» System Architecture - A look at Display cards «

Video introduction

Video or graphics circuitry, usually fitted to a card but sometimes found on the motherboard itself, are responsible for creating the picture displayed by a monitor. On early text-based PCs this was a fairly mundane task. However, the advent of graphical operating systems dramatically increased the amount of information needing to be displayed to levels where it was impractical for it to be handled by the main processor. The solution was to off-load the handling of all screen activity to a more intelligent generation of controllers.

As we went beyond monochrome monitors, passing the heady days of 256 colours and into the graphically intense games and programs of today, so the need for ever faster memory increased.

80's
Up the late 80's a maximum of 256k was the standard, with an 8 bit controller.

90's
As memory prices began to fall, this begin to rise until, by around 1995, 1Mb with 16 bit controller became the standard, rising to 4Mb and 32 bit - 64 bit controllers by 1997 and climbing to 8Mb-16Mb and 64bit -128 bit as entry level as the century closes.

00's
As we approach the millenium the entry level for video memory look set to rise to 32Mb as standard with 196 bit - 256 bit controllers. High end card - largely for CAD stations - already offer 196Mb memory or more, while the new nVidia GeoForce chipset - aimed at playing games, supports 128Mb of video memory and utilitizes the world first GPU - graphics processor unit.

"DRam"
Dynamic Ram was the earliest memory used for video cards and remained the most widely used type until late 1996 when it was largely replaced by the faster EDO Ram. Traditionally supplied in 4 megabit (512k) chips with a 16 bit interface. To get the maximum data transfer speed between a 64 bit controller and ram, four chips were needed, meaning 2Mb of memory.

"EDO"
Extended Data Out Ram offered faster refresh rates and screen redraws and soon replaced the existing standard.

"VRam"
Video Ram was horrendously expensive and remained the province of graphic designers and CAD users. A special type of dual-ported DRAM Dual ported, it could be updated from the computer and refresh the screen at the same time. Favoured by Diamond.

"WRam"
Another dual ported ram, with additional hardware functions for screen manipulation within the chip. Favoured by Matrox

"SD-RAM"
Synchronous DRam.

"SG-RAM"
SDRam with additional graphics functions in hardware. Favoured by Matrox.

"MD-RAM"
Mosys Dram. This is Dram organised in 'chunks' to increase bandwidth. Favoured by Tseng, amongst others.

"RD-RAM / DRDRam"
Direct Rambus Dram uses a special interface with a low pin count, allowing for high speed and low cost. (Used for Nintendos Ultra 64)

The table below summarises the characteristics of six popular types of memory used in graphics subsystems:

Colour depth
Each pixel of a screen image is displayed using a combination of three different colour signals. The precise appearance of each pixel is controlled by the intensity of these three beams of light and the amount of information that is stored about a pixel determines its colour depth. The more bits that are used per pixel, the finer the colour detail of the image.

The table below shows the colour depths in current use:

Resolutions generally fall into predefined sets and the table below shows the series of video standards since CGA, the first to support colour/graphics capability

The table below shows the colour depths, subject to the amount of video memory, colours and resolution used:

Get the Anatomy of a video card