Enhanced Graphics Adapter

The Enhanced Graphics Adapter EGA short, is a IBM in 1984, created for the IBM PC AT graphics card standard.


The EGA standard superseded CGA and early 1990s was in turn replaced by VGA. In Germany the first EGA graphics card 1985, under the name HR- graphics adapter on the market. As a consequence, but also offered more and more other companies, such as Paradise or Genoa, EGA compatible graphics cards.

Back in 1987, offered the most EGA cards far more power than the IBM EGA standard. The original IBM EGA card had 64 kilobytes video RAM to which memory expansion cards, however, were available; later versions and versions from other manufacturers brought up to 256 kilobytes of video RAM. For resolutions up to 800 × were achieved even before the start of the VGA standard 600 pixels. For the user this additional service, however, was of only limited use, since with cards from other manufacturers often lacked the necessary drivers for audio application. For example, if someone wanted to use AutoCAD on a PC with Genoa card and AutoCAD brought no suitable driver for this Genoa card, then no additional features were available on the standard EGA - performance addition.

EGA cards possessed 16 kilobytes of additional ROM memory. This includes its own BIOS, the new graphics functions can be controlled via the programs, because the system BIOS only MDA and CGA supported. The BIOS code - run at system boot - as for example in SCSI host controllers or certain network cards. The BIOS thus appears with his routines usually from the segment address C000hex into memory address space and runs the software interrupt 10hex, the routines for video output holds to their own routines and extends it. Around EGA - specific functions This concept has been retained in almost all later PC graphics solutions; to date can be found in PCs, the video BIOS, but almost only use the modern operating systems during startup, separated from the main BIOS to the segment address C000hex, even if the graphics hardware is often not performed as a separate plug-in card.

The well-known by the MDA and CGA card Motorola 6845 comes with the EGA card no longer used because it has some no longer outdated restrictions, such as a limited 7- bit line counter, which had the consequence that the memory layout in the graphics modes was complicated by CGA and Hercules card. The programming interface of the CRTC on the EGA card is but a lot of the of the 6845 ajar, so that programs that screen to change pages or cursor positioning have performed with direct port access, running smoothly on the EGA card. This is not true for the timing registers that are provided to protect against erroneous programming with a software controlled write protection. Programs that attempt to adjust the video mode, the CGA card at the BIOS over, do not work correctly with EGA cards.

The EGA standard was followed in April 1987 by IBM Personal System / 2 (PS / 2) introduced VGA standard. The first had only the consequence that the EGA cards that had priced occupied until then the upper class, slowly also for Central and then available from circa 1989 in low-budget PCs were. Only with the advent of Windows 3.0 from 1991 EGA gradually disappeared.

Original placement plan

The component layout of the original IBM EGA card was as follows:

  • 22: Jumper
  • 23: SN74LS04N inverter
  • 24: Socket for future extensions
  • 25: SN74LS32N OR
  • 26: 74LS151PC multiplexer
  • 27: 74LS367APC bus line driver
  • 28: BECKMANN 898-1 -R4 -7K, 8432
  • 29: 74LS86PC XOR
  • 30: SN74LS74AN, J429AJD D flip-flop
  • 31: 74LS11PC, 11422 AND
  • 32: SN74LS32N, I8442B OR
  • 33: SN74LS244N, I8439AL bus driver
  • 34: SN74LS245N, I8441D bidirectional transceiver
  • 35: N823137N, K8326
  • 36: P8436, DM74LS125AN line driver
  • 37: 01,503,247
  • 38: Jumper
  • 39: SN74LS175N, 8442 D flip-flop
  • 40: Jumper
  • 41: DIP switch
  • 42: AUX jacks
  • 43: 9-pin monitor connector

More details

After the planned IBM EGA standard, up to 16 colors from a palette of 64 colors and four -bit color depth with an image resolution of 640 × 350 pixels realized. EGA cards also contain a 16-color versions of the CGA resolutions 640 × 200 and 320 × 200, which were represented by the CGA card because of the limited memory size only with 2 or 4 colors. In conjunction with the IBM EGA monitor 5154 are only 16 colors shown in the modes with 200 lines, because the monitor is operating at a 200 -line signal in a CGA compatibility mode. These resolutions are IBM overlooking the home user market even been already implemented in the built- graphics system of the IBM PCjr, but the memory layout of the graphics memory at the CGA card is greatly different from that of the PCjr graphics system, so that these modes are not software-compatible.

The EGA video memory is built internally into four image planes each can be addressed individually via register access pretty easy. By default, the graphics modes selected from each of these 4 levels a bit the color. In some drawing programs but the decomposition into up to 4 independent monochrome levels offered, their superposition sequence can be chosen arbitrarily by the clever choice of the figure of the 16 possible color values ​​on the monitor colors.

The construction of the image memory in 4 levels one bit per pixel level and is used in graphics memory. These are at the base equipment of 64 kilobytes 16 kilobytes per level, so 128 kilobits. That's not enough for the 640 × 350 graphics mode, the required 224 kilo pixels. Therefore, the option is provided to link each two levels logically, so that instead of 4 levels with 128 kilobits only 2 levels with it but 256 kilobits available. This leads to a 640 × 350 graphics mode with only 4 colors that can be freely chosen, however, from the 64 different EGA colors.

In text mode support EGA cards 40 or 80 columns of text and 25 ( 8 × 14 pixels per character) or 43 (8 × 8 pixels per character) lines of text. In contrast to MDA, HGC and CGA, which only have hard-coded character sets, the EGA standard allows first custom character sets. These may each contain 256 characters each having 1 to 32 pixels, and 8 pixels height width. Using custom fonts text modes with up to 70 lines are thus possible, but these are a result of only 5 pixels high notes hard to read.

Bit 3 in the attribute byte controls - in addition to the foreground color - to select the corresponding character from two character sets. But by default contain both character sets identical characters. This is used, among other things, the text console when Linux screen fonts are loaded with more than 256 characters (where the color palette is automatically adjusted so that the foreground colors 8-15 are identical to the colors 0 to 7. Thus then just eight different foreground colors available ).

To use the EGA colors, one needed a special EGA monitor. These monitors received digital signals, which is why they were called by the used logic "TTL monitors " (9-pin D- sub connector). The differences between CGA and EGA monitors:

  • CGA monitor: 15.75 kHz line frequency, EGA monitor: additional 21.85 kHz
  • CGA monitor: 200 visible lines, EGA monitor: additional 350 visible lines
  • CGA monitor: 4 digital signals for controlling 16 colors, EGA monitor: 6 digital signals for controlling 64 colors
  • Similarities: 9 pin connector plug ( but with different occupancy), 60 Hz frame rate.
  • EGA monitors could represent CGA EGA modes as an EGA card.

It is possible to use an EGA card instead of a the introduction of the EGA card still quite expensive EGA monitor with a CGA monitor, or even a monochrome monitor. In this case, the functionality of the graphics card is limited by the monitor. With a CGA monitor accounts for all modes that use 350 lines at a line frequency of 21.85 kHz. This applies obviously to 640 × 350 graphics mode, but also the text modes are of the EGA card displayed by default with 640 × 350 pixels ( and a 8x14 pixel character cell ) when an EGA monitor is connected. With a CGA monitor, however, only the lower quality 8x8 cell is 640 × 200 pixels available, and the 43- line mode is not required. Furthermore, reduces the color selection of 64 colors to 16 colors; simultaneous representability of 16 colors in the mode of 320 × 200 or 640 × 200 pixels is not affected by the CGA display.

With an MDA monitor, the EGA card text mode with a 9x14 character box and 720 × 350 pixels available, or a graphics mode with 640 × 350 pixels. The color selection is limited by the monitor to 3 intensities (including black). The graphics mode, as a fourth color, the average intensity flashing available.

An IBM System can be equipped with CGA and MDA card at the same time, which actually used because of the limited text quality of the CGA card (especially if it was operated with a composite video monitor), and the lack of graphics capability of the MDA card been. The EGA card can be combined except as a single graphics card to work with a CGA or MDA card, for reasons of software compatibility, but the configuration of the EGA card, in which she replaced an MDA card, only with a monochrome monitor offered, the configuration in which it replaces a CGA card, only with a color monitor (CGA or EGA ). The combination of two EGA card is not possible.

The screen memory of the EGA card is, in contrast to the former RAM main memory, dual-ported, which means that the same programs can read and write to the screen memory, while the hardware accesses this memory. This eliminated the problem of the so-called CGA snow where concurrent access the monitor image was disturbed briefly at the appropriate place. For the storage of the image data and bit - pattern of the screen memory in four is roughly equal parts, called split plan.

The EGA card used as a PC interface is a 9- pin Sub- D connector on the computer side but a DB connector is used to avoid confusion with the RS- 232 interface. The pins are assigned as follows: