Description of Video Ram Required for Higher Resolutions (132328)

SUMMARY

This article describes the memory requirements for different video resolutions in Windows.

   Colors       16        256       32K***    64K***    16.7 Million***
   --------------------------------------------------------------------
   Resolution
   640x480      256K*     256K      1 MB      1 MB      1 MB
   800x600      256K      512K      1 MB      1 MB      1.5 MB
   1024x768     512K      1 MB      1.5 MB    1.5 MB    2.5 MB
   1280x1024    1 MB**    1.5 MB    2.5 MB    2.5 MB    4 MB

* Most VGA cards are shipped with 256 KB of RAM installed.

** 640 KB actually needed. 1 MB required due to upgrade requirements on video cards.

*** External RAMDAC (RAM digital-to-analog converter) required for these colors.

MORE INFORMATION

Most current video chip sets have an internal ability to display 256 simultaneous colors. This requires 3 analog-to-digital converters to convert the bit planes in video memory to analog signals for the color guns in the monitor. This function is provided by the RAMDACs.

To use more than the basic number of colors typically requires an external RAMDAC. The "High Sierra" RAMDAC is a relatively small 16-pin DIP (dual in-line package) that supports up to 32,000 colors (or 15-bit color). The newer 24-bit (16.7 million colors) RAMDACs are sometimes larger than the actual video chip set.

Speed and resolution have a different meaning for RAMDACs and the actual video chip set. The speed of the chip set determines what type of performance is delivered, and resolution is the number of pixels it can process. For RAMDACs, resolution refers to the number of colors, not the number of pixels on the screen. The RAMDAC's resolution is its ability to differentiate between subtle shades of color.

It is difficult to produce a high-speed and high-resolution RAMDAC. These chips are not only large, but produce a significant amount of heat and add considerably to the cost of the video board. Many systems simply include empty sockets for optional RAMDACs. Without these chips, however, the boards are simply 256-color boards.

The amount of RAM required for each resolution-and-color combination can be determined using the following formula:

   ((horizontal * vertical) / 8) * number of bits for color

For example,

   ((640 * 480) /8 ) * 24 (16.7 million colors) = 921,600 bytes or 1 MB

The following table lists the number of colors converted to bits:

   Number of colors   Bits
   -----------------------
   16                 4
   256                8
   32K                15
   64K                16
   16.7 M             24