Types of Video Display Controllers
Video Display controllers can be (arbitrarily) divided in several different types (here listed from simple to complex);
- Video shifters, or "Video shift register based systems" (there is no generally agreed upon name for these type of devices) are the most simple type of video controllers; they are, (directly or indirectly) responsible for the video timing signals, but they normally do not access the Video RAM directly. They get the video data from the main CPU, a byte at a time, and convert it to a serial bitstream (hence the technical name "Video shifter"). This serial data stream is then used, together with the synchronisation signals, to output a (colour) video signal. The main CPU needs to do the bulk of the work. Normally these chips only support a very low resolution Raster graphics mode.
- A CRTC, or Cathode Ray Tube Controller, generates the video timings and reads video data from a RAM attached to the CRTC, to output it via an external character generator ROM (for text modes) or directly (for high resolution graphics modes) to the video output shift register. Because the actual capabilities of the video generator depend to a large degree on the external logic, video generator based on a CRTC chip can have a wide range of capabilities. From very simple (text mode only) systems to very high resolution systems supporting a wide range of colours. Sprites however are normally not supported by these systems.
- Video interface controllers are much more complex than CRT controllers, and the external circuitry that is needed with a CRTC is embedded in the video controller chip. Sprites are often supported, as are (RAM based) character generators and video RAM dedicated to colour attributes and pallette registers (Color lookup tables) for the high-resolution and/or text-modes.
- Video coprocessors have their own internal CPU dedicated to reading (and writing) their own video RAM, and converting the contents of this video RAM to a video signal. The main CPU can give commands to the coprocessor, for example to change the video modes or to manipulate the video ram contents. The video coprocessor also controls the (most often RAM based) character generator, the colour attribute RAM, Palette registers and the Sprite logic (as long as these exist of course).
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