Video display controller

A video display controller (VDC), also called a display engine or display interface, is an integrated circuit which is the main component in a video-signal generator, a device responsible for the production of a video signal in a computing or game system. Some VDCs also generate an audio signal, but that is not their main function. VDCs were used in the home computers of the 1980s and also in game consoles and arcade games.

The VDC is the main component of the video signal generator logic, responsible for generating the timing of video signals such as the horizontal and vertical synchronization signals and the blanking interval signal. Sometimes other supporting chips were necessary to build a complete system, such as RAM to hold pixeldata, ROM to hold character fonts, or some discrete logic such as shift registers.

Most often the VDC chip is used as an extension to a host computer system, and is completely integrated to the host, with the memory holding the graphics data appearing in the same memory map of the main CPU. This allows the CPU to write new data to memory which the VDC then reads and displays. In some cases, the VDC functions as a coprocessor that can manipulate the RAM contents independently. Due to the low performance of early dynamic RAM, some VDCs used an entirely separate DRAM pool that it could read without interrupting or being interrupted by the CPU. Later systems used dual-ported video RAM to avoid the performance issues, allowing the CPU to read or write memory from one port, while at the same time the VDC reads it from the other port.