Specifications

Video Display Adapters
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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 dis-
play every pixel on the screen, and the resolution determines the number of total pixels. For example,
a screen resolution of 1024×768 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 num-
ber of bits required to represent each pixel, you have the amount of memory the adapter needs to dis-
play that resolution. Here is how the calculation works:
1024×768 = 786432 pixels×24 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 1024×768 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 publish-
ing, and Web page design.
Table 15.12 Video Display Adapter Minimum Memory Requirements for 2D
Operations
Color Memory
Resolution Depth Max. Colors Required Memory Used
640×480 16-bit 65,536 1MB 614,400 bytes
640×480 24-bit 16,777,216 1MB 921,600 bytes
640×480 32-bit 4,294,967,296 2MB 1,228,800 bytes
800×600 16-bit 65,536 1MB 960,000 bytes
800×600 24-bit 16,777,216 2MB 1,440,000 bytes
800×600 32-bit 4,294,967,296 2MB 1,920,000 bytes
Chapter 15
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