Color Graphics Adapter
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The Color Graphics Adapter (CGA), introduced in 1981, was IBM's first color graphics card (originally sold under the name "Color/Graphics Monitor Adapter"), and the first color computer display standard for the IBM PC.
When IBM introduced its PC in 1981, the CGA standard, though introduced at the same time, was used relatively little at first. Most people bought PCs for business computing. For gaming, other computers were much more popular; and at that time color graphics were considered to have little more than toy value. Thus, most early PC buyers opted for the cheaper text-only Monochrome Display Adapter (MDA) instead of CGA.
In 1982 came the introduction of the Hercules Graphics Card, which offered monochrome-only graphics at a much higher resolution than the CGA card and was more compatible with MDA, further eroding CGAs market share. Things changed in 1984 when IBM introduced the PC AT and the Enhanced Graphics Adapter (EGA). Along with this move, the price of the older CGA card was lowered considerably; it now became an attractive low-cost solution and was soon adopted by the new PC cloning companies as well. Entry-level non-AT PCs with CGA graphics sold very well during the next few years, and consequently there were many games released for such systems, despite their limitations. CGA's popularity started to wane after VGA became IBM's high-level solution and EGA the entry-level solution in 1987.
The standard IBM CGA graphics card was equipped with 16 kilobytes of video memory. The CGA card featured several graphics and text modes. The highest resolution of any mode was 640×200, and the highest color depth supported was 4-bit (16 colors). The best known mode, used for most CGA games, displayed 4-color graphics at a resolution of 320×200.
While 4-color display was generally considered the limit for graphics on CGA, there were several ways (some official, some not) to display more colors.
The CGA color palette
| | |
|---|---|
| 0 — black | 8 — (dark) gray |
| 1 — blue | 9 — bright blue |
| 2 — green | 10 — bright green |
| 3 — cyan | 11 — bright cyan |
| 4 — red | 12 — bright red |
| 5 — magenta | 13 — bright magenta |
| 6 — brown | 14 — yellow |
| 7 — white (light gray) | 15 — bright white |
The CGA card worked with CRT RGBI color monitors. It was based around the Motorola MC6845 display controller and had a palette of 16 colors. Red, green and blue corresponded to the three cathode rays and black meant all rays were almost off. Cyan was a mix between the blue and green rays, magenta was a mix between blue and red and orange-brown was a mix between green and red. White (or light gray) was a mix between all three rays.
The remaining 8 colors were achieved by turning on an "intensifier" bit, giving a brighter version of each color, although the dark gray color was indistinguishable from black with many monitors. CGA's "RGB plus intensity bit" design was also called RGBI.
The Commodore 128 used the same method of transmitting colors on its RGBI output and thus could use the same monitors and display the same 16 colors.
There is some confusion regarding color #6 on RGBI monitors: If one strictly follows the RGBI color model, color #6 would appear as dark yellow (#AAAA00) (see below). However, IBM chose to include additional circuitry in the 5153 color monitor to detect color #6 and lower the green component to yield a more pleasing brown-tone (#AA5500), so most "CGA compatible" monitors do the same. As to why IBM chose to do this, the predominant theory is that IBM wanted to make the colors more closely match the colors of the 3270 mainframe terminal, specifically the 3279. (This information has not yet been verified by multiple sources, however.)
Standard text modes
CGA offered two text modes:
- 40×25 characters in up to 16 colors. Each character was a pattern of 8×8 dots. The effective screen resolution in this mode was 320×200 pixels (a pixel aspect ratio of 1:1.2), though individual pixels could not be addressed independently. The choice of patterns for any location was thus limited to one of the 256 available characters, the patterns for which were stored in a ROM chip on the card itself. The display font in text mode was therefore fixed and could not be changed (although when using the original IBM CGA in an original IBM PC it was possible to select one of two different fonts -- normal or thin --, by changing a jumper. Many clones didn't offer this possibility). This mode allowed each character a foreground and a background color, both of which could be freely chosen from the entire CGA palette (see table) — e.g. red on yellow text for one character, white on black for the next and cyan on gray for yet another. The card had sufficient video RAM for 8 different text pages in this mode.
- 80×25 characters in up to 16 colors. Each character was again an 8×8 dot pattern (the same character set was used as for 40x25), in a pixel aspect ratio of 1:2.4. The effective screen resolution of this mode was 640×200 pixels. Again, the pixels could not be individually addressed. Since there were twice as many characters on the screen in this mode, the card had enough video RAM just for 4 different text pages.
Standard graphics modes
| Fixed CGA 4-color palette #1 | ||
| default | 5 — magenta | |
| 3 — cyan | 7 — white (light gray) | |
| Fixed CGA 4-color palette #2 | ||
| default | 4 — red | |
| 2 — green | 6 — brown (orange) | |
CGA offered two commonly-used graphics modes:
- 320×200 pixels, as with the 40×25 text mode. In the graphics mode, however, each pixel could be addressed independently. The tradeoff was that only 4 colors could be displayed at a time. These four colors could not be freely chosen from the 16 CGA colors — there were only two official palettes for this mode:
- #Magenta, cyan, white and background color (black by default).
- #Red, green, brown/yellow and background color (black by default).
- The 1:1.2 pixel aspect ratio needed to be taken into account when drawing large geometrical shapes on the screen.
- 640×200 pixels, as with the 80×25 text mode. All pixels could be addressed independently. This mode was monochrome, offering only black and white as colors (though this could be changed), with a pixel aspect ratio of 1:2.4.
The composite video modes
Little-known to many was an additional 160×200 graphics mode (pixel aspect ratio 1.67:1), which could utilize 16 different colors. The colors in Composite color mode did not use the same color table as the CGA color set, but was more like that of the Apple II's "double-hi-res" mode, as both used a similar technique (composite signal color artifacting). Composite color mode was rarely utilized in software: No BIOS support led to poor programmer adoption, and the cost of an IBM system was high enough that most people who could afford it could also afford the more expensive RGB monitor IBM offered. Only a handful of software titles used the 160×200 graphics mode, most of them games.A common misconception is that Composite color mode was supported on some RGB-equipped machines, but this is a direct contradiction to how composite color mode technically functioned. A more likely explanation for the misconception is that people confuse the 160×200 modes of the PCjr/Tandy 1000 and Amstrad CPCs as being the same thing (they are identical in resolution, but not color and memory organization). Attempting to set Composite color mode on any card with an RGB monitor attached would appear identical to the 640×200 graphics mode.
Further graphics modes and tweaks
| "Tweaked" 3rd Fixed CGA 4-color palette | ||
| default | 4 - red | |
| 3 — cyan | 7 — white (light gray) | |
A number of official and unofficial features existed that could be exploited to achieve better graphics on a monitor.
- In 320×200 graphics mode, the background color, which defaulted to black on mode initialization, could be changed to any of the other 15 colors of the CGA palette. This allowed for some variation, as well as flashing effects, as the background color could be changed without having to redraw the screen.
- In 640×200 graphics mode, the foreground color could be changed from its usual white to any of the other 15 colors.
- In text mode, the border color (displayed outside the regular display area) could be changed from its usual black to any of the other 15 colors.
- A third 320×200 4-color palette was achieved by enabling the monochrome bit while in color graphics mode. This switched the current graphics palette to red, cyan, white and the background color.
- Through precision timing, it was possible to switch to another palette while the screen content was still being drawn, allowing the use of any one of the 3 palettes per scanline. The best example of this in use is the game California Games [link] when run on a stock 4.77 MHz 8088. (Running it on a faster computer did not produce the effect, as producing the timing necessary to switch palettes at predetermined locations was extremely sensitive to machine speed.)
- Additional colors were often approximated using dithering, although the low resolution made it very apparent.
The 160×100 16 color mode
Technically, this mode was not a graphics mode, but a tweak of the 80×25 text mode. The character cell height register was changed to display only 2 lines per character cell instead of the normal 8 lines. This quadrupled the number of text rows displayed from 25 to 100. These "tightly squeezed" text characters were not full characters. The system only displayed their top two lines of pixels (8 each) before moving on to the next row.
| Character 221. |
| 221 with blue text and red background color. |
| 221 with red text and blue background color. |
| Character 222. |
Because each character could be assigned different foreground and background colors, it could be colored (for example) blue on the left (foreground color) and bright red on the right (background color). This could be reversed by swapping the foreground and background colors.
Using either character 221 or 222, each half of each truncated character cell could thus be treated as an individual pixel— making 160 horizontal pixels available per line. Thus, 160×100 pixels at 16 colors, with an aspect ratio of 1:1.2, were possible.
|
More detail could be achieved in this mode by using other characters, combining ASCII art with the aforesaid technique.
The same text cell height reduction technique could also be used with the 40×25 text mode. This only made sense when using ASCII art, because without it the resulting resolution would only have been 80×100 [link] [link] [link].
Bugs and errata
CGA's most noticeable hardware bug was snow in 80×25 text mode. The display RAM on the original IBM CGA card was not dual-ported — read and write access was not possible simultaneously. As such, random pixels were displayed whenever display memory was written to by the CPU at the same time as being read by the display hardware. This bug was fixed in most third-party clones, but still existed in some iterations (such as the AT&T PC 6300 display adapter).For programmers, another annoyance was that CGA display memory in graphics modes was interlaced. Normally, video memory is strictly linear: the next row of display data corresponds to the next row of pixels. But with CGA, the next row of display data corresponded to the row of pixels two rows down. This continued until the end of the screen and only with the second half of display data were the in-between rows addressed. So the first half of display memory was for rows 0, 2, 4, etc., until the end of the screen and the second half of CGA RAM was for rows 1, 3, 5, etc. This added calculation steps to most CGA graphics operations if the programmer wanted to avoid visual artifacts when updating the screen.
| Dark Yellow | ||
| #AAAA00 | ||
As previously mentioned, IBM designed the 5153 CGA monitor to intentionally darken color index #6 from dark yellow to brown; however, some clone monitors did not have this circuitry. On such monitors, or 5153 monitors where this circuitry had failed, color index #6 would remain dark yellow (see color example).
The total amount of video memory on a CGA card (16384 total bytes) is not fully utilised by all BIOS-initiated video modes (40×25 and 80×25 text modes, 320×200 and 640×200 graphics modes). Only by setting up video modes manually using CGA port writes can all 16384 bytes be displayed as pixel elements simultaneously.
Screenshots
Competing adapters
CGA had two main competitors:- For business and word processing use, IBM launched its Monochrome Display Adapter (MDA) at the same time as CGA. The MDA produced significantly higher resolution text display in 80×25 mode, rendering each character in a box of 9×14 pixels, of which 7×11 were the character itself. This was clearly superior than the CGA's 8×8 dots text character matrix. Because of this and CGA's higher price at the time, MDA was often preferred for business use.
- In 1982, the non-IBM Hercules Graphics Card (HGC) was introduced. It offered an MDA-compatible high resolution text mode and a monochrome graphics mode. The black and white graphics mode had a resolution of 720×348 pixels, which was better than even the highest resolution CGA could offer. Thus, even without a color capability of any kind, the Hercules adapter's offer of better monochrome graphics and its ability to work with less expensive monochrome monitors made it a desirable choice for many. As early as 1985, emulator memory-resident programs such as SIMCGA were available, allowing the display of CGA graphics mode data in Hercules graphics modes (the result looking like crude dithering). The HGC was arguably the most commonly-utilized card connected to monochrome monitors throughout the IBM PC's life.
The CGA card was succeeded in the consumer space by IBM's Enhanced Graphics Adapter (EGA) card, which supported most of CGA's modes, and added an additional resolution (640×350) as well as a software-selectable palette of 16 colors out of 64 in both text and graphics modes.
References
- [ZiffDavis-Net site with info on old display adapters, including CGA] (note: inaccurately states Hercules cards offered 4 colors.)
- [Comprehensive FAQ on old display adapters] (Usenet archive, some details speculated and/or inaccurate)
- [IBM PC-Compatible CGA Video Reference] — includes technical details
- [CGA monitor calibration] — Technical information on the IBM 5153 monitor's color decoding and calibration
- [IBM Color Graphics Adapter (CGA)] (Thor Kildsen)
- This article was originally based on material from the Free On-line Dictionary of Computing.
See also
External links
| Computer display standard |
| Video hardware |
|---|
| MDA | Hercules | CGA | EGA | VGA | MCGA | 8514 | XGA |
| Display resolutions ([Image]) |
| QVGA | VGA | SVGA | XGA | SXGA | SXGA+ | UXGA | QXGA | QSXGA | QUXGA | HXGA | HSXGA | HUXGA |
| Widescreen variants |
| WXGA | WSXGA/WXGA+ | WSXGA+ | WUXGA | WQXGA | WQSXGA | WHXGA | WHSXGA | WHUXGA | WQUXGA |
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