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Video Memory Usage
The main use of the video memory is as the frame buffer. This is the place where the information is stored about the video image itself. Each pixel on the screen has associated with it typically 4 to 32 bits of data that represent its color and intensity. The video processor (along with the system CPU) manipulate this data to change the screen image and the RAMDAC reads it and sends it to the monitor.
Higher end video cards also use the video memory for computation work. In particular, cards that support 3D functions such as filtering need to use memory for this work. Different video cards use different methods to decide how to allocate memory for these functions: some will use whatever memory is "left over" after the necessary space for the frame buffer has been set aside, while others will pre-allocate the calculation memory and then limit the size of the frame buffer accordingly. See here for memory requirements for the frame buffer for various resolutions and color depths.
Since some cards use their video memory for both the frame buffer and additional calculations, they may have more than one type of video memory. The dual porting advantages of VRAM are of course very helpful for the frame buffer, but of no value in memory being used internally for calculations. So, since VRAM costs more than single-ported memory technologies, manufacturers put some VRAM on the card for the frame buffer, and some "regular" memory on it for other purposes.
There are also two situations under which the video card can use the main system RAM as well. First, some discount PCs are designed to use part of the main system memory for their frame buffer, saving cost but leading to poor performance. Second, newer AGP video systems can use the main system memory for doing 3D processing or other work.
Next: DRAM Rated Speed