Enhanced Graphics Adapter
The Enhanced Graphics Adapter ( EGA ) is a graphics card standard created by IBM in 1984 for the IBM PC AT .
history
The EGA standard replaced CGA and was in turn replaced by VGA in the early 1990s . In Germany , the first EGA graphics card came onto the market in 1985 under the name HR graphics adapter . As a result, however, more and more other companies, for example Paradise or Genoa, offered EGA- compatible graphics cards.
As early as 1987, most EGA cards offered far more performance than the IBM EGA standard. The original IBM EGA card had 64 kilobytes of graphics memory , but memory expansion cards were also available; later versions and versions from other manufacturers brought with them up to 256 kilobytes of graphics memory. This enabled resolutions of up to 800 × 600 pixels to be achieved even before the start of the VGA standard. For the user, however, this additional service could only be used to a limited extent, as cards from other manufacturers often lacked the necessary drivers for the software used. If, for example, someone wanted to use AutoCAD on a computer with a Genoa card and AutoCAD did not bring a suitable driver for this Genoa card, then no additional features could be used beyond the standard EGA performance.
EGA cards had 16 kilobytes of additional ROM memory. This contains its own BIOS, which programs can use to control the new graphics functions, since the system BIOS only supports MDA and CGA. The BIOS code is executed - similar to, for example, SCSI host controllers or certain network cards - when the system is booted. With its routines, the BIOS usually fades into the memory address space from the segment address C000 hex , transfers the software interrupt 10 hex , which holds routines for video output, to its own routines and expands these with EGA-specific functions. This concept was retained in almost all later PC graphics solutions; The graphics BIOS can still be found in PCs today, but modern operating systems almost only use it during the start-up process, separated from the main BIOS at the segment address C000 hex , even if the graphics hardware is often no longer implemented as a separate plug-in card.
The Motorola 6845 , known from the MDA and CGA cards, is no longer used with the EGA card because it has some outdated restrictions such as a line counter limited to 7 bits, which meant that the memory layout in the graphics modes was complicated by CGA and Hercules card. The programming interface of the cathode ray tubes running controller (CRTC) VDC on the EGA card but the leaning heavily of 6845, so that programs that Aspect switching or cursor positioning have done with direct port access, running on the EGA card problems. However, this does not apply to the timing registers, which are provided with a software-controlled write protection to protect against incorrect programming. Programs that try to change the video mode of the CGA card bypassing the BIOS do not work correctly with EGA cards.
The EGA standard was followed in April 1987 by the VGA standard introduced by IBM with Personal System / 2 (PS / 2) . Initially, however, this only had the consequence that the EGA cards, which until then had occupied the upper class in terms of prices, slowly became available for PCs in the middle and from around 1989 the lower price class. It was only with the advent of Windows 3.0 in 1991 that EGA gradually disappeared.
Original equipment plan
The assembly plan of the original EGA card from IBM looked like this:
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more details
According to the EGA standard provided by IBM, up to 16 colors from a palette of 64 colors and four bit color depth are realized with an image resolution of 640 × 350 pixels . EGA cards also contain 16-color versions of the CGA resolutions 640 × 200 and 320 × 200, which were only displayed with 2 or 4 colors by the CGA card due to the limited memory size. In connection with the IBM EGA monitor 5154, only 16 colors are displayed in the modes with 200 lines, since the monitor works in a CGA compatibility mode with a 200 line signal. With a view to the home user market, IBM has already implemented these resolutions in the built-in graphics system of the IBM PCjr , but the memory layout of the graphics memory on the CGA card is very different from that of the PCjr graphics system, so that these modes are not software-compatible .
The EGA video memory is built up internally in 4 image levels, each of which can be easily addressed individually via register access. By default, the color is selected from one bit each of these 4 levels in the graphic modes. In some drawing programs, however, the division into up to 4 independent monochrome levels is offered, the order of superimposition of which can be freely selected by cleverly choosing the mapping of the 16 possible color values onto the monitor colors.
Due to the structure of the image memory in 4 levels, one bit per level and pixel is used in the graphics memory. With the basic configuration of 64 kilobytes, that is 16 kilobytes per level, i.e. 128 kilobits. That is not enough for the 640 × 350 graphics mode, which requires 224 kilopixels. Therefore, the option is provided to logically link two levels so that instead of 4 levels with 128 kilobits, only 2 levels with 256 kilobits are available. This leads to a 640 × 350 graphics mode with only 4 colors, which, however, can be freely selected from the 64 different EGA colors.
In text mode , EGA cards support 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 enables user-defined character sets for the first time. These can each contain 256 characters, each 1 to 32 pixels high and 8 pixels wide. With user-defined character sets, text modes with up to 70 lines are possible, but these are difficult to read because the characters are only 5 pixels high.
Bit 3 in the attribute byte controls - in addition to the foreground color - the selection of the associated character from two character sets. By default, however, both character sets contain identical characters. This is used, among other things, by the Linux text console when screen fonts with more than 256 characters are loaded (the color palette is automatically adjusted so that foreground colors 8 to 15 are identical to colors 0 to 7. This means that there are only eight different foreground colors available).
In order to be able to use the EGA colors, a special EGA monitor was required . These monitors received digital signals, which is why they were called " TTL monitors" according to the logic used (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: an additional 350 visible lines
- CGA monitor: 4 digital signals to control 16 colors, EGA monitor: 6 digital signals to control 64 colors
- Similarities: 9-pin connector (but with different assignment), 60 Hz image frequency.
- EGA monitors could display CGA as well as EGA modes of an EGA card.
It is possible to use an EGA card with a CGA monitor or even a monochrome monitor instead of an EGA monitor, which was still quite expensive when the EGA card was introduced. In this case, the functionality of the graphics card is limited by the monitor. With a CGA monitor, all modes that use 350 lines at a line frequency of 21.85 kHz are eliminated. This obviously applies to the 640 × 350 graphics mode, but the EGA card also displays the text modes by default with 640 × 350 pixels (and an 8x14 pixel character cell) when an EGA monitor is connected. With a CGA monitor, on the other hand, only the poorer quality 8x8 cell with 640 × 200 pixels is available, and the 43-line mode is no longer available. Furthermore, the color selection is reduced from 64 colors to 16 colors; the simultaneous display of 16 colors in the modes with 320 × 200 or 640 × 200 pixels is not impaired by the CGA monitor.
With an MDA monitor, the EGA card provides a text mode with a 9x14 character box and 720 × 350 pixels, or a graphics mode with 640 × 350 pixels. The color selection is limited to 3 intensities (including black) by the monitor. The graphics mode makes the middle intensity available as the 4th color, flashing.
An IBM system can be equipped with CGA and MDA cards at the same time, which is actually used because of the limited text quality of the CGA card (especially if it was operated with a composite monitor) and the lack of graphics capability of the MDA card has been. In addition to being the only graphics card, the EGA card can also be combined with a CGA or MDA card, but for reasons of software compatibility the configuration of the EGA card, in which it replaces an MDA card, is only possible with a monochrome monitor offered the configuration in which it replaces a CGA card, only with a color monitor (CGA or EGA). It is not possible to combine two EGA cards.
The EGA card controls access to your screen memory. For this purpose, either a program or the hardware of the graphics card can access the screen memory during a clock cycle. During the display phase of the image, the hardware of the graphics card occupies 2 out of 5 clock cycles with low and 4 out of 5 clock cycles with high image resolution. The remaining clock cycles are available to the program. This eliminates the problem of the so-called CGA snow, where the monitor image is briefly disturbed at the corresponding point when simultaneously accessed. To store the image data as a bit pattern, the screen memory is divided into four areas of equal size, so-called planes .
The EGA card uses a 9-pin Sub-D connector as the PC interface , but a DB socket is used on the computer to avoid confusion with the RS-232 interface. The pins are assigned as follows:
| Pin | function |
|---|---|
| 1 | Dimensions |
| 2 | Red LSB |
| 3 | Red MSB |
| 4th | Green MSB |
| 5 | Blue MSB |
| 6th | Green LSB |
| 7th | Blue LSB |
| 8th | Sync + |
| 9 | Sync - |
Individual evidence
- ↑ Hans-Peter Messmer: PC hardware - structure, functionality, programming. 5th edition. Bonn, 1998.
- ↑ IBM Enhanced Graphics Adapter. (PDF; 9.1 MB) p. 3 , accessed on August 24, 2020 (English).
- ↑ IBM Enhanced Graphics Adapter. (PDF; 9.1 MB) p. 20 , accessed on August 24, 2020 (English).
literature
- Manfred Michael: Enhanced Graphics Adapter, the installation and programming of the IBM-HR and compatible graphics cards . 2nd edition, Markt & Technik Verlag AG, Haar near Munich 1987, ISBN 3-89090-462-9
- Scott Mueller: Upgrading and Repairing PCs, Second Edition , Que Books, Indianapolis 1992, ISBN 0-88022-856-3