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Frame Buffer Memory Requirements
The frame buffer is the video memory that is used to hold the video image displayed on the screen. The amount of memory required to hold the image depends primarily on the resolution of the screen image and also the color depth used per pixel. The formula to calculate how much video memory is required at a given resolution and bit depth is quite simple:
Of course it isn't really that simple (heh, you didn't think it would be, did you? :^) ). You usually need more memory than this formula computes. One major reason is that video cards are only available in certain memory configurations; you can't order a card with 1.7 MB of memory, as they couldn't build it even if they wanted to. This is the result of how memory is manufactured and how it is accessed; most conventional memory is organized into large banks and it isn't practical to arrange the card to use amounts of memory that aren't whole megabytes. In particular, there are several popular resolution and color depth combinations that require an amount of memory just over 2 MB; with conventional memory types these all require a card with 4 MB of video RAM, much of which is wasted. (A newer memory technology, MDRAM, is designed to use many smaller banks of memory so that cards with different amounts of memory can be manufactured.)
Another reason why more memory is often required is that many video cards, especially high end accelerators and 3D cards, use memory for computation as well as for the frame buffer, so they can use much more memory than is needed strictly to hold the screen image.
Finally, some video cards can only do memory addressing 8, 16 or 32 bits at a time; they cannot do 24-bit access to memory. Memory is commonly structured to allow access in amounts of bits that are straight powers of two, so some cards can't deal with 24-bit values. This means that for these cards, 32 bits of memory are required to use true color. The extra 8 bits per pixel are generally wasted when this is the case. As it happens, due to the "rounding up" that occurs because video cards use memory amounts in whole megabytes, there is no difference between 24-bit and 32-bit memory for 800x600 and 1024x768 resolutions. Both require 2 MB of video RAM or 4 MB of video RAM, respectively (see the table immediately below).
The table below shows, in binary megabytes, the amount of memory required for the frame buffer for each common combination of screen resolution and color depth. In parentheses, the smallest industry standard video memory configuration required to support the combination is shown, based on conventional video memory technology (not MDRAM):