Video display control system

摘要

A video display control system displays a video image composed of a plurality of display elements on a screen of a video display unit. The system comprises a memory (VRAM) for storing a plurality of color codes each representing at least one display element and a video display controller (VDP). The VDP comprises a mode register for selecting one of normal display and transparency processing modes, display processing circuit for reading the color codes from the VRAM, a backdrop color register for storing a color code such as one representing a backdrop color, a detection circuit for detecting a predetermined color code from the color codes read by the display processing circuit, and a selector controlled by an output of the detection circuit. In the normal display mode, the selector outputs all color codes read by the display processing circuit to the display unit. In the transparency processing mode, the selector outputs the color codes read by the display processing circuit to the display unit when the predetermined color code is not detected, and outputs the color code contained in the backdrop color register to the display unit when the predetermined color code is detected. The VDP further comprises another detection circuit for detecting a second predetermined color code from the color codes outputted from the selector to output a control signal. And if the second predetermined color code is detected in the transparency processing mode, the display unit displays an image in accordance with an external video signal.