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, colour 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 colours could be assigned a unique RGB colour code via a palette mechanism in the 640×350 high-resolution mode; EGA let you choose the displayed colours out of a total of 64 palette colours (two bits per pixel for red, green and blue). EGA also included full 16-colour versions of the CGA 640×200 and 320×200 graphics modes; only the 16 CGA/RGBI colours 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 colour in the 640×200 and 320×200 modes). Eventually, most EGA cards and clones would come with a full 256 KB 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.
See also Professional Graphics Controller, a much more powerful graphics adapter with an equally short marketing life.
Pinout and Signal Specification
| Pin | Signal |
|---|---|
| Pin 1 | Ground |
| Pin 2 | Red Intensity |
| Pin 3 | Red |
| Pin 4 | Green |
| Pin 5 | Blue |
| Pin 6 | Green Intensity |
| Pin 7 | Blue Intensity |
| Pin 8 | Horizontal Sync |
| Pin 9 | Vertical Sync |
| Type | Digital, TTL |
| Resolution | 640h x 350v x 16c, other modes available |
| H-freq | 15.7/21.8kHz |
| Colors | 16/64 |
See also
References
- Mueller, Scott (1992) Upgrading and Repairing PCs, Second Edition, Que Books, ISBN 0-88022-856-3
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