Video Hardware, Part 3 - Video RAM Speed

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Video cards with the same type of 3D graphics processor chip (GPU) onboard might use different speeds of memory. For example, two cards that use the NVIDIA GeForce FX 5200—the Prolink PixelView and the Chaintech A-FX20—use different memory speeds. The Prolink card uses 4ns memory, whereas the Chaintech card uses 5ns memory.

Sometimes, video card makers also match different memory speeds with different versions of the same basic GPU, as with ATI's Radeon 9800 XT and 9800 Pro. The 9800XT has a core clock speed of 412MHz, versus the 9800 Pro's 380MHz clock speed. Thus, the 9800 XT supports memory running at 730MHz, whereas the 9800 Pro supports memory running at 680MHz. By using the slower Pro part, vendors can use slower memory to create a less expensive graphics card.

Unless you dig deeply into the technical details of a particular 3D graphics card, determining whether a particular card uses SDRAM, DDR SDRAM, DDR-II, SGRAM, or GDDR-3 can be difficult. Because none of today's 3D accelerators feature upgradeable memory, I recommend that you look at the performance of a given card and choose the card with the performance, features, and price that's right for you.

RAM Calculations

The amount of memory a video adapter needs to display a particular resolution and color depth is based on a mathematical equation. A location must be present in the adapter's memory array to display every pixel on the screen, and the resolution determines the number of total pixels. For example, a screen resolution of 1024x768 requires a total of 786,432 pixels.

If you were to display that resolution with only two colors, you would need only 1 bit of memory space to represent each pixel. If the bit has a value of 0, the dot is black, and if its value is 1, the dot is white. If you use 24 bits of memory space to control each pixel, you can display more than 16.7 million colors because 16,777,216 combinations are possible with a 4-digit binary number (2 ²⁴=16,777,216). If you multiply the number of pixels necessary for the screen resolution by the number of bits required to represent each pixel, you have the amount of memory the adapter needs to display that resolution. Here is how the calculation works:

1024x768 = 786432 pixelsx24 bits per pixel

= 18,874,368 bits

= 2,359,296 bytes

= 2.25MB

As you can see, displaying 24-bit color (16,777,216 colors) at 1024x768 resolution requires exactly 2.25MB of RAM on the video adapter. Because most adapters support memory amounts of only 256KB, 512KB, 1MB, 2MB, or 4MB, you would need to use a video adapter with at least 4MB of RAM onboard to run your system using that resolution and color depth.

To use the higher-resolution modes and greater numbers of colors common today, you would need much more memory on your video adapter than the 256KB found on the original IBM VGA. Table 15.12 shows the memory requirements for some of the most common screen resolutions and color depths used for 2D graphics operations, such as photo editing, presentation graphics, desktop publishing, and Web page design.

Table 15.12 Video Display Adapter Minimum Memory Requirements for 2D Operations

Resolution	Color Depth	Max. Colors	Memory Required	Memory Used
640x480	16-bit	65,536	1MB	614,400 bytes
640x480	24-bit	16,777,216	1MB	921,600 bytes
640x480	32-bit	4,294,967,296	2MB	1,228,800 bytes
800x600	16-bit	65,536	1MB	960,000 bytes
800x600	24-bit	16,777,216	2MB	1,440,000 bytes
800x600	32-bit	4,294,967,296	2MB	1,920,000 bytes
1024x768	16-bit	65,536	2MB	1,572,864 bytes
1024x768	24-bit	16,777,216	4MB	2,359,296 bytes
1024x768	32-bit	4,294,967,296	4MB	3,145,728 bytes
1280x1024	16-bit	65,536	4MB	2,621,440 bytes
1280x1024	24-bit	16,777,216	4MB	3,932,160 bytes
1280x1024	32-bit	4,294,967,296	8MB	5,242,880 bytes
1400x1050	16-bit	65,536	4MB	2,940,000 bytes
1400x1050	24-bit	16,777,216	8MB	4,410,000 bytes
1400x1050	32-bit	4,294,967,296	8MB	5,880,000 bytes
1600x1200	16-bit	65,536	4MB	3,840,000 bytes
1600x1200	24-bit	16,777,216	8MB	5,760,000 bytes
1600x1200	32-bit	4,294,967,296	8MB	7,680,000 bytes

From this table, you can see that a video adapter with 4MB can display 65,536 colors in 1600x1200 resolution mode, but for a true color (16.8 million colors) display, you would need to upgrade to 8MB. In most cases you can't add memory to your video card—you would need to replace your current video card with a new one with more memory.

3D video cards require more memory for a given resolution and color depth because the video memory must be used for three buffers: the front buffer, back buffer, and Z-buffer. The amount of video memory required for a particular operation varies according to the settings used for the color depth and Z-buffer. Triple buffering allocates more memory for 3D textures than double-buffering but can slow down performance of some games. The buffering mode used by a given 3D video card usually can be adjusted through its properties sheet.

Table 15.13 lists the memory requirements for 3D cards in selected modes. For memory sizes used by other combinations of color depth and Z-buffer depth, see the eTesting Labs' Memory Requirements for 3D Applications Web site at the following address:

http://www.etestinglabs.com/benchmarks/3dwinbench/d5memfor3d.asp

Table 15.13 Video Display Adapter Memory Requirements for 3D Operations

Resolution	Color Depth	Z-Buffer Depth	Buffer Mode	Actual Memory Used	Onboard Video Memory Size Required
640x480	16-bit	16-bit	Double	1.76MB	2MB
			Triple	2.34MB	4MB
	24-bit	24-bit	Double	2.64MB	4MB
			Triple	3.52MB	4MB
	32-bit	32-bit	Double	3.52MB	4MB
			Triple	4.69MB	8MB
800x600	16-bit	16-bit	Double	2.75MB	4MB
			Triple	3.66MB	4MB
	24-bit	24-bit	Double	4.12MB	8MB
			Triple	5.49MB	8MB
	32-bit	32-bit	Double	5.49MB	8MB
			Triple	7.32MB	8MB
1024x768	16-bit	16-bit	Double	4.50MB	8MB
			Triple	6.00MB	8MB
	24-bit	24-bit	Double	6.75MB	8MB
			Triple	9.00MB	16MB
	32-bit 1	32-bit	Double	9.00MB	16MB
			Triple	12.00MB	16MB
1280x1024	16-bit	16-bit	Double	7.50MB	8MB
			Triple	10.00MB	16MB
	24-bit	24-bit	Double	11.25MB	16MB
			Triple	15.00MB	16MB
	32-bit	32-bit	Double	15.00MB	16MB
			Triple	20.00MB	32MB
1600x1200	16-bit	16-bit	Double	10.99MB	16MB
			Triple	14.65MB	16MB
	24-bit	24-bit	Double	16.48MB	32MB
			Triple	21.97MB	32MB
	32-bit	32-bit	Double	21.97MB	32MB
			Triple	29.30MB	32MB

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Note -Although 3D adapters typically operate in a 32-bit mode (refer to Table 15.12), this does not necessarily mean they can produce more than the 16,277,216 colors of a 24-bit true-color display. Many video processors and video memory buses are optimized to move data in 32-bit words, and they actually display 24-bit color while operating in a 32-bit mode, instead of the 4,294,967,296 colors you would expect from a true 32-bit color depth.

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If you spend a lot of time working with graphics and want to enjoy 3D games, you might want to invest in a 32-bit 3D video card with at least 64MB or more of RAM. Although 2D operations can be performed with as little as 4MB of RAM, 32-bit color depths for realistic 3D operation with large Z-buffers use most of the RAM available on a 16MB card at 1024x768 resolution; higher resolutions use more than 16MB of RAM at higher color depths. Today's video cards provide more RAM and more 2D/3D performance for less money than ever before. Note that recent and current-model video cards don't use socketed memory anymore, so you must be sure to buy a video card with all the memory you might need now and in the future. Otherwise, you must replace a card with inadequate memory.

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Note -If your system uses integrated graphics and you have less than 256MB of RAM, you might be able to increase your available graphics memory by upgrading system memory (system memory is used by the integrated chipset). Most recent Intel chipsets with integrated graphics automatically detect additional system memory and adjust the size of graphics memory automatically. Refer to Table 15.8 for details.

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Windows Can't Display More Than 256 Colors -If you have a video card with 1MB or more of video memory, but the Windows Display Settings properties sheet won't allow you to select a color depth greater than 256 colors, Windows might have misidentified the video card during installation or the video driver installation might be corrupted. To see which video card Windows recognizes, click the Advanced Properties button, and then click the Adapter tab if necessary. Your adapter type might be listed either by the video card's brand name and model or by the video card's chipset maker and chipset model.

If the card model or chipset appears to be incorrect or not specific enough, click Change and see what other drivers your system lists that appear to be compatible, or use a utility program provided by your video card/chipset maker to identify your card and memory size. Then, manually select the correct driver if necessary. If the video card model or chipset appears to be correct, open the System properties sheet, locate the Device Manager, and remove the display adapter listing. Restart the computer and Windows will redetect the video card/chipset and install the correct driver.

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Next: Video Bus Width >>
 

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