| Upgrading Video with a Video Acceleration Board |
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6.2 |
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6.2.4 | Understanding RAMDAC and video memory |
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The display information stored and
manipulated in the video memory is in the standard binary format of 1s
and 0s. These binary patterns control the resolution and color of each
pixel on the video display screen. However, monitors are analog, not
digital devices. In order for the monitor to work, the digital
information in the video memory must be translated into analog form
for export to the monitor screen. This is the role of the Random Access
Memory Digital-to-Analog Converter
(RAMDAC) chip.
The RAMDAC chip reads the video memory content, converts it to analog, and sends it over a cable to the video monitor. The quality of this chip impacts the quality of the image, speed of the refresh rate, and maximum resolution capability. Refresh rate refers to the number of times per second that the video display screen can be redrawn. Video Memory Resolution is commonly expressed as a pair of numbers. Each pair of numbers represents the maximum possible number of dots on a horizontal axis and the maximum possible number dots on a vertical axis. The basic VGA resolution of 640 by 480 means that there are 640 possible dots on the horizontal axis, and 480 possible dots on the vertical axis. Enhanced VGA has a resolution of 800 by 600 dots. Super VGA has a resolution of 1024 by 768 dots. From these examples, it is easy to see that the higher the resolution, the more memory required to draw the image. However, the higher the resolution, the sharper and clearer the image. When an image is displayed in color (or grayscale), a certain number of bits must be assigned per dot (per pixel) to achieve a given color depth or possible number of colors. The more bits assigned, the more colors that can be presented. To see how video memory impacts the execution of graphics applications, return to the previous example. Imagine once again that a graphics application is being used to draw a box on the monitor screen. When the system CPU issues a series of commands to draw a box, it issues commands to the video processor chip to draw the box.
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