The Enhanced Graphics Adapter (EGA) is the IBM PC computer display standard specification located between CGA and VGA in terms of color and space resolution. Introduced in October 1984^[1] by IBM for its new PC-AT, EGA produces a display of 16 simultaneous colors from a palette of 64 at a resolution of up to 640×350 pixels. The EGA card includes a 16 KB ROM to extend the system BIOS for additional graphics functions, and includes the Motorola MC6845 video address generator as used in the CGA.

Each of the 16 colors can be assigned a unique RGB color code via a palette mechanism in the 640×350 high-resolution mode; the 64 palette colors are a balanced RGB color set comprising all possible combinations of two bits per pixel for red, green and blue. EGA also includes 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. EGA 4-bit (16 colors) graphic modes are also notable by a sophisticated use of bit planes and mask registers^[2] together with CPU bitwise operations,^[3] which constitutes an early graphics accelerator inherited by VGA and numerous compatible hardware.

The original CGA modes are also present, though EGA is not 100% hardware compatible with CGA. EGA can drive an MDA monitor by a special setting of switches on the board; only 640×350 high-resolution monochrome graphics and the standard MDA text mode are available in this mode.

EGA cards use 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, only enough for 4 colors in high-resolution graphics. Expansion to 256 KB required a daughterboard. Eventually, most EGA cards and clones would come with 256 KB of memory. A few third-party EGA clones (notably the ATI Technologies and Paradise boards) feature 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.

Sample of text mode characters with cursor.

Color palette

The EGA palette allows all 16 CGA colors to be used simultaneously, and it allows substitution of each of these colors with any one 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.

Specifications

The EGA uses a female 9-pin D-subminiature (DE-9) connector which looks identical to the CGA connector. The hardware signal interface, including the pin configuration, is largely compatible with CGA. The differences are in the repurposing of three pins for the EGA's secondary RGB signals: the CGA Intensity pin (pin 6) has been changed to Secondary Green (Intensity); the second ground of CGA (pin 2) has been changed to Secondary Red (Intensity), and pin 7 (Reserved on the CGA) is used for Secondary Blue (Intensity). If the EGA is operated in the modes having the same scan rates as CGA, a connected CGA monitor should operate correctly, though if the monitor connects pin 2 to ground, the shorting of the EGA's Secondary Red (Intensity) output to ground could conceivably damage the EGA adapter. Similarly, if the CGA monitor is wired with pin 2 as its sole ground (which is poor design), it will not work with the EGA, though it will work with a CGA. Finally, because of the use of the CGA's Intensity pin as Secondary Green, on a CGA monitor connected to an EGA, all CGA colors will display correctly, but all other EGA colors will incorrectly display as the standard CGA color which has the same values for the g, R, G, and B bits (ignoring the r and b bits.) Conversely, an EGA monitor should work with a CGA adapter, but the Secondary Red signal will be grounded (always 0) and the Secondary Blue will be floating (unconnected), causing all high-intensity CGA colors except brown to display incorrectly and all colors to perhaps (but probably not) have a blue tint due to the indeterminate state of the unconnected Secondary Blue.

Connector

Female DE-9, on EGA (computer).
Pin numbers (looking at socket): top row is pins 1-5, bottom row is pins 6 to 9, both numbered from right to left in this illustration.

Signal

See also

• Graphics card
• Graphic display resolutions
• 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
• VGA compatible text mode — EGA’s own modes are just a subset, and all features are nearly the same

References

• Mueller, Scott (1992). Upgrading and Repairing PCs (second ed.). Que Books. ISBN 0-88022-856-3.