Editing Enhanced Graphics Adapter (section)

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

For modes new to the EGA, the video memory begins at address A0000h and occupies 64&nbsp;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 be used simultaneously with a CGA in the same computer.

EGA's native graphics modes are [[planar (computer graphics)|planar]], as opposed to the interleaved CGA and [[Hercules Graphics Card|Hercules]] modes. Video memory is divided into four "planes" (except {{resx|640×350}}&nbsp;×&nbsp;2, which has one plane), one for each component of the RGBI color space. Each pixel is represented by one bit in each plane. If a bit in the red plane is on, 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 on, it would become white, and so forth.

Planes are different sizes depending on the mode:
{| class="wikitable"
|+ EGA video plane sizes
! Mode
! Plane size
|-
| 200-line modes,<br/>{{resx|640×350}}&nbsp;×&nbsp;2 mode
| 8 KB
|-
| {{resx|640×350}} with 64 KB RAM
| 16 KB
|-
| {{resx|640×350}} with 128 KB RAM
| 32 KB
|}
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<!--Though internally, the EGA always reads all 4 planes at once and stores the byte read from each plane in the Graphics Controller data latch for that plane, regardless of which plane is selected to be read on the [CPU] I/O bus-->; however, the programmer may set the control registers on the card to select which planes are written to and write to several at once. An exception is read mode&nbsp;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.