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[ The PC Guide | Systems and Components Reference Guide | Video Cards | Video Card Overview ]

Video Memory (Frame Buffer)

The screen image that you see on your monitor can contain a fair bit of information: at the upper end, a 1600x1200 pixel screen display in true color contains almost 6 MB of data! And this is just for the displayed image, not for the data itself that the image represents.

In the early days of PCs the amount of information displayed was much, much less. A screen of monochrome text, for example, needs only about 2 KB of space. Special parts of the upper memory area (UMA) were dedicated to holding this video data. The processor would compute what needed to be displayed, would put it into this area, and then the video card would read it and display it.

As the need for video memory increased into the megabyte range, it began to make more sense to put the memory on the video card itself. In fact, to preserve existing PC design limitations, it was necessary (there simply isn't any more space in the UMA to hold the bigger screen images). The memory that holds the video image is sometimes called the frame buffer. A big advantage of having the memory on the video card is that it can be customized to the task at hand for greater efficiency, instead of using regular system RAM. The memory on the video card comes in many different sizes and flavors, and new technologies to improve performance are being invented all the time.

Some motherboard designs integrate the video chipset into the motherboard itself, and then use part of the system RAM for the frame buffer. This is called unified memory architecture. This is done for only one reason: cost savings. The result is almost always much lower video performance, because in order to use higher resolutions and refresh rates, the video memory needs to have much higher performance than the RAM normally used for the system. A similar-sounding but different system is used by the new Accelerated Graphics Port (AGP), which lets the video processor access the system memory for doing graphics calculations, but keeps a dedicated video memory for the frame buffer. This allows for more flexible memory use without sacrificing performance and is becoming a new standard in the PC world.

Video memory issues are discussed in detail here.

Next: The RAMDAC

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