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Video RAM Requirements for Various Display Resolutions and Color Depths*

Color Depth / Resolution 256 colors (8 bpp) 65536 colors (16 bpp) 16777216 colors (24 bpp)

640x480 0.5MB 1.0MB 1.0MB

800x600 0.5MB 1.0MB 1.5MB

1024x768 1.0MB 1.5MB 2.5MB

1152x864 1.0MB 2.0MB 3.0MB

1280x1024 1.5MB 2.5MB 4.0MB

1600x1200 2.0MB 4.0MB 5.5MB

*Note: Rounded up to the nearest 0.5MB.

The following table provides a good guideline for estimating the capabilities of a given video

source or for determining the memory requirements for a new source.

NOTE:

When video RAM is organized to provide 8bpp, each 8-bit pixel value in memory functions

as an index into an array of 256 palette registers. The color value presented to the monitor

(either directly or by way of a D/A converter) is that contained in the indexed palette register.

The palette registers can contain color values with more than 8-bits (typically 18-bits). Thus,

while it is possible to display only 256 distinct colors, these colors constitute a subset of a

potentially much larger color set (typically a set of 262,144 distinct colors.) Any 256-member

subset of this larger set of colors may be displayed by changing the palette register values.

On the other hand, when video RAM is organized to provide 16 or 24bpp, each pixel value in

memory becomes a direct representation of a color. In this case, the palette registers are

not used, and the RAM data are fed directly (or by way of a D/A converter) to the monitor.

NOTE:

LCD monitors are not subject to “flicker” when displaying imagery at low refresh rates (vertical

sync rates) as are CRT monitors. Therefore, there is no advantage to running a 3000 series

monitor at a refresh rate higher than 60Hz. In fact, limiting the refresh rate to 60Hz could be

of benefit in some cases. Whenever the refresh rate is increased for a given resolution, the

video source is required to run at correspondingly higher pixel clock rate. For this reason,

some older video sources might not be able to support a specific high resolution at a refresh

rate above 60Hz, but might be able to support that resolution at 60Hz.

3.2

Setting up the Video Source Cont.

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