Enhanced Graphics Adapter

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Enhanced Graphics Adapter
Release date 1984
Cards
Entry-level IBM EGA card, Chips and Technologies, ATI EGA Wonder
Mid-range ATI EGA Wonder 800
High-end ATI EGA Wonder 800+
History
Predecessor Monochrome Display Adapter, Color Graphics Adapter
Successor Video Graphics Array
IBM EGA card - 64kB version
A non-IBM EGA card

The Enhanced Graphics Adapter (EGA) is an IBM PC computer display standard specification which is between CGA and VGA in terms of color and space resolution. Introduced in September 1984[1][2] by IBM shortly after (but not exclusively 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[citation needed]. EGA 4-bit (16 colors) graphic modes are also notable for a sophisticated use of bit planes and mask registers[3] together with CPU bitwise operations,[4] which constitutes an early graphics accelerator inherited by VGA and numerous compatible hardware.

EGA is dual-sync; it scans at 23Khz when 350-line modes are used and 15Khz when 200-line modes are used. 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 original IBM EGA card had 64k of onboard RAM and required a daughterboard to add an additional 64k (cards with 64k are limited to 4 colors when 640x350 mode is used). All third-party cards came with 128k already installed and some even 256k, allowing multiple graphics pages. 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 in 1987 of MCGA with the PS/2 computer line[5] and VGA.

Shortly before the introduction of VGA, Genoa Systems introduced a half-size graphics card built around a proprietary chip set, which they called Super EGA (later cards supporting an extended version of the VGA were similarly named Super VGA).[6]

Output capabilities[edit]

VGA text sample animation.gif
Sample of text mode characters
with cursor.
Screenshot of the Arachne web browser using the 640x350 graphics mode. The screenshot contains 14 colors.

EGA supports:

  • 640×350x16 (from a 6 bit palette of 64 colors)
  • 640x350x2
  • 640×200x16
  • 320x200x16

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 640×350)

Extended graphics modes of third party boards:

  • 640×400
  • 640×480
  • 720×540

Color palette[edit]

EGA Color Table
The full 64-color EGA palette
Screen color test of EGA 16 colors
Screen color test of EGA in CGA 16-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. The extended color palette cannot be used in 200-line modes.

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)
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

Specifications[edit]

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.

Almost all EGA cards have DIP switches on the back of the card to select the monitor type. If CGA is selected, the card will operate in 200-line mode and use 8x8 characters in text mode. 350-line modes cannot be accessed, nor any of the extended color palette. If EGA is selected, the card will operate in 350-line mode and use 8x16 text. When 200-line modes are set, it will sync down to 15Khz. If MDA is selected, the card also uses 350 lines and 8x16 text, but the only graphics mode that can be accessed is 640x350x2.

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.[7]

Memory mapping[edit]

EGA graphics modes are planar, as opposed to the interlaced CGA and Hercules modes. The video memory is divided into four pages (except 640x350x2, which has two pages) one for each component of the RGBI color space, and each bit represents 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. The planes are 8k in size (200-line modes and 640x350x2), 16k (640x350 with 64k video RAM), or 32k (640x350 with 128k video RAM) and reside at Segment A000 (color text and CGA modes reside in Segment B800 and monochrome text at B000 for backwards compatibility) in the CPU's address space. They are bank-switched and only one plane can be "seen" at once, however the user can still set the control registers on the EGA card to select which planes are written to. Thus, it's possible to write to all of them at once even though just one plane can be read from at any given moment.

Software support[edit]

Commercial software began supporting EGA by 1986 and Sierra's King's Quest 3 was one of the earliest games to use it. Most software made up to 1991 can run in EGA, although the vast majority of commercial games used 320x200x16 for reasons of compatibility with CGA and Tandy and to support users who did not own a proper EGA monitor. The 350-line modes were mostly used by freeware/shareware games and application software, although SimCity is a notable example of a commercial game that runs in 640x350x16 mode.

Connector[edit]

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.

DE9 Diagram.svg

Pin assignments
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

Signal[edit]

Type Digital, TTL
Resolution H × V 640x350, other modes available
H-freq 15.7 or 21.8 kHz
V-freq 60 Hz
Colors 6-bit (64)

See also[edit]

References[edit]

  1. ^ High-Resolution Standard Is Latest Step in DOS Graphics Evolution, InfoWorld, June 26, 1989, p. 48
  2. ^ News Briefs, Big Blue Turns Colors, InfoWorld, Oct 8, 1984
  3. ^ Complete Instructions to BLOAD and BSAVE EGA and VGA Screens, Microsoft
  4. ^ Abrash, Michael (2001). "Chapter 43: Bit-plane animation". Graphics Programming Black Book. Coriolis Group Books. p. 1342. ISBN 1-57610-174-6. Retrieved March 2010. 
  5. ^ Scott Mueller, Upgrading and Repairing PCs, Tenth Edition, Que,1998, 0-7897-1636-4 page 515
  6. ^ Hardware, Genoa Systems Ready to Ship $449 Half-Size Graphics Card, InfoWorld, February 23, 1987
  7. ^ IBM Options and Adapters, Volume 1, "Enhanced Color Display", Page 4: "When operating in Mode 1, the display maps the 4 input bits into 16 of the possible 64 colors as shown in the following chart.". August 2nd, 1984.
Notes