Color Graphics Adapter

Color Graphics Adapter
Release date 1981 (1981)
Architecture Motorola MC6845, ATI CW16800
Cards
Entry-level ATi Graphics Solution Rev 3, ATi Color Emulation Card, Tseng Labs ColorPAK,
Mid-range ATi Graphics Solution plus, ATi Graphics Solution Plus SP, ATi Graphics Solution SR, Number Nine Graphics System
High-end ATi Small Wonder Graphics Solution, Tseng Labs EVA/480
Enthusiast ATi Small Wonder Graphics Solution with game port
History
Successor

Plantronics Colorplus
Tandy Graphics Adapter
Enhanced Graphics Adapter
Multi-Color Graphics Array

Professional Graphics Controller
Original IBM Color Graphics Adapter

The Color Graphics Adapter (CGA), originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter,[1] introduced in 1981, was IBM's first graphics card and first color display card for the IBM PC. For this reason, it also became that computer's first color computer display standard.

The standard IBM CGA graphics card was equipped with 16 kilobytes of video memory and could be connected either to a dedicated direct-drive CRT monitor using a 4-bit digital (TTL) "RGBI"[2] interface, such as the IBM 5153 color display, or to an NTSC-compatible television or composite video monitor via an RCA connector.[3] The RCA connector provided only baseband video, so to connect the CGA card to a standard television set required a separate RF modulator.[4]

Built around the Motorola MC6845 display controller,[5] the CGA card featured several graphics and text modes. The highest display resolution of any mode was 640×200, and the highest color depth supported was 4-bit (16 colors).

Output capabilities

CGA supports:

(The pixel ratio stems from rendering said amount of pixels on a 4:3 screen, a monitor ratio typical of that time.)

Text modes:

Extended graphics modes:

IBM intended that CGA be compatible with a home television set. The 40x25 text and 320x200 graphics modes are usable with a television, and the 80x25 text and 640x200 graphics modes are intended for a monitor.[5]

Color palette

Despite varying bit depths among the CGA graphics modes (see below), CGA processes colors in its palette in four bits, yielding 24 = 16 different colors. The four color bits are arranged according to the RGBI color model: the lower three bits represent red, green, and blue color components; a fourth "intensifier" bit, when set, increases the brightness of all three color components (red, green, and blue).[6] In graphics modes, colors are set per-pixel; in text modes, colors are set per-character, with an independent foreground and background color for each character.

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

With an RGBI monitor

These four bits are passed on unmodified to the DE-9 connector at the back of the card, leaving all color processing to the RGBI monitor connected to it. With respect to the RGBI color model described above, the monitor would use approximately the following formula to process the digital four-bit color number to analog voltages ranging from 0.0 to 1.0:

red   := 2/3×(colorNumber & 4)/4 + 1/3×(colorNumber & 8)/8
green := 2/3×(colorNumber & 2)/2 + 1/3×(colorNumber & 8)/8
blue  := 2/3×(colorNumber & 1)/1 + 1/3×(colorNumber & 8)/8
Dark Yellow
6 #AAAA00

Color 6 is treated differently; when using the formula above, color 6 would become dark yellow, as seen to the left, but in order to achieve a more pleasing brown tone, special circuitry in most RGBI monitors, including the IBM 5153 color display,[7] makes an exception for color 6 and changes its hue from dark yellow to brown by halving the analogue green signal's amplitude:

if colorNumber = 6 then green := green / 2

It is this "RGBI with tweaked brown" palette, shown in the complete palette to the right, that all later PC graphics standards such as EGA and VGA have retained for compatibility as a power-on default setting of their internal palette registers and/or DAC registers.

With a composite color monitor/television set

For the composite output, these four-bit color numbers are encoded by the CGA's onboard hardware into an NTSC-compatible signal fed to the card's RCA output jack. For cost reasons, this is not done using an RGB-to-YIQ converter as called for by the NTSC standard, but by a series of flip-flops and delay lines.[8][9] Consequently, the hues seen are lacking in purity; notably, both cyan and yellow have a greenish tint, and color 6 again looks dark yellow instead of brown. The relative luminances of the colors produced by the composite color-generating circuit differ between CGA revisions: they are identical for colors 1-6 and 9-14 with early CGAs produced until 1983,[10] and are different for later CGAs due to the addition of additional resistors.[11]

CGA's 16 colors when using the NTSC output (post-1983 card revision)

RGBI monitor availability

When the CGA was introduced in 1981, IBM did not offer an RGBI monitor.[12] Instead, customers were supposed to use the RCA output with an RF modulator (that they obtained separately, from a third party) to connect the CGA to their television set.[13] The IBM 5153 Personal Computer Color Display would not be introduced until March 1983.[14] Resulting from the lack of available RGBI monitors in 1981 and 1982, many users would use simpler RGB monitors (without provisions for the "intensifier" bit), reducing the number of available colors to eight, and displaying both colors 6 and 14 as yellow.[12] This is relevant insofar as if an application or game programmer used either one of these configurations, they will have expected color 6 to look dark yellow instead of brown.

Standard text modes

CGA offers four BIOS text modes (called alphanumeric modes in IBM's documentation):

BIOS Modes 0 & 1 select 40 column text modes. The difference between these two modes can only be seen on a composite monitor; mode 0 disables the color burst, making colors appear in grayscale. Mode 1 enables the color burst, allowing for color. Mode 0 and Mode 1 are functionally identical on RGB monitors and on later adapters that emulate CGA without supporting composite color output.

BIOS Modes 2 & 3 select 80 column text modes. As with the 40-column text modes, Mode 2 disables the color burst in the composite signal and Mode 3 enables it.

In every text mode, each character has a background and a foreground color—e.g. red on yellow text for one character, white on black for the next, etc. While the same 4-bit nybble value used for the foreground color would normally allow all 16 colors to be used for the background color, the most significant bit of the background nybble is alternatively used to denote whether or not the character should blink (a hardware effect offered by CGA independent of the CPU). When a character is blinking, its foreground dots alternate between the foreground and background color, so that the during the blink off period, the character cell is filled with the background color (exactly like a space character).[15] All blinking characters on the screen blink in sync. The blinking attribute effect is enabled by default and the high-intensity background effect is disabled; disabling blinking is the only way to freely choose the latter eight-color indexes (8-15) for the background color.

Notably, the GW-BASIC and, later, Microsoft QBASIC (a lesser derivative of Microsoft QuickBASIC) programming language interpreters included with MS-DOS (which was the de facto PC OS while the CGA was popular) supported all the text modes of the CGA with full color control, but did not provide a normal means through the BASIC language to switch the CGA from blink mode to 16-background-color mode, though it would be possible by directly programming the hardware registers using the OUT statement of the BASIC language. In BASIC, foreground text color numbers 16-31 are the blinking versions of colors 0-15, respectively, but background colors 8-15 are identical to colors 0-7 respectively.

Standard graphics modes

CGA offers two commonly-used BIOS graphics modes (sometimes called all points addressable by IBM):

#Palette 1Palette 1 in
high intensity
0 default default
1 3 — cyan 11 — light cyan
2 5 — magenta 13 — light magenta
3 7 — light gray 15 — white (high intensity)
#Palette 0Palette 0 in
high intensity
0 default default
1 2 — green 10 — light green
2 4 — red 12 — light red
3 6 — brown 14 — yellow
  1. Magenta, cyan, white and background color (any of the 16 colors, black by default).
  2. Red, green, brown/yellow and background color (any of the 16 colors, black by default).
By setting the high-intensity bit, brighter versions of these modes can be accessed.
The 1:1.2 pixel aspect ratio needs to be taken into account when drawing large geometrical shapes on the screen.

BIOS Modes 4 & 5 set up the 320x200 graphics modes. Similar to the text modes, Mode 4 enables the composite color burst bit, Mode 5 disables it. Unlike the text modes, disabling the composite color burst bit (which setting Mode 5 does) in 320x200 affects the colors displayed on an RGB monitor with the IBM CGA card and true compatibles (see below.)

BIOS Mode 6 sets up the 640x200 graphics mode. This mode disables the composite color burst signal by default. The BIOS does not provide an option to turn the color burst on in 640x200 mode, and the user must write directly to the mode control register to enable it.

In text mode, font bitmap data comes from the character ROM on the card, which is only available to the card itself. In graphics modes, text output by the BIOS uses two separate tables. The first half of the character set (characters numbered 0 through 127, corresponding to 7-bit ASCII with some added graphical symbols) is supplied by a table in the BIOS ROM chip on the computer's mainboard at the fixed address F000:FA6E (the table is still present at this location even in modern PC BIOSes; unlike the font ROM on the CGA card itself that is used for the text modes, this table provides only the "thick" font shapes, not the "thin" ones). The second half of the set (characters numbered 128 through 255, corresponding to the international, block-graphics and mathematics characters) is supplied by the location pointed to by interrupt vector 1F (the vector itself is found at memory address 0000:007C; this is not in fact a real interrupt vector, since the vector does not point to executable machine code, as real interrupt vectors on the PC's Intel 8086 CPU do). The second half of the character set is ordinarily absent (the vector 1F does not point to actual font data), and trying to display it will result in garbage or blank characters. The character data may be placed into memory manually by the user, or by a utility such as GRAFTABL.

Further graphics modes and tweaks

#3rd palette3rd Palette in
high intensity
0 default default
1 3 — cyan 11 — light cyan
2 4 — red 12 — light red
3 7 — light gray 15 — white (high intensity)

A number of official and unofficial features exist that can be exploited to achieve special effects.

Some of these above tweaks can even be combined. Examples can be found in several games.[18] Most software titles did not use these possibilities, but there were a few impressive exceptions.

160×100 16 color mode

Title screen of PakuPaku, a Pac-Man clone that uses 160×100 mode
A single big "pixel" in 160×100 mode. This is the two top rows of half of character 221. Note the eight constituent non-square pixels and the overall 1:1.2 aspect ratio.

Technically, this mode is not a graphics mode, but a tweak of the 80×25 text mode. The character cell height register is changed to display only two lines per character cell instead of the normal eight lines. This quadruples the number of text rows displayed from 25 to 100. These "tightly squeezed" text characters are not full characters. The system only displays their top two lines of pixels (eight 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 of code page 437 consists of a box occupying the entire left half of the character matrix. (Character 222 consists of a box occupying the entire right half.)

Because each character can be assigned different foreground and background colors, it can be colored (for example) blue on the left (foreground color) and bright red on the right (background color). This can be reversed by swapping the foreground and background colors.

Using either character 221 or 222, each half of each truncated character cell can 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, are possible.

Although a roundabout way of achieving 16-color graphics display, this works quite well and the mode is even mentioned (although not explained) in IBM's official hardware documentation.[19]

More detail can be achieved in this mode by using other characters, combining ASCII art with the aforesaid technique.

Because the CGA has 16 KiB (16384 bytes) of graphics memory, not 16000, it is just as easy to set the number of lines in this mode to 102 instead of 100 for a resolution of 160×102 (16320 pixels). This uses extra video memory that is 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 can 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.[20][21][22]

Special effects on composite color monitors

Using the NTSC TV-out instead of an RGBI monitor not only made for less attractive colors, as described above, but as is common with NTSC composite video, the separation between luminance and chrominance is far from perfect, yielding cross-color artifacts, or color "smearing". This is especially a problem with 80-column text:

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

It is for this reason that each of the text and graphics modes described above exists twice: once as the normal "color" version and once as a "monochrome" version; the "monochrome" version of each mode would turn off the NTSC color decoding in the viewing monitor completely, resulting in a black-and-white picture, but also no color bleeding, hence, a sharper picture. On RGBI monitors, the two versions of each mode are identical, with the exception of the 320x200 graphics mode, where the "monochrome" version produces the third palette, as described above.

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 artifact colors. Both the standard 320×200 four-color and the 640×200 color-on-black graphics modes could be used with this technique.

Internal operation

Direct colors are the normal 16 colors as described above under "The CGA color palette".

Artifact colors are seen because the composite monitor's NTSC chroma decoder misinterprets some of the luminance information as color, as stated before. By carefully placing pixels in appropriate patterns, the skilled programmer produces particular cross-color artifacts yielding the desired color; either from purely black-and-white pixels in 640×200 mode, or resulting from a combination of direct and artifact colors in 320×200 mode, as seen in these pictures.

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), plus the ability to set the foreground color in 640×200 mode freely, each one of these parameters results in a different set of artifact colors, making for a total gamut of over 100 colors, of which 16 can be displayed at the same time.

Later demonstrations by enthusiasts have increased the maximum number of colors the CGA is known to produce in a single image to approximately a thousand. Aside of artifacting, this technique involves the text mode tweak which quadruples its rows, thus offering the benefit of 16 foreground and 16 background colors. Certain ASCII characters such as U and ‼ are then used to produce the necessary patterns, which result in non-dithered images with an effective resolution of 80×100 on a composite monitor.[23]

Availability and caveats

The 320×200 variant of this technique (see above) is how the standard BIOS-supported graphics mode looks on a composite color monitor. The 640×200 variant however requires 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.

The modern, games-centric PC emulator DOSBox includes a CGA mode, which can emulate a composite monitor (in graphics modes). As of December 2012, the latest official version will emulate the more common 640×200 composite mode and its set of 16 artifact colors; support for the more complex 320×200 variant has been added to the DOSBox codebase for the next official build.

Resolution and usage

Composite artifacting, whether used intentionally or as an unwanted artifact, reduces the effective horizontal resolution to a minimum of 160 pixels, more for black-on-white or white-on-black text, without changing the vertical resolution. The resulting composite video display with "artifacted" colors was thus sometimes described as a 160x200/16-color "mode", though technically it was a method, not a mode.

The low resolution of this composite color artifacting method led to it being used almost exclusively in games. Many of the more high-profile titles optionally, sometimes exclusively, offering graphics optimized for composite color monitors. Ultima II, the first game in the game series to be ported to IBM PC, used CGA composite graphics. King's Quest I was innovative in its use of 16-color graphics on the PC, PCjr and Tandy 1000; even CGA owners could enjoy the 16-color graphics by using a composite color monitor or television, thanks to programmers exploiting the inaccuracies of composite NTSC chroma decoding. Selecting 'RGB mode' at the title screen would instead result in the usual CGA graphics mode limited to 4 colors. In this mode, dithering was employed to simulate extra colors.

Limitations, bugs and errata

Video timing on the CGA is provided by the Motorola 6845 video controller. This integrated circuit was originally designed only for character-based alphanumeric (text) displays and can only address a maximum of 128 character rows. To realize graphics modes with 200 scanlines on the CGA, the MC6845 is programmed with 100 character rows per picture and two scanlines per character row. Because the video memory address output by the MC6845 is identical for each scanline within a character row, the CGA must use the MC6845's "row address" output (i.e. the scanline within the character row) as an additional address bit to fetch raster data from video memory.[24] This implies that unless the size of a single scanline's raster data is a power of two, raster data can not be laid out continuously in video memory. Instead, graphics modes on the CGA first put only the even-numbered scanlines continuously in a first block of video memory, then a second block of odd-numbered scanlines starting at video memory position 8,192. This arrangement results in additional overhead in graphics modes for software that manipulates video memory.

Even though the MC6845 video controller can provide the timing for interlaced video, the CGA's circuitry aligns the synchronization signals in such a way that scanning is always progressive. Therefore, it is impossible to double the vertical resolution to 400 scanlines using a standard 15 kHz monitor.

The higher bandwidth used by 80-column text mode results in random short horizontal lines appearing onscreen (known as "snow") if a program writes directly to video memory. The BIOS avoids the problem by only accessing the memory during horizontal retrace, or by temporarily turning off the output during scrolling; while causing the display to blink, IBM decided that doing so was better than snow.[5] The "snow" problem does not occur on any other video adapter, or on most CGA clones.

In the 80-column text mode, the pixel clock is doubled, and all the synchronization signals are output for twice the number of clock cycles in order to last for their proper duration. The composite output's color burst signal circuit is an exception: because it still outputs the same number of cycles now at twice the clock rate, the color burst signal produced is too short for most monitors, yielding no or unstable color. Hence, IBM documentation lists the 80-column text mode as a "feature" only for RGBI and black-and-white composite monitors.[25] Stable color can still be achieved by setting the border color to brown, which happens to produce a phase identical to the correct color burst signal and serves as a substitute for it.

Software support

CGA was widely supported in PC software up until the 1990s. Some of the software that supported the board was:

Competing adapters

BYTE in January 1982 described the output from CGA as "very good—slightly better than color graphics on existing microcomputers".[12] CGA had two main competitors:

Other alternatives:

The CGA card was succeeded in the consumer space by IBM's Enhanced Graphics Adapter (EGA) card, which supports most of CGA's modes and adds an additional resolution (640×350) as well as a software-selectable palette of 16 colors out of 64 in both text and graphics modes. Along with this move, the price of the older CGA card was lowered considerably; it became an attractive low-cost option 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 CGA's limitations. CGA's popularity started to wane after VGA became IBM's high-level standard and EGA the entry-level standard in 1987. However, most software made up to 1990 supported it.

Specifications

Connector

The Color Graphics Adapter uses a standard DE-9 connector for direct-drive video (to an RGBI monitor). The connector on the card is female and the one on the monitor cable is male.

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

See also

References

  1. ↑ ; cf. section 1-133, "Color/Graphics Adapter", page 143 of ibm_techref_v202_1.pdf
  2. ↑ Red, Green, Blue, Intensity
  3. ↑ A. Kumar (2002). Encyclopaedia of Management of Computer Hardware. Anmol Publications. p. 1050. ISBN 978-81-261-1030-8.
  4. ↑ (There was also a connector on the original IBM CGA cards for an add-on RF modulator unit installed inside the computer case, but no such device was offered by IBM, and it would have had to provide its own output connector separate from the ones on the CGA itself.)
  5. 1 2 3 Bradley, David J. (September 1990). "The Creation of the IBM PC". BYTE. pp. 414–420. Retrieved 2 April 2016.
  6. ↑ The color brown, represented by R=1, G=1, B=0, I=0, is an exception; whereas a straight interpretation of these bit values would resolve this color as dark yellow, the intensity of the green component is halved, to produce brown, for only this one 4-bit value. See this page for details. This special RGBI interpretation for brown is performed in the monitor; the IBM 5153 monitor designed for the CGA performs it, but some early third-party monitors did not.
  7. ↑ International Business Machines Corporation (1983): IBM Personal Computer XT Technical Reference Manual, pages D-42 to D-43.
  8. ↑ Dean et al. (1984): Composite video color signal generation from digital color signals. U.S. Patent #4,442,428
  9. ↑ International Business Machines Corporation (1983): IBM Personal Computer XT Technical Reference Manual, page D-40.
  10. ↑ IBM Personal Computer (PDF) (Technical Reference). IBM Personal Computer Hardware Reference Library (revised ed.). April 1983. p. D-50.
  11. ↑ IBM Color/Graphics Monitor Adapter (PDF) (Technical Reference). IBM Options and Adapters. p. 32.
  12. 1 2 3 Williams, Gregg (January 1982). "A Closer Look at the IBM Personal Computer". BYTE. p. 36. Retrieved 19 October 2013.
  13. ↑ International Business Machines Corporation (1982): You & Your IBM Personal Computer. Sales Brochure, page 4.
  14. ↑ International Business Machines Corporation (1983): Announcement Letter Number 183-002 - IBM COLOR DISPLAY, 5153. Dated February 4, 1983. http://www-01.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_ca/2/897/ENUS183-002/index.html&lang=en&request_locale=en
  15. ↑ As a consequence of this, the blink bit has no effect on a space character. The same is true of any character that has the same color for its foreground and background, as such a character also always appears the same as a blank space.
  16. ↑ mobygames.com
  17. ↑ mobygames.com
  18. ↑ mobygames.com
  19. ↑ cf. http://vintageibm.net/yahoo_site_admin/assets/docs/techrefv202.zip , section/page 1-142, "Color/Graphics Adapter", page 152 of ibm_techref_v202_1.pdf
  20. ↑ oldskool.org
  21. ↑ oldskool.org
  22. ↑ oldskool.org
  23. ↑ "CGA in 1024 Colors - a New Mode: the Illustrated Guide.". A blog entry by the creators of the demo "8088 MPH" explaining this technique.
  24. ↑ IBM Enhanced Graphics Adapter (PDF) (Technical Reference). IBM Options and Adapters. August 2, 1984. p. 41.
  25. ↑ IBM Color/Graphics Monitor Adapter (PDF) (Technical Reference). IBM Options and Adapters. p. 7.
  26. ↑ Curran, Lawrence J., Shuford, Richard S. (November 1983). "IBM's Estridge". BYTE. pp. 88–97. Retrieved 19 March 2016.
  27. ↑ Stark, Craig L. (1984-10-02). "Paradise Graphics Card: It's Easier Being Green". PC Magazine. p. 59. Retrieved 25 October 2013.

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