windows include any point or character positions occupying the column of the screen which is addressed by the said portion of the X-axis group of bits, the other being a window Y-axis projection memory which is addressed by at least a more significant portion of a Y-axis group of bits and which stores for each of the resulting possible Y-axis addressing values a window Y-axis occupation word indicating which of the windows include any point or character positions occupying the line of the screen which is addressed by the said portion of the Y-axis group of bits; intersection logic means connected to receive the window X-axis and Y-axis occupation words to generate a third occupation word indicating which of the windows include the point or character position currently being scanned by the screen controller; and a priority encoder determining an order of window superposition and connected to receive the said third occupation word to deliver a number representative of the window which is to be displayed at the currently scanned screen position, said number being used to select a particular page of the image memory to be read by means of the address word generated by the screen controller.