Apparatus for controlling the background and foreground colors displayed

Abstract

Apparatus for controlling the colors displayed by a raster graphic system. Information stored at each addressable location of a RAM includes a set of behavior bits and a set of control bits. These bits are read out of memory during each memory read cycle. The control bits are stored in a shift register and the behavior bits are applied to an escape code detector and may be stored in a foreground or a background behavior register if enabled by the detector. One control bit is shifted out of the shift register each pixel clock pulse. This control bit determines the register from which the behavior bits are selected to form a color index. The index includes behavior bits from the selected register and the control bit for the pixel being scanned. This index is applied to a color look-up memory which produces color control signals which are applied to D/A converters, the outputs of which control the color and intensity of each pixel of the raster. A certain set of behavior bits, an escape code, causes the detector to inhibit both behavior registers from storing the escape code and enables the background behavior register to store the next set of behavior bits read out of memory. Thereafter the escape code detector enables the foreground register to store behavior bits produced from memory during each memory read cycle until the next escape code is detected.