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【标准】计算机视频显示标准介绍(英文) - Color Graphics Adapter (CGA)

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【标准】计算机视频显示标准介绍(英文)
MDA
HGC
CGA
EGA
PGC
8514
VGA
MCGA
SVGA
XGA
SXGA
QXGA
HXGA
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Color Graphics Adapter (CGA)

IBM came back to the fore when color started to appear in computer displays. The CGA standard, introduced in 1981 and primative by today's standards, was still color; even if only 16 of them. Because the first PCs were for business, the color did not first catch on and the MDA monochrome standard we more often used. As prices came down and clones of the IBM PC were introduced, CGA became more of a standard.

The CGA card came with 16 KB of video memory and supported several different modes:

  • Text mode which included 80x25 text (like the MDA system) in 16 colors. The resolution, however was lower as each character was made up of 8x8 pixels instead of the MDA's 9x14 pixels. A 40x25 text mode was also supported in 16 colors. In both, the foreground and background colors could be changed for each character.
  • Monochrome graphics mode which displayed graphics at 640x200 pixels. This was lower than the Hercules card but seemed to serve the purpose for an initial release and this was quickly replaced with the EGA standard.
  • Color graphics mode which came in two flavors: a 320x200 pixel mode with four colors and a lesser-used resolution of 160x200 in 16 colors. The four-color mode only had two official palettes to choose from:
    • Magenta, cyan, white and background color (black by default).
    • Red, green, brown/yellow and background color (black by default).

The 16-color graphic mode used a composite color mode instead of the 16 colors of the CGA text above. Because the color technique was not supported in the BIOS there was little adoption of that mode except by some games.

The CGA color palette was based on the Motorola MC6845 display controller. Red, green, and blue were created by the three cathode rays with black being an absence of cathode rays. The other colors were mixes of two different colors and white used all three color beams. An "intensifier" bit gave a brighter version of the basic 8 colors for a total of 16. There was one exception to this. In the normal RGB model color #6 should be a dark yellow (#AAAA00) however IBM changed the monitor circuitry to detect it and lower its green component to more closely match a brown (#AA5500) color. Other monitor makers mimiced this which is why the intense version of #6, brown, turned out to be a bright yellow as the intense version was not so modified. There is no clear reason expressed why IBM did this but it's speculated they wanted to match 3270 mainframe colors. So, the colors appeared as...

  • Color 0 - Black - #000000 and the intense version, color 8 - Dark Grey - #555555
  • Color 1 - Blue - #0000AA and the intense version, color 9 - Bright Blue - #5555FF
  • Color 2 - Green - #00AA00 and the intense version, color 10 - Bright Green - #55FF55
  • Color 3 - Cyan - #00AAAA and the intense version, color 11 - Bright Cyan - #55FFFF
  • Color 4 - Red - #AA0000 and the intense version, color 12 - Bright Red - #FF5555
  • Color 5 - Magenta - #AA00AA and the intense version, color 13 - Bright Magenta - #FF55FF
  • Color 6 - Brown - #AA5500 and the intense version, color 14 - Bright Yellow - #FFFF55
    Color 6 in some clone monitors -Yellow - #AAAA00
  • Color 7 - Light Grey - #AAAAAA and the intense version, color 15 - Bright White - #FFFFFF (which you won't see because the background here is white)

There were several tweaks to the CGA text and graphics systems which resulted in different default background colors, different colored borders, and other tweaks which gave the appearance of the CGA system having more than the graphic modes above; but, these were all tweaks and not changes to the basic system itself.

Refresh rate for CGA monitors was increased to 60 Hz as a result of eyestrain complaints from the MDA 50 Hz rate. (The higher the refresh rate the less likely pixels on the screen will flicker as the phosphor is refreshed at a faster rate.)

See here for more details and pictures.

But, the low resolution of CGA begged for higher resolutions. To fill those demands IBM developed EGA...