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AGP Video System Memory Access
One of the key features of AGP is the ability to share the main system memory with the video chipset. The reason that this has been incorporated into the design is to allow the video subsystem to have access to larger amounts of memory for 3D and other processing, without requiring that large quantities of special video memory be put on the video card for this purpose. Right now the memory on the video card is shared between the frame buffer and the other uses that the video card has for memory. Since the frame buffer requires high-performance memory technologies, most cards use this better technology (such as VRAM) for all of the memory on the video card, which is usually overkill for the parts other than the frame buffer.
Note that AGP is not the same as the ill-fated unified memory architecture (UMA). Under UMA, all of the video card's memory, including the frame buffer, is taken from main system memory. Under AGP, the frame buffer remains on the video card, where it belongs. The frame buffer is the most important part of the video memory and it requires the highest performance, so it makes sense to leave it on the video card so special video-specific technologies like VRAM can be used.
What AGP does is to allow the video processor to access the system memory for other tasks that require memory, such as texturing and other 3D operations. The theory is that this memory isn't as crucial as the frame buffer, and doing this design means that higher-end video cards can be made more inexpensively by saving on the cost of dedicated video card memory that is only being used for these operations.
Next: AGP Requirements