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 interface, such as the IBM 5153 color display, or to an NTSC-compatible television or composite video monitor via an RCA connector.[2] The RCA connector provided only baseband video, so to connect the CGA card to a standard television set required a separate RF modulator unless the TV had an RCA jack though with the former combined with an amplifier sometimes was more practical since one could then hook up an antenna to the amplifier and get wireless video.[3]

Built around the Motorola MC6845 display controller,[4] 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).

Color Graphics Adapter
Release date1981
ArchitectureMotorola MC6845, ATI CW16800
Cards
Entry-levelIBM Color Graphics Adapter, ATi Graphics Solution Rev 3, ATi Color Emulation Card, Tseng Labs ColorPAK,
Mid-rangeATi Graphics Solution plus, ATi Graphics Solution Plus SP, ATi Graphics Solution SR, Number Nine Graphics System
High-endATi Small Wonder Graphics Solution, Tseng Labs EVA/480
EnthusiastATi Small Wonder Graphics Solution with game port
History
SuccessorPlantronics Colorplus

PCjr/Tandy Graphics Adapter
Enhanced Graphics Adapter
Multi-Color Graphics Array

Professional Graphics Controller
IBM Color Graphics Adapter
Original IBM Color Graphics Adapter

Output capabilities

CGA supports:

  • 320×200 in 4 colors from a 16 color hardware palette. Pixel aspect ratio of 1:1.2.
  • 640×200 in 2 colors. Pixel aspect ratio of 1:2.4

(The pixel aspect ratio stems from rendering said distribution of pixels on a screen with 4:3 proportions, typical of monitors at the time.)

Text modes:

  • 40×25 with 8×8 pixel font (effective resolution of 320×200)
  • 80×25 with 8×8 pixel font (effective resolution of 640×200)

Extended graphics modes:

  • 160×100 16 color mode
  • Artifact colors using a NTSC monitor (16 colors from more than 100 possible)

IBM intended that CGA be compatible with a home television set. The 40×25 text and 320×200 graphics modes are usable with a television, and the 80×25 text and 640×200 graphics modes are intended for a monitor.[4]

Cga p0

Cga p0.png CGA 320×200 in 4 colors palette 0 (red, yellow, green, black background)

Cga p1

Cga p1.png CGA 320×200 in 4 colors palette 1 (cyan, magenta, white, black background)

Cga p3

Cga p3.png CGA 320×200 in 4 colors 3rd palette (tweaked), (cyan, red, white, black background)

Cga 640x200

Cga 640x200.png CGA 640×200 in 2 colors(1-bit)

Cga 150x100

Cga 150x100.png CGA 160×100 16 color mode(4-bit)

CGA Partial Mandelbrot Set

CGA Partial Mandelbrot Set.png A partial Mandelbrot set rendered in CGA palette 1

Arachne CGA Mode

Arachne CGA Mode.svg Screenshot of Arachne displaying its embedded frames and tables test pages in CGA 640×200 mode

Paku-paku5-dos

Paku-paku5-dos.png PakuPaku in 160×100 16 color mode

CGA program interface

CGA program interface.png PCPaint in 320×200 3rd palette low intensity, showing a typical low resolution interface. Note the use of dithering to overcome the CGA palette limitations.

CGA 640x200 game

CGA 640x200 game.png SimCity in 640×200 monochrome. Note the use of dithering to simulate gray tones and non-square pixel ratio that deforms the fonts.

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).[5] 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
#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,[6] 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.[7][8] 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,[9] and are different for later CGAs due to the addition of additional resistors.[10]

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

As noted however, this method only works on NTSC television sets, PAL TVs do not display the colors as expected when connected to the composite output, as PAL's superior color separation prevents artifacting from occurring.

RGBI monitor availability

When the CGA was introduced in 1981, IBM did not offer an RGBI monitor.[11] 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.[12] The IBM 5153 Personal Computer Color Display would not be introduced until March 1983.[13] 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.[11] 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):

  • 40×25 characters in up to 16 colors. Each character is a pattern of 8×8 dots. The effective screen resolution in this mode is 320×200 pixels (a pixel aspect ratio of 1:1.2), though individual pixels cannot be addressed independently. The choice of patterns for any location is thus limited to one of the 256 available characters, the patterns for which are stored in a ROM chip on the card itself. The display font in text mode (the hardware code page 437 character set) is therefore fixed and cannot be changed (although when using the original IBM CGA, it is possible to select one of two different fonts—normal or thin—by changing a jumper. Many clones didn't offer this possibility). The card has sufficient video RAM for eight different text pages in this mode. 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.
  • 80×25 characters in up to 16 colors. Each character is again an 8×8 dot pattern (the same character set is used as for 40×25), in a pixel aspect ratio of 1:2.4. The effective screen resolution of this mode is 640×200 pixels. Again, the pixels cannot be individually addressed. Since there are twice as many characters on the screen in this mode, the card has enough video RAM for just four different text pages. 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).[14] 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 graphics modes in two resolutions:

  • 320×200 pixels, as with the 40×25 text mode. In the graphics mode, however, each pixel can be addressed independently. The tradeoff is that only four colors can be displayed at a time. Also, only one of the four colors can be freely chosen from the 16 CGA colors. The 1:1.2 pixel aspect ratio needs to be taken into account when drawing large geometrical shapes on the screen.

BIOS Mode 4 offers two palettes (differing in the presence or absence of the blue color component): green/red/brown and cyan/magenta/white. By setting the high-intensity bit, brighter versions of these modes can be accessed. The background can be set to any of the 16 CGA colors, but is black by default.

As with Modes 0 and 2, Mode 5 disables the color burst bit to allow colors to appear in grayscale on Composite monitor. Unlike the text modes, disabling the composite color burst bit affects the colors displayed on an RGB monitor with the IBM CGA card and true compatibles. Some programmers use Mode 5 as an unofficial third palette on RGB monitors: cyan/red/white and the background color. The intense versions of these colors can also be used and the background color may be changed, but the palette cannot be switched to palettes 0 or 1 without enabling the composite color signal again. Notably, it is not mentioned in the IBM Technical Reference manual, and some CGA clones may not support it.

  Mode 4 Mode 5
Palette 0 Palette 1 low intensity high intensity
# low intensity high intensity low intensity high intensity
0 default default default default default default
1 2 — green 10 — light green 3 — cyan 11 — light cyan 3 — cyan 11 — light cyan
2 4 — red 12 — light red 5 — magenta 13 — light magenta 4 — red 12 — light red
3 6 — brown 14 — yellow 7 — light gray 15 — white 7 — light gray 15 — white
  • 640×200 pixels, as with the 80×25 text mode. All pixels can be addressed independently. This mode is monochrome with a pixel aspect ratio of 1:2.4. By default the colors are black and bright white, but the foreground color can be changed to any other color of the CGA palette. This can be done at runtime without refreshing the screen. The background color cannot be changed from black on a true IBM CGA card. BIOS Mode 6 sets up the 640×200 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 640×200 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

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

  • In 320×200 graphics mode, the background color (which also affects the border color), which defaults to black on mode initialization, can be changed to any of the other 15 colors of the CGA palette. This allows for some variation, as well as flashing effects, as the background color can be changed without having to redraw the screen (i.e. without changing the contents of the video RAM.)
  • In text mode, the border color (displayed outside the regular display area—into the overscan area) can be changed from its usual black to any of the other 15 colors.
  • Through precision timing, it is possible to switch to another palette while the screen is being scanned(drawn), allowing the use of any one of the six palettes per scanline. The best example of this in use is the game California Games[15] when run on a stock 4.77 MHz 8088. (Running it on a faster computer does not produce the effect, as the method the programmers used to switch palettes at predetermined locations is extremely sensitive to machine speed.) The same can be done with the background color, to create the river and road in Frogger.[16] Another documented example of the technique is in Atarisoft's port of Jungle Hunt to the PC.
  • Additional colors are often approximated using dithering, although the low resolution makes it very apparent. In particular, many Sierra games use palette 0 at low intensity and dark blue as the background color. This gives it the three primary RGB colors to work with (as well as brown).

Some of these above tweaks can even be combined. Examples can be found in several games.[17] Most software titles did not use these possibilities, but there were a few impressive exceptions.

160×100 16 color mode

Paku Paku
Title screen of PakuPaku, a Pac-Man clone that uses 160×100 mode
Single pixel in CGA 160x100 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.

 ASCII.221.character.svg   Character 221. 
 Half-block.character.blue.red.svg   221 with blue text and red background color. 
 Half-block.character.red.blue.svg   221 with red text and blue background color. 
 ASCII.222.character.svg   Character 222. 

Character 221 of the CGA character set 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.[18]

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 (16,384 bytes) of graphics memory, not 16,000, 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- a fear that is not unfounded as it was later found that certain compatibles have cards that either glitch or ignore any attempt to put the device into this mode.

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.[19][20][21]

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:

CGA CompVsRGB 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 320×200 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.

CGA CompVsRGB 320p0

320×200 palette 0

CGA CompVsRGB 320p1

320×200 palette 1

CGA CompVsRGB 640

640×200

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.[22]

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 160×200/16-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.

Microsoft Decathlon RGBvsComposite

Microsoft Decathlon - Top: Game in composite mode, Bottom: Game in RGB mode, Left: with RGB monitor, Right: with composite monitor

KQ CompVsRGB

King's Quest -Top: Game in composite mode, Bottom: Game in RGB mode, Left: with RGB monitor, Right: with composite monitor

Ultima2 CompVsRGB

Ultima II - Left: with RGB monitor, Right: with composite monitor

High color depth

CGA-1024-color-mode
1024 colors in composite mode

By taking advantage of the color smearing, the NTSC color clock and a method similar to that used in the 16 color mode, it's possible to display over 16 colors in composite monitors.

160 cycles of the NTSC color clock occur during the each line's pixel period so in 40 column mode each pixel occupies half a cycle and in 80 column mode each pixel uses a quarter of a cycle. Limiting the character display to the upper or upper two scanlines, and taking advantage of the pixel arrangement in certain characters of the codepage 437, it is possible to display up to 1024 colors.[23] This technique was used in the demo 8088 MPH.[24]

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.[25] This implies that unless the size of a single scanline's raster data is a power of two, raster data cannot 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.[4] 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.[26] 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".[11] PC Magazine disagreed, reporting in June 1983 that "the IBM monochrome display is absolutely beautiful for text and wonderfully easy on the eyes, but is limited to simple character graphics. Text quality on displays connected to the color/graphics adapter ... is at best of medium quality and is conducive to eyestrain over the long haul".[27] In a retrospective commentary, Next Generation also took a negative view on the CGA, stating, "Even for the time (early 1980s), these graphics were terrible, paling in comparison to other color machines available on the market."[28]

CGA had two main competitors:

  • For business and word processing use, IBM announced its Monochrome Display Adapter (MDA) at the same time as CGA. MDA was much more popular than CGA at first.[29] The author of an internal IBM publication stated in October 1981 that he had planned to purchase the CGA adapter but changed his mind after seeing it ("rough") and MDA ("beautiful") display text, observing that "you stare at text a whole lot more than you stare at color graphics".[30] 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 and more clearly separated characters than the CGA's 8×8 dots text character matrix allowed. Because of this, MDA was the preferred choice for business use. Also, IBM initially manufactured the MDA card as a printer port/MDA combo card. This meant that users wishing to connect printers to their original IBM PC would have to pay for the MDA card anyway (initially $335), while the CGA card (initially $300) could be left out to save money. While including the CGA card and connecting an existing TV set for use as a monitor allowed users to forgo the purchase of a monitor, this was not significantly cheaper than buying a monochrome monitor (initially $345) and leaving out the CGA card. Also, 80-column text was almost unusable on color composite displays, and the IBM model 5153 CGA color video display that was required to fully exploit the CGA card's capabilities was even more expensive. Since a great many PCs were sold to businesses, the sharp, high-resolution monochrome text was more desirable for running applications.
  • In 1982, the non-IBM Hercules Graphics Card (HGC) was introduced, the first third-party video card for the PC. In addition to an MDA-compatible text mode, it offered a monochrome graphics mode. With a resolution of 720×348 pixels, it had a higher resolution than that produced by CGA. The Hercules' combination of sharp monochrome text and graphics capabilities made it ideal for running software such as Lotus 1-2-3 that supported business graphics. Some games also had Hercules support, and most others could be made to work with HGC via SimCGA, a TSR which would reformat the CGA graphics memory to HGC format in the background.

Other alternatives:

  • The IBM PCjr (1984) and the compatible Tandy 1000 (1985) featured onboard "extended CGA" video hardware that extended video RAM beyond 16 kB, thus allowing 16 colors at 320×200 resolution and four colors at 640×200 resolution (later Tandys also had a 640×200 mode with 16 colors). Because the Tandy 1000 long outlived the PCjr, the video modes became known as "Tandy Graphics Adapter" or "TGA", and were very popular for games during the 1980s. Similar but less widely used was the Plantronics Colorplus.
  • In 1984, IBM also introduced the Professional Graphics Controller, a high-end graphics solution intended for e.g. CAD applications. It was mostly backwards compatible with CGA. The PGC did not see widespread adoption due to its $4,000 price tag, and was discontinued in 1987.
  • Paradise Systems introduced in 1984 the first successful CGA-compatible card for MDA monitors. It displayed CGA's 16 colors in shades of monochrome. Because it was hardware-compatible with CGA, the Paradise card did not need special software support or additional drivers.[31]
  • Another extension in some CGA-compatible chipsets (including those in the Olivetti M24, AT&T 6300, the DEC VAXmate, and some Compaq and Toshiba portables) is a doubled vertical resolution. This gives a higher quality 8×16 text display and an additional 640×400 graphics mode.

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.

DE9 Diagram

DE9 Diagram
Pin assignments
Pin Function
1 Ground
2 Ground
3 Red
4 Green
5 Blue
6 Intensity
7 Reserved
8 Horizontal Sync
9 Vertical Sync

Warning of 12V in connector

The pin 7 may be used by card for providing 12V to monitor. It should be not connected to most monitors.[32]

Signal

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

See also

References

  1. ^ [1]; cf. section 1-133, "Color/Graphics Adapter", page 143 of ibm_techref_v202_1.pdf
  2. ^ A. Kumar (2002). Encyclopaedia of Management of Computer Hardware. Anmol Publications. p. 1050. ISBN 978-81-261-1030-8.
  3. ^ (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.)
  4. ^ a b c Bradley, David J. (September 1990). "The Creation of the IBM PC". BYTE. pp. 414–420. Retrieved 2 April 2016.
  5. ^ 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.
  6. ^ International Business Machines Corporation (1983): IBM Personal Computer XT Technical Reference Manual, pages D-42 to D-43.
  7. ^ Dean et al. (1984): Composite video color signal generation from digital color signals. U.S. Patent #4,442,428
  8. ^ International Business Machines Corporation (1983): IBM Personal Computer XT Technical Reference Manual, page D-40.
  9. ^ IBM Personal Computer (PDF) (Technical Reference). IBM Personal Computer Hardware Reference Library (revised ed.). April 1983. p. D-50.
  10. ^ IBM Color/Graphics Monitor Adapter (PDF) (Technical Reference). IBM Options and Adapters. p. 32.
  11. ^ a b c Williams, Gregg (January 1982). "A Closer Look at the IBM Personal Computer". BYTE. p. 36. Retrieved 19 October 2013.
  12. ^ International Business Machines Corporation (1982): You & Your IBM Personal Computer. Sales Brochure, page 4.
  13. ^ 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
  14. ^ 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.
  15. ^ mobygames.com
  16. ^ mobygames.com
  17. ^ mobygames.com
  18. ^ 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
  19. ^ oldskool.org
  20. ^ oldskool.org
  21. ^ oldskool.org
  22. ^ "CGA in 1024 Colors - a New Mode: the Illustrated Guide". A blog entry by the creators of the demo "8088 MPH" explaining this technique.
  23. ^ VileR (2015-04-15). "8088 MPH: CGA in 1024 Colors - a New Mode: the Illustrated Guide". int10h.org. Retrieved 2018-05-04.
  24. ^ "1K colours on CGA: How it's done". Reenigne Blog. 2015-04-08. Retrieved 2018-04-27.
  25. ^ IBM Enhanced Graphics Adapter (PDF) (Technical Reference). IBM Options and Adapters. August 2, 1984. p. 41.
  26. ^ IBM Color/Graphics Monitor Adapter (PDF) (Technical Reference). IBM Options and Adapters. p. 7.
  27. ^ Fastie, Will (June 1983). "The Graphical PC". PC Magazine.
  28. ^ "The Next Generation 1996 Lexicon A to Z". Next Generation. No. 15. Imagine Media. March 1996. p. 31.
  29. ^ Curran, Lawrence J.; Shuford, Richard S. (November 1983). "IBM's Estridge". BYTE. pp. 88–97. Retrieved 19 March 2016.
  30. ^ Dievendorff, Dick (1981). IBM Personal Computer Questions and Answers. IBM. p. 25.
  31. ^ Stark, Craig L. (1984-10-02). "Paradise Graphics Card: It's Easier Being Green". PC Magazine. p. 59. Retrieved 25 October 2013.
  32. ^ http://nemesis.lonestar.org/reference/video/cga.html
Notes

External links

BIOS color attributes

BIOS Color Attribute is an 8 bit value where the low 4 bits represent the character color and the high 4 bits represent the background color. For example, to print a white character 'A' with black background, the "BIOS Color Attribute" would be set to the hexadecimal value 0x0F. The highest bit of the color attribute, which is also the highest bit of the background color can take over two functions. It can either have no influence on the background color making text blink when set, effectively limiting the available background colors to only eight, or if intensive background colors are enabled the full 16 colors become available but blinking is no longer available. This behavior can be changed, i.e., using BIOS 80x86 interrupt 0x10, function 0x1003. This 16 colour palette is often used in console programs (e.g. cmd in Windows) and sometimes for chat in games (e.g. Minecraft).

BSAVE

A BSAVE Image (aka "BSAVED Image") as it is referenced in a graphics program is an image file format created usually by saving raw video memory to disk (sometimes but not always in a BASIC program using the BSAVE command).The BASIC BSAVE command is a general command meant for dumping ranges of memory addresses to disk. Data could be recalled using the counterpart BLOAD command. Some platforms provided a BRUN command that would immediately attempt to execute the loaded RAM image as a program.

BSAVE was in general use as a file format when the IBM PC was introduced. It was also in general use on the Apple II in the same time period. Although the commands were available on the Commodore PET line, they were removed from the later (and more popular) Commodore 64 and VIC-20 computers. In 1985 the Commodore 128 was released with Commodore BASIC version 7.0 which restored the BSAVE and BLOAD commands.

Compaq Deskpro

The Compaq Deskpro was a line of business-oriented personal computers manufactured by Compaq, then discontinued after the merger with Hewlett-Packard. Models were produced containing microprocessors from the 8086 up to the x86-based Intel Pentium 4.

The original Compaq Deskpro (released in 1984), available in several disk configurations, was an XT-class PC equipped with an 8 MHz 8086 CPU and Compaq's unique display hardware that combined Color Graphics Adapter graphics with high resolution Monochrome Display Adapter text. As a result, it was both considerably faster than the IBM PC and had a much better quality text display compared to an IBM PC equipped with graphics. Like the earlier Compaq Portable, its hardware and BIOS was 100% compatible with the IBM PC Model 5150 (a characteristic that few other PC workalikes shared so soon after the establishment of the IBM standard).

In 1985, Compaq released the Deskpro 286, which looks quite similar to the IBM PC/AT. Then in 1987, the Deskpro 386 was launched after Intel released its 80386 microprocessor, beating IBM by 7 months on their comparable 386 computer, and thus making a name for themselves.The DeskPro 386/25 was released August 28, 1989 and cost $7,999.The form factor for the Compaq Deskpro is mostly the desktop model which lies upon a desk, with a monitor placed on top of it. Compaq has produced many tower upright models that have been highly successful in sales, and are usually convertible to a desktop form factor. An SFF (small form factor) desktop version was also produced during the Deskpro's lifetime.

The many different models include the :

Deskpro 286e

Deskpro 386 released as the first true 32 bit computer with 386 processor.

Deskpro 386S (Second Generation 386 introducing 16 bit EISA slots)

Deskpro XE 486 ISA and IDE

Deskpro XL High-End workstation with EISA and SCSI either and 486, Pentium, Pentium Pro

Deskpro M 386, 486 and 586 early Pentium models

Deskpro 2000 Pentium 1, Pentium Pro and Pentium 2

Deskpro 4000 Pentium 1 with MMX & Pentium 2

Deskpro 6000 Pentium 1, Pentium Pro and Pentium 2 and scsi

Deskpro DX

Deskpro EXD, SB , EN , ENL: Pentium III based

Deskpro EVO500 series, the last of the range with Pentium 4 processors

Friendlyware

Friendlyware was a set of 30 computer programs that were written in BASIC and bundled with certain IBM personal computers. They were a collection of business, game and personal analysis software that pushed the power of BASIC to the maximum.

Friendlyware was published in 1983 by a company called Friendlysoft. Friendlyware was one of the first pieces of software to implement a boss key.There were several releases of Friendlyware. The original "demo" copy came on a 5.25" floppy with a green label. The demo copy (Friendlyware I) had one arcade game (Brick Out) and other "family" software, including strategy games like Reversi. The Friendlyware Arcade pack came on a floppy with a red label, and contained eight additional arcade style games reminiscent of the early 1980s. The BusinessWare release came with a blue label, and contained simple business software.

The FriendlySoft company was started by an investor named Michael Yaw, who also owned several Domino's Pizza stores. The software was written by a team of four programmers, who were paid royalties on the sales. Development was done on some of the earlier IBM PCs, which were so new at the time that they had to be ordered. One of the computers purchased was a PC with a CGA card (Color Graphics Adapter), which was an innovation. Over 29,000 copies of the original FriendlyWare were eventually sold.

GRiDPad

The GRiDPad was a pen computing tablet built by GRiD Systems Corporation in 1989. It is regarded as the first commercially successful tablet computer. Jeff Hawkins went on to use the GRiDPad as a predecessor for his best known-invention, the Palm Pilot.

Hercules Graphics Card

The Hercules Graphics Card (HGC) is a computer graphics controller made by Hercules Computer Technology, Inc. that combines IBM's text-only MDA display standard with a bitmapped graphics mode. This allows the HGC to offer both high quality text and graphics from a single card. The HGC was very popular, and became a widely supported de facto display standard on IBM PC compatibles connected to a monochrome monitor. The HGC standard was used long after more technically capable systems had entered the market, especially on dual-monitor setups.

IBM JX

The IBM JX (or JXPC) was a personal computer released in 1984 into the Japanese, Australian and New Zealand markets. Designed in Japan, it was based on the technology of the IBM PCjr and was designated the IBM 5511. It was targeted in the Australasian market towards the public education sector rather than at consumers, and was sold in three levels: JX (64 KiB), JX2 (128 KiB) and JX3 (256 KiB). Upgrades were available to both 384 KiB and 512 KiB.

IBM Monochrome Display Adapter

The Monochrome Display Adapter (MDA, also MDA card, Monochrome Display and Printer Adapter, MDPA) is IBM's standard video display card and computer display standard for the PC introduced in 1981. The MDA does not have any pixel-addressable graphics modes. It has only a single monochrome text mode (PC video mode 7), which can display 80 columns by 25 lines of high resolution text characters or symbols useful for drawing forms.

IBM Personal Computer XT

The IBM Personal Computer XT, often shortened to the IBM XT, PC XT, or simply XT, is a version of the IBM PC with a built-in hard drive. It was released as IBM Machine Type number 5160 on March 8, 1983. Apart from the hard drive, it was essentially the same as the original PC, with only minor improvements. The XT was mainly intended as an enhanced IBM PC for business users. Later floppy-only models would effectively replace the original model 5150 PC. A corresponding 3270 PC featuring 3270 terminal emulation was released later in October 1983. XT stands for eXtended Technology.

List of common resolutions

This article lists computer monitor screen resolutions that are defined by standards or in common use. Most of them use certain preferred numbers.

Monochrome monitor

A monochrome monitor is a type of CRT computer monitor which was very common in the early days of computing, from the 1960s through the 1980s, before color monitors became popular. They are still widely used in applications such as computerized cash register systems, owing to the age of many registers. Green screen was the common name for a monochrome monitor using a green "P1" phosphor screen.Abundant in the early-to-mid-1980s, they succeeded Teletype terminals and preceded color CRTs and later LCDs as the predominant visual output device for computers.

Plantronics Colorplus

The Plantronics Colorplus is a graphics card for IBM PC computers, first sold in 1982. It is a superset of the then-current CGA standard, using the same monitor standard and providing the same pixel resolutions. It was produced by Frederick Electronics, of Frederick, Maryland.

The Colorplus has twice the memory of a standard CGA board (32k, compared to 16k). The additional memory can be used in graphics modes to double the color depth, giving two additional graphics modes—16 colors at 320×200 resolution, or 4 colors at 640×200 resolution.

It uses the same Motorola MC6845 display controller as the previous MDA and CGA adapters.The original card also includes a parallel printer port.

Professional Graphics Controller

Professional Graphics Controller (PGC, often called Professional Graphics Adapter and sometimes Professional Graphics Array) is a graphics card manufactured by IBM for PCs. It consists of three interconnected PCBs, and contains its own processor and memory. The PGC was, at the time of its release, the most advanced graphics card for the IBM XT and aimed for tasks such as CAD.Introduced in 1984, the Professional Graphics Controller offered a maximum resolution of 640×480 with 256 colors at a refresh rate of 60 Hertz—a higher resolution and color depth than CGA and EGA supported. This mode is not BIOS-supported. It was intended for the computer-aided design market and included 320 kB of display RAM and an on-board Intel 8088 microprocessor. The 8088 was placed directly on the card to permit rapid updates of video memory. Other cards forced the PC's CPU to write to video memory through a slower ISA bus. While never widespread in consumer-class personal computers, its US $4,290 list price compared favorably to US$50,000 dedicated CAD workstations of the time. It was discontinued in 1987 with the arrival of VGA and 8514.

RGBI

RGBI could refer to:

Rio Grande Bible Institute

RGBI interface — Red, Green, Blue, Intensity, as in an RGBI cathode ray tube monitor interface; cf. Color Graphics Adapter

RG color space

The RG or red-green color space is a color space that uses only two colors, red and green. It is an additive format, similar to the RGB color model but without a blue channel. Thus, blue is said to be out of gamut. This format is not in use today, and was only used on two-color Technicolor and other early color processes for films; by comparison to a full spectrum, its poor color reproduction made it undesirable. The system cannot create white naturally, and many colors are distorted.

Snake (video game genre)

Snake is the common name for a video game concept where the player maneuvers a line which grows in length, with the line itself being a primary obstacle. The concept originated in the 1976 arcade game Blockade, and the ease of implementing Snake has led to hundreds of versions (some of which have the word snake or worm in the title) for many platforms. After a variant was preloaded on Nokia mobile phones in 1998, there was a resurgence of interest in the snake concept as it found a larger audience. There are over 300 Snake-like games for iOS alone.

Tandy Graphics Adapter

Tandy Graphics Adapter (TGA) is a computer display standard for an IBM PC compatible video subsystem that improved on IBM's Color Graphics Adapter (CGA) technology. Whereas CGA could display only four colors at a time at a screen resolution of 320×200 pixels, a TGA system could display up to 16 colors. While not strictly an adapter—the TGA hardware was available only integrated onto computer motherboards, not on a separate card—TGA is so called to parallel CGA, to which TGA is related and with which it competed. TGA is also known as Tandy graphics.

IBM premiered the new video adapter in the PCjr home computer it announced in 1983. At that time, the company called the main LSI chip of the video subsystem the Video Gate Array (not to be confused with the later Video Graphics Array or VGA standard), or CGA Plus. The PCjr video subsystem incorporated the Motorola 6845 "CRT Controller" (CRTC), an integrated circuit for controlling video raster timing, which is also used in the IBM MDA and CGA video adapters. Thus the PCjr video subsystem consists mainly of the VGA and 6845 chips, along with relatively few discrete logic chips. This was the most integrated IBM PC video system to date. It was also the last to use the Motorola 6845, as IBM switched to their own custom CRTC for their next PC video standard, the Enhanced Graphics Adapter or EGA.

The PCjr was not popular, but the Tandy 1000 family of home computers are compatible with its video standard and Texas Instruments SN76489-based enhanced sound, and sold well. With built-in joystick ports, the 1000 was the best platform for IBM PC-compatible games before the VGA era, and the combination of its graphics and sound became known as "Tandy compatible"; 28 of 66 games that Computer Gaming World tested in 1989 supported Tandy graphics. The later Tandy 1000 SL and TL models were equipped with an enhanced version of the CGA Plus adapter, capable of displaying 16 colors at an improved resolution of 640×200.

Toshiba T1000

The Toshiba T1000 was a laptop computer manufactured by the Toshiba Corporation in 1987. It had a similar specification to the IBM PC Convertible, with a 4.77 MHz 80C88 processor, 512 kB of RAM, and a monochrome CGA-compatible LCD. Unlike the Convertible, it includes a standard serial port and parallel port, connectors for an external monitor, and a real-time clock.

Unusually for an IBM compatible PC, the T1000 contained a 256 kB ROM with a copy of MS-DOS 2.11. This acted as a small, read-only hard drive. Alternative operating systems could still be loaded from the floppy drive, or (if present) the ramdisk.

Along with the earlier T1100 and T1200 systems, the Toshiba T1000 was one of the early computers to feature a "laptop" form factor and battery-powered operation.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.