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Encyclopedia > Color Graphics Adapter
The 640×200 2 color mode with its default foreground color — Arachne Internet suite.
The 640×200 2 color mode with its default foreground color — Arachne Internet suite.
The 320×200 4 color mode.
The 320×200 4 color mode.

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. Image File history File links Arachne_CGA_Mode. ... Image File history File links Arachne_CGA_Mode. ... Arachne is a full-screen Internet suite containing a graphical web browser, email client, and dialer. ... Internet suite - Wikipedia /**/ @import /skins-1. ... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... International Business Machines Corporation (IBM, or colloquially, Big Blue) (NYSE: IBM) (incorporated June 15, 1911, in operation since 1888) is headquartered in Armonk, New York, USA. The company manufactures and sells computer hardware, software, and services. ... Various computer display standards or display modes have been used in the history of the personal computer. ... IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ...


The standard IBM CGA graphics card was equipped with 16 kilobytes of video memory, and could be connected either to a NTSC-compatible monitor or TV via an RCA jack, or to a dedicated RGBI CRT monitor. Based around the Motorola MC6845 display controller, 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). A kilobyte (derived from the SI prefix kilo-, meaning 1000) is a unit of information or computer storage equal to the decimal 1024 bytes (2 to the 10th power, or 1,024 bytes based in the binary system). ... VRAM an acronym for Video RAM. Generally a term used in computers to describe RAM dedicated to the purpose of displaying bitmap graphics in raster graphics hardware. ... NTSC is the analog television system in use in the United States, Canada, Japan, South Korea, the Philippines, Mexico, and some other countries, mostly in the Americas (see map). ... Cathode ray tube employing electromagnetic focus and deflection Cutaway rendering of a color CRT: 1. ... Motorola Inc. ... The MC6845 was a video address generator first introduced by Motorola and used in the CGA and EGA video adapters, Amstrad CPC and BBC Micro. ... A text mode program communicates with the user by only displaying text and possibly a limited set of predefined semi-graphical characters, which allow the drawing of rudimentary boxes around portions of text, either to highlight the content or to simulate widget or control interface objects found in GUI programs. ...

Contents

The CGA color palette

Full CGA 16-color palette
0 black
#000000
8 (dark) gray
#555555
1 blue
#0000AA
9 bright blue
#5555FF
2 green
#00AA00
10 bright green
#55FF55
3 cyan
#00AAAA
11 bright cyan
#55FFFF
4 red
#AA0000
12 bright red
#FF5555
5 magenta
#AA00AA
13 bright magenta
#FF55FF
6 brown
#AA5500
14 yellow
#FFFF55
7 white (light gray)
#AAAAAA
15 bright white
#FFFFFF

The CGA's maximum color depth of four bits results in a palette of 16 colors. The lower three bits, representing red, green and blue, corresponded to the three cathode rays, with black meaning all three cathodes were almost off. Cyan was a mixture of blue and green, magenta was blue and red, and orange-brown was green and red. White (or light gray) used all three cathodes.


The remaining 8 colors were achieved by turning on a fourth "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.


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 40×25), 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.

Top: jumper block on IDE hard drive with shunt; bottom: assorted shunts In electronics and particularly computing, a jumper is two or more connecting points that can be conveniently shorted together to set up or adjust a printed circuit board, such as a computers motherboard. ...

Standard RGB graphics modes

Index Palette #1 Palette #2
0 default default
1 3 — cyan 2 — green
2 5 — magenta 4 — red
3 7 — white 6 — brown
Castle Master using palette 2 in high intensity.
Castle Master using palette 2 in high intensity.
Alley Cat using palette 1 in high intensity.
Alley Cat using palette 1 in high intensity.

CGA offered two commonly-used graphics modes: Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Castle Master is a 1990 computer game written by British developers Teque Software Development and published by Incentive Software. ... Screenshot of a copyrighted game (IBMs Alley Cat). ... Screenshot of a copyrighted game (IBMs Alley Cat). ... Alley Cat is a 1984 computer game for the PC and Atari 800 created by Bill Williams of SynSoft, in which the player controls an alley cat whose object is to perform certain tasks within the homes of strangers. ...

  • 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:
    1. Magenta, cyan, white and background color (black by default).
    2. Red, green, brown/yellow and background color (black by default).

By setting the high-intensity bit, brighter versions of these modes could be accessed.

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 with a pixel aspect ratio of 1:2.4. By default the colors were black and white, but the foreground color (white) could be changed to any other color of the CGA palette. This could be done at runtime without refreshing the screen.

In text mode, font bitmap data came from the character ROM on the card, which was only available to the card itself. In graphics modes, text output by the BIOS used two separate tables: The first half of the character set (128 characters) was supplied by a table in the BIOS at F000:FA6E, and the second half was supplied by the location pointed to by interrupt 1F (0000:007C). The second half of the character set would display as blanks (or garbage, depending on implementation) unless they were explicitly defined, usually by a utility such as GRAFTABL or by the calling program.


Further RGB graphics modes and tweaks

Index 3d palette
0 default
1 3 — cyan
2 4 — red
3 7 — white (light gray)

A number of official and unofficial features existed that could be exploited to achieve better graphics on an RGBI 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 6 palettes per scanline. The best example of this in use is the game California Games [1] when run on a stock 4.77 MHz 8088. (Running it on a faster computer did not produce the effect, as the method the programmers used to switch palettes at predetermined locations was extremely sensitive to machine speed.) The same could be done with the background color, to create the river and road in Frogger [2].
  • Additional colors were often approximated using dithering, although the low resolution made it very apparent. In particular, the game King's Quest used palette 2 at high intensity and high intensity blue as the background colour. This gave it the three primary RGB colours to work with (as well as yellow).

Some of these above tweaks could even be combined. Examples could be found in several games [3]. Most software titles did not use these possibilities, but there were a few impressive exceptions. California Games is a 1987 Epyx sports video game for many home computers and video game consoles. ... Frogger is an arcade game introduced in 1981. ... This article or section should be merged with Dither An illustration of dithering. ... Kings Quest IV screenshot Kings Quest is an adventure game series made by the American computer game company Sierra On-Line (currently known as Sierra Entertainment). ... REDIRECT RGB color model ...


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. 

Character 221 in the extended ASCII character set consisted of a box occupying the entire left half of the character matrix. (Character 222 consisted of a box occupying the entire right half.) I created this file, which is intended to depict the ASCII character 221 This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... I created this file, which is intended to depict the ASCII character 222 This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ...


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.

A single big "pixel" in 160×100 mode. This is the two top rows of half of character 221. Note the 8 constituent pixels and the overall 1:1.2 aspect ratio.

Although a roundabout way of achieving 16 color graphics display, this worked quite well [4] and the mode was even mentioned (although not explained) in IBM's official hardware documentation. Image File history File links Single_pixel_in_CGA_160x100_mode. ...


More detail could be achieved in this mode by using other characters, combining ASCII art with the aforesaid technique. ASCII art, an artistic medium relying primarily on computers for presentation, consists of pictures pieced together from characters (preferably from the 95 printable characters defined by ASCII). ...


Because the CGA had 16384 bytes of graphics memory, not 16000, it was just as easy to set the number of lines in this mode to 102 instead of 100 for a resolution of 160×102. This used extra video memory that was normally unused. However, most games did not do this, perhaps out of fear it would only work on some monitors but not others.


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 [5] [6] [7].


Composite video display

While connecting a dedicated RGBI color monitor was the more common configuration, it was also possible to connect an NTSC-compatible composite color monitor, or even a normal TV set, to the CGA's RCA output jack. As is common with NTSC composite video, the separation between luminance and chrominance was far from perfect, yielding cross-color artifacts, or color "smearing". This was especially a problem with 80-column text:

80-column text on RGB (left) vs. composite monitor (right)
80-column text on RGB (left) vs. composite monitor (right)

For this reason, using an RGBI color monitor was the preferred configuration. Image File history File links Size of this preview: 800 × 250 pixelsFull resolution (1280 × 400 pixel, file size: 177 KB, MIME type: image/png)CGA 80-column text display on RGB vs. ... Image File history File links Size of this preview: 800 × 250 pixelsFull resolution (1280 × 400 pixel, file size: 177 KB, MIME type: image/png)CGA 80-column text display on RGB vs. ...


A flaw turned into an advantage

However, programmers soon found out that this flaw could be turned into an asset, as distinct patterns of high-resolution dots would "smear" into consistent areas of solid colors, thus allowing the display of completely new colors. Since these new colors are the result of cross-color artifacting, they are often called artifact colors. Both the standard 320×200 four-color and the 640×200 black-and-white graphics modes could be used with this technique:

Thus, with the choice of 320×200 vs. 640×200 mode, the choice of palette (1 or 2) and the freely-selectable color 0 in 320×200 modes (see above), each one of these parameters resulted in a different set of artifact colors, making for a total gamut of well over a hundred colors, of which 16 could be displayed at the same time.


Availability

The 320×200 variant of this technique (see above) was just how the standard BIOS-supported graphics mode looked on a composite color monitor. The 640×200 variant however required modifying a bit (color burst disable) directly in the CGA's hardware registers, as a result, it is usually referred to as a separate "mode", often just as "the" composite color mode, since its more distinctive set of artifact colors led it to being more commonly used than the 320×200 variant.


Being completely dependent on the NTSC encoding/decoding process, composite color artifacting is not available on an RGBI monitor, nor is it emulated by EGA, VGA or contemporary graphics adapters.


Resolution and usage

Due to the relationship between the CGA's pixel clock and the NTSC color subcarrier, the effective horizontal resolution is reduced to 160 pixels of any color, or 320 pixels when limiting oneself to black and white pixels.


This low resolution led to composite color artifacting being used almost exclusively in games, with many of the more high-profile titles optionally, sometimes exclusively, offering graphics optimized for composite color monitors:

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). Programers dealt with this by polling a hardware port which changed value during the video retrace, and writing updates in a burst during this interval.


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
6 #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.


Specifications

Connector

The Color Graphics Adapter uses a standard DE-9 connector. A male DE-9 connector. ...

Pin assignments
Pin Function
1 Ground
2 Ground
3 Red
4 Green
5 Blue
6 Intensity
7 Reserved
8 Horizontal Sync
9 Vertical Sync

Signal

Type Digital, TTL
Resolution 640h × 200v, 320h × 200v
H-freq 15.75 kHz
V-freq 60 Hz
Colors 16

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 a 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 produced sharper characters than the CGA's 8×8 dots text character matrix allowed. 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. In addition to an MDA-compatible text mode, it offered a monochrome graphics mode. With a resolution of 720×348 pixels, the graphics mode was better than what CGA could produce. 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).

A less widely-used competitor was the Plantronics Colorplus, a CGA-compatible card which doubled the video RAM to 32k, thus allowing 16 colors at 320×200 resolution and 4 colors at 640×200 resolution. The "extended CGA" modes provided by the IBM PCjr and Tandy 1000 were similar to these modes. Green screen driven by a Monochrome Display Adapter The Monochrome Display Adapter (MDA, also MDA card, Monochrome Display and Printer Adapter, MDPA) introduced in 1981 was IBMs standard video display card and computer display standard for the PC. The MDA did not have any graphics mode of any kind... Year 1982 (MCMLXXXII) was a common year starting on Friday (link displays the 1982 Gregorian calendar). ... The Hercules Graphics Card (HGC) was a mid-1980s computer graphics controller which through its popularity became a de-facto display standard. ... DosBox emulates the familiar command line interface of DOS. An emulator duplicates (provide an emulation of) the functions of one system with a different system, so that the second system behaves like (and appears to be) the first system. ... This article or section should be merged with Dither An illustration of dithering. ... Plantronics is a hardware company based in Santa Cruz, California, that specializes in lightweight headsets and is the market leader worldwide[1]. // The company was incorporated as Pacific Plantronics on May 18, 1961. ... The Plantronics Colorplus was a graphics card for IBM PC computers, first sold in 1982. ... A PCjr with the revised keyboard and a third-party floppy drive (attached to the top of the computer). ... The Tandy 1000 was a line of more or less IBM PC compatible home computer systems produced by the Tandy Corporation for sale in its Radio Shack chain of stores. ...


Another extension in some CGA-compatible chipsets (including those in the Olivetti M24, the DEC VAXmate, and some Compaq and Toshiba portables) was a doubled vertical resolution. This gave a higher-quality text display and an extra 640×400 graphics mode. Olivetti Lettera 22, 1950 Ing. ... The DEC logo Digital Equipment Corporation was a pioneering American company in the computer industry. ... VAXmate was a personal computer produced by Digital Equipment Corporation. ... Compaq Computer Corporation is an American personal computer company founded in 1982, and now a brand name of Hewlett-Packard. ... Toshiba Corporations headquarters (Center) in Hamamatsucho, Tokyo Toshiba Corporation sales by division for year ending March 31, 2005 Toshiba Corporation ) (TYO: 6502 ) is a Japanese multinational conglomerate manufacturing company, headquartered in Tokyo, Japan. ...


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. The Enhanced Graphics Adapter (EGA) is the IBM PC computer display standard specification located between CGA and VGA in terms of graphics performance (that is, colour and space resolution). ...


Market penetration

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 brands of home 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. The home computer is a consumer-friendly word for the second generation of microcomputers (the technical term that was previously used), entering the market in 1977 and becoming common during the 1980s. ... Green screen driven by a Monochrome Display Adapter The Monochrome Display Adapter (MDA, also MDA card, Monochrome Display and Printer Adapter, MDPA) introduced in 1981 was IBMs standard video display card and computer display standard for the PC. The MDA did not have any graphics mode of any kind...


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 CGA's market share. The HGC was arguably the most commonly-utilized card connected to monochrome monitors throughout the IBM PC's life. The Hercules Graphics Card (HGC) was a mid-1980s computer graphics controller which through its popularity became a de-facto display standard. ...


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 IBM AT, more formally known as the PC AT or PC/AT, was IBMs third-generation PC, designed around the Intel 80286 microprocessor and released in 1984. ... The Enhanced Graphics Adapter (EGA) is the IBM PC computer display standard specification located between CGA and VGA in terms of graphics performance (that is, colour and space resolution). ...


References

This article does not cite any references or sources. ...

See also

A representation of additive color mixing—In CRT based (analog electronics) television three color electron guns are used to stimulate such an arrangement of phosphorescent coatings of the glass, the resultant reemission of photons providing the image seen by the eye. ... A graphics/video/display card/board/adapter is a computer component designed to convert the logical representation of visual information into a signal that can be used as input for a display medium. ... GeForce 6600GT (NV43) GPU Radeon 9800 Pro (R350) GPU Intel GMA X3000 IGP “GPU” redirects here. ... ^ What is CVBS video format - aus. ...

External links


  Results from FactBites:
 
Color Graphics Adapter - Wikipedia, the free encyclopedia (1898 words)
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.
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).
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.
  More results at FactBites »

 
 

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