Video display controller


A video display controller, 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 TV 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 games 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 pixel data, ROM to hold character fonts, or some discrete logic such as shift registers.
Most often the VDC chip is completely integrated in the logic of the main computer system, 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 at the same time the VDC reads it.

VDC vs. GPU

The difference between a display controller, a graphics accelerator, blitter, and a graphics processing unit is often blurred by examples that are called one but have some features of another class. It is not always clear when a particular video chip is a video display controller and when it is a video display processor. For example, the TMS9918 may be referred to either way, depending on the source.
Generally, a canonical VDC would not include any internal capability to perform graphics operations on its own, the ability to draw lines or fill areas for instance, and relies on the host CPU to perform these sorts of operations by performing those calculations and depositing the results in memory for the VDC to read. Blitters add some of these features, which requires additional registers and some math hardware on the IC to run these routines. Some chips called VDCs in the 1980 do include some of these features, which makes them technically closer to blitters, and the choice of terminology was largely due to the manufacturer preferring one term over the other.
VDCs and blitters were popular during the 1980s and into the 1990s. These VDCs often had special hardware for the creation of sprites. Early examples include the Atari's ANTIC and the Texas Instruments TMS9918. Later designs generalized sprites by implementing Bit Blitter functions, including the VDP2 32-bit background and scroll plane video display processor of the Sega Saturn and the Lisa chip that was used for the improved graphics of the later generation Amiga computers.
Graphics processing units take this evolution a step further, including not only more powerful math hardware, typically multiple floating point units, but also include the ability to send custom routines to the GPU to be run locally. This means that a GPU not only has built-in routines for "draw line" or "rotate camera", but can also be sent entire programs like "draw airplane", which can then be performed with a single line of code. This allows the host program to dramatically lower the amount of information that has to be sent back and forth between the GPU and the host and can offer large performance improvements.
GPUs became popular during the 1990s, including the S3 ViRGE, the Matrox Mystique, and the Voodoo Graphics; though earlier examples such as the NEC μPD7220 had already existed for some time. Modern systems all use a GPU-like display systems, sometimes built into the CPU, sometimes a separate IC on a larger motherboard, and sometimes on a separate card.

Types

Video display controllers can be divided in several different types, listed here from simplest to most complex;
  • Video shifters, or "video shift register based systems", 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 synchronization signals to output a 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 RAM attached to the CRTC to output it via an external character generator ROM or directly 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 simple text-mode only systems to 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 character generators and video RAM dedicated to colour attributes and palette registers for the high-resolution or text modes.
  • Video coprocessors have their own internal CPU dedicated to reading 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 character generator, the colour attribute RAM, palette registers, and the sprite logic.

    List of example VDCs

Examples of video display controllers are:
Video shifters
  • The RCA CDP1861 was a very simple chip, built in CMOS technology to complement the RCA 1802 microprocessor, it was mainly used in the COSMAC VIP. It could only support a very low resolution monochrome graphic mode.
  • The Television Interface Adaptor is the custom video chip that is the heart of the Atari 2600 games console, a primitive chip that relied on the 6502 microprocessor to do most of the work, also was used to generate the audio.
CRT Controllers
  • The Intel 8275 CRT controller was used in the Convergent Technologies AWS / Burroughs B20, along with some S-100 bus systems.
  • The Motorola 6845 is a video address generator first introduced by Motorola and used for the Amstrad CPC, and the BBC Micro. It was also used for almost all the early video adapters for the PC, such as the MDA, CGA and EGA adapters. The MDA and CGA use an actual Motorola chip, while the EGA has a custom IBM chipset of five LSI chips; one of those chips includes IBM's reimplementation of the CRTC, which operates like an MC6845 but differs in a few register addresses and functions so it is not 100% compatible. In all later VGA compatible adapters the function of the 6845 is still reproduced inside the video chip, so in a sense all current IBM PC compatible PCs still incorporate the logic of the 6845 CRTC.
Video interface controllers
Video coprocessors
  • The ANTIC was an early video system chip used in Atari 8-bit computers. It could read a "Display list" with its own built in CPU and use this data to generate a complex video signal.
  • The TMS9918 is known as the Video Display Processor and was first designed for the Texas Instruments TI-99/4, but was later also used in systems like the MSX, ColecoVision, Memotech MTX series, and for the Sega SG-1000 and SC-3000. The Master System uses an enhanced VDP based on the TMS9918, and the Sega 315-5313 VDP used in the Sega Genesis and some arcade machines is a further advancement of the Master System VDP with the original TMS9918 modes removed.
  • The NEC μPD7220. Used in some high-end graphics boards for the IBM PC in the mid 80s, notably in products from Number Nine Visual Technology.
  • The RP2C02 or RP2C07 was a video coprocessor designed by Ricoh for Nintendo's use in the Famicom and Nintendo Entertainment System. It was connected to 2048 bytes of dedicated video RAM, and had a dedicated address bus that allowed additional RAM or ROM to be accessed from the game cartridge. A scrollable playfield of 256×240 pixels was supported, along with a display list of 64 OBJs, of which 8 could be displayed per scanline.
  • The Yamaha V9938 is an improved version of the TMS9918, and was mainly used in the MSX2.
  • The Yamaha V9958 is the Video Display Processor mainly used in the MSX2+ and MSX turboR computers.
  • The VLSI VS21S010D-L is a 128kB SPI/parallel SRAM with an integrated video display controller with variable-bit-depth pixels and a block-move blitter.
  • The Thomson EF936x series of Graphic Display Processor, which offers a draw rate of 1 million pixels per second and resolutions up to 1024×512.