Enhanced Graphics Adapter
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The Enhanced Graphics Adapter (EGA) is the IBM PC computer display standard specification located between CGA and VGA in terms of graphics performance (that is, color and space resolution). Introduced in 1984 by IBM for its new PC-AT, EGA produced a display of 16 colors at a resolution of up to 640×350 pixels. The EGA card included a 16 kilobyte ROM to extend the system BIOS for additional graphics functions and included the Motorola MC6845 video address generator.
Each of the 16 colors could be assigned a unique RGB color code via a palette mechanism in the 640×350 high-resolution mode; EGA let you choose the displayed colors out of a total of 64 palette colors (two bits per pixel for red, green and blue). EGA also included full 16-color versions of the CGA 640×200 and 320×200 graphics modes; only the 16 CGA/RGBI colors are available in these modes. The original CGA modes are also present, though EGA isn't 100% hardware compatible with CGA. EGA can drive an MDA monitor by a special setting of switches on the board; only 640×350 high-res is available in this mode.
EGA cards used the PC ISA bus, and were available starting in 8-bit versions. The base IBM EGA card came with 64 kilobytes of video memory installed, actually just enough to handle monochrome high-resolution graphics (but allowing for full color in the 640×200 and 320×200 modes). Eventually, most EGA cards and clones would come with a full 256 KiB of memory. A few third-party EGA clones (notably the ATI Technologies and Paradise boards) featured a range of extended graphics modes (e.g. 640×400, 640×480 and 720×540), as well as automatic monitor type detection, and sometimes also a special 400-line interlace mode for use on CGA monitors.
The EGA standard was made obsolete by the introduction of VGA by IBM in April 1987 with the PS/2 computer line. Utilities such as VGA2EGA helped late adopters to view VGA images without upgrading hardware.
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[edit] The EGA color palette
The EGA palette allows all 16 CGA colors to be used simultaneously, and it allows each of these colors to be selected from a total of 64 colors (two bits each for red, green and blue.) This also allows the CGA's alternate brown color to be used without any additional display hardware. The later VGA standard built on this by allowing each of the 64 colors to be further customized.
When selecting a color from the EGA palette, two bits are used for the red, green and blue channels. This allows each channel a value of 0, 1, 2 or 3. To select the color magenta, the red and blue values would be medium intensity (2, or 10 in binary) and the green value would be off (0). When calculating the intended value in the 64-color EGA palette, the binary number of the intended entry is of the form "rgbRGB" where a lowercase letter is the least significant bit of the channel intensity and an uppercase letter is the most significant bit. For magenta, the most significant bit in the red and blue values is a 1, so the uppercase R and B placeholders would become 1. All other digits are zeros, giving the binary number 000101 for the color magenta. This is 5 in decimal, so setting a palette entry to 5 would result in it being set to magenta. All the color values for the default colors are listed in the table on the right.
| Default EGA 16-color palette (set up to match the standard CGA colors) |
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|---|---|---|
| Color | rgbRGB | Decimal |
| 0 — black (#000000) | 000000 | 0 |
| 1 — blue (#0000AA) | 000001 | 1 |
| 2 — green (#00AA00) | 000010 | 2 |
| 3 — cyan (#00AAAA) | 000011 | 3 |
| 4 — red (#AA0000) | 000100 | 4 |
| 5 — magenta (#AA00AA) | 000101 | 5 |
| 6 — brown (#AA5500) | 010100 | 20 |
| 7 — white / light gray (#AAAAAA) | 000111 | 7 |
| 8 — dark gray / bright black (#555555) | 111000 | 56 |
| 9 — bright blue (#5555FF) | 111001 | 57 |
| 10 — bright green (#55FF55) | 111010 | 58 |
| 11 — bright cyan (#55FFFF) | 111011 | 59 |
| 12 — bright red (#FF5555) | 111100 | 60 |
| 13 — bright magenta (#FF55FF) | 111101 | 61 |
| 14 — bright yellow (#FFFF55) | 111110 | 62 |
| 15 — bright white (#FFFFFF) | 111111 | 63 |
[edit] Specifications
[edit] Connector
Pin numbers (looking at socket):
| Pin | Name | Function |
|---|---|---|
| 1 | GND | Ground |
| 2 | SR | Secondary Red (Intensity) |
| 3 | PR | Primary Red |
| 4 | PG | Primary Green |
| 5 | PB | Primary Blue |
| 6 | SG | Secondary Green (Intensity) |
| 7 | SB | Secondary Blue (Intensity) |
| 8 | H | Horizontal Sync |
| 9 | V | Vertical Sync |
[edit] Signal
| Type | Digital, TTL |
|---|---|
| Resolution | 640h x 350v x 16c, other modes available |
| H-freq | 15.7/21.8 kHz |
| V-freq | 60 Hz |
| Colors | 16/64 |
[edit] See also
- Graphics card
- Graphics processing unit
- List of display interfaces
- List of monochrome and RGB palettes — 6-bit RGB section
- List of 16-bit computer hardware palettes — EGA section
- Professional Graphics Controller
[edit] References
- Mueller, Scott (1992) Upgrading and Repairing PCs, Second Edition, Que Books, ISBN 0-88022-856-3
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