Enhanced Graphics Adapter

EGA supports:

• 640×350 w/16 colors (from a 6 bit palette of 64 colors), pixel aspect ratio of 1:1.37.
• 640×350 w/2 colors, pixel aspect ratio of 1:1.37.
• 640×200 w/16 colors, pixel aspect ratio of 1:2.4.
• 320×200 w/16 colors, pixel aspect ratio of 1:1.2.

Text modes:

• 40×25 with 8×8 pixel font (effective resolution of 320×200)
• 80×25 with 8×8 pixel font (effective resolution of 640×200)
• 80×25 with 8×14 pixel font (effective resolution of 640×350)
• 80×43 with 8×8 pixel font (effective resolution of 640×344)

Extended graphics modes of third party boards:

• 640×400
• 640×480
• 720×540

EGA color table

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.

Standard EGA monitors do not support use of the extended color palette in 200-line modes, because the monitor cannot distinguish between being connected to a CGA card or being connected to an EGA card outputting a 200-line mode. EGA redefines some pins of the connector to carry the extended color information. If the monitor were connected to a CGA card, these pins would not carry valid color information, and the screen might be garbled if the monitor were to interpret them as such. For this reason, standard EGA monitors will use the CGA pin assignment in 200-line modes so the monitor can also be used with a CGA card.

Some EGA monitors are switchable, meaning they can be set up to use the full palette even in 200-line modes, often through a mechanical switch. Only a few commercial games were released with support for the extended color palette in 320×200 or 640×200 (including the DOS version of Super Off Road.)

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. For instance, 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).

The table below displays an example palette matching the standard 16 CGA colors, with their representations in hex, decimal and binary. The binary representation is in the form "rgbRGB" where the lowercase letters are the low-intensity bits, and uppercase letters are high-intensity bits.

• 
  Full 64-color EGA palette
• 
  Full 64-color EGA palette
• 
  Screen color test with custom EGA palette
• 
  Screen color test with standard 16-color palette

The EGA uses a female nine-pin D-subminiature (DE-9) connector for output, identical to the CGA connector, and the signal standard and pinout is backwards-compatible with CGA, allowing EGA monitors to be used on CGA cards and vice versa.

When operating in EGA modes, three pins are repurposed for EGA's secondary RGB signals:

When operating in 200-line CGA modes, the EGA card is fully backwards compatible with a standard IBM CGA monitor, however, third-party monitors had varying compatibility.

Front and rear views of the TVM MD-3, a third party EGA monitor. DE-9 input, mode switch, contrast and brightness controls at front, V-Size and V-Hold knobs at rear.

Third-party monitors sometimes connected pin two to ground internally. When connected to an EGA card, this shorts the EGA's secondary red output to ground and can damage the card. Also, some monitors were wired with pin two as their sole ground, and these will not work with the EGA.

Conversely, an EGA monitor should work with a CGA adapter, but if it is not set to CGA mode, the secondary red signal will be grounded (always zero) and the secondary blue will be floating (unconnected), causing all high-intensity colors except brown to display incorrectly, and all colors to potentially have a variable blue tint due to the indeterminate state of the unconnected secondary blue.

The IBM 5154 EGA monitor has a special IBM 5153 CGA compatibility mode when operating with CGA sync signals, and it will automatically change to the CGA pinout to avoid all of the mentioned problems when operating in this mode.[19]

For color text and CGA graphics modes, video memory is mapped to 16 KB of addresses beginning at address B8000h, and in monochrome (MDA-compatible) text mode, video memory occupies 16 KB beginning at B0000h. These address mappings are for backwards compatibility.

For modes new to the EGA, the video memory begins at address A0000h and occupies 64 KB. The different base addresses for color vs. monochrome modes makes it possible for an EGA to be used simultaneously with a monochrome graphics card in the same computer, or for an EGA in MDA text mode to used simultaneously with a CGA in the same computer.

EGA's native graphics modes are planar, as opposed to the interlaced CGA and Hercules modes. Video memory is divided into four pages (except 640×350×2, which has two pages), one for each component of the RGBI color space, with each bit representing one pixel. If a bit in the red page is enabled, but none of the equivalent bits in the other pages are, a red pixel will appear in that location on-screen. If all the other bits for that particular pixel were also enabled, it would become white, and so forth.

Planes are different sizes depending on the mode:

All planes reside at segment A000 in the CPU's address space. They are bank-switched, and only one plane can be read on the CPU bus at once, however, the programmer may set the control registers on the card to select which planes are written to, which permits writing to several at once. An exception is read mode 1, in which all four planes are read and compared with programmed "Color Compare" data, and a byte indicating the result of comparing all four planes can be read on the I/O bus.