The Enhanced Graphics Adapter (EGA) is an IBM PC graphics adapter and de facto computer display standard from 1984 that superseded the CGA standard introduced with the original IBM PC, and was itself superseded by the VGA standard in 1987. In addition to the original EGA card manufactured by IBM, many compatible third-party cards were manufactured, and EGA graphics modes continued to be supported by VGA and later standards.
History
EGA was introduced in October 1984 by IBM, shortly after its new PC/AT. The EGA could be installed in previously released IBM PCs, but required a ROM upgrade on the mainboard.
Chips and Technologies' first product, announced in September 1985, was a four chip EGA chipset that handled the functions of 19 of IBM's proprietary chips on the original Enhanced Graphics Adapter. By that November's COMDEX, more than a half dozen companies had introduced EGA-compatible boards based on C&T's chipset.
Between 1984 and 1987, several third-party manufacturers produced compatible cards, such as the Autoswitch EGA or Genoa Systems' Super EGA chipset. Later cards supporting an extended version of the VGA were similarly named Super VGA.
The EGA standard was made obsolete in 1987 by the introduction of MCGA and VGA with the PS/2 computer line.
Adoption
Commercial software began supporting EGA soon after its introduction, with The Ancient Art of War, released in 1984. Microsoft Flight Simulator v2.12, Jet, Silent Service, and Cyrus, all released in 1985, offered EGA support, along with Windows 1.0. Sierra's King's Quest III, released in 1986, was one of the earliest mainstream PC games to use it.
By 1987, EGA support was commonplace. Most software made up to 1991 could run in EGA, although the vast majority of commercial games used 320 × 200 with 16 colors for backwards compatibility with CGA and Tandy, and to support users who did not own an enhanced EGA monitor. 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 640 × 350 with 16 colors mode.
Hardware design
The original IBM EGA was an 8-bit PC ISA card with 64 KB of onboard RAM. An optional daughter-board (the Graphics Memory Expansion Card) provided a minimum of 64 KB additional RAM, and up to 192 KB if fully populated with the Graphics Memory Module Kit. Without these upgrades, the card would be limited to four colors in 640 × 350 mode.
Output was via direct-drive RGB, as with the CGA, but no composite video output was included. MDA and CGA monitors could be driven, as well as newly released enhanced color monitors for use specifically with EGA.
EGA-specific monitors used a dual-sync design which could switch from the 15.7 kHz of 200-line modes to 21.8 kHz for 350-line modes.
Many 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. If EGA is selected, the card will operate in 350-line mode and use 8×14 text.
Some third-party cards using the EGA specification were sold with the full 128 KB of RAM from the factory, while others included as much as 256 KB to enable multiple graphics pages, multiple text-mode character sets, and large scrolling displays. A few third-party cards, such as the ATI Technologies EGA Wonder, built on the EGA standard to additionally offer features such as extended graphics modes as high as 800 × 560 and automatic monitor type detection.
Capabilities
EGA produces a display of up to 16 colors (using a fixed palette, or one selected from a gamut of 64 colors (6-bit RGB), depending on mode) at several resolutions up to 640 × 350 pixels, as well as two monochrome modes at higher resolutions. EGA cards include a ROM to extend the system BIOS for additional graphics functions, and a custom CRT controller (CRTC).
The IBM EGA CRTC supports all of the modes of the IBM MDA and CGA adapters through specific mode options, but it is not fully register-compatible with the Motorola MC6845 used in those cards, so software that directly programs the registers to select modes may produce different results on the EGA.
Supported resolutions are 320 × 200 and 640 × 200 (on a CGA or EGA monitor), 720 × 350 and 640 × 350 (on an MDA monitor) and 320 × 350 and 640 × 350 (on an EGA monitor). EGA scans at 21.8 kHz when 350-line modes are used and 15.7 kHz when 200-line modes are used.