The Color Graphics Adapter , also known as the Color/Graphics Adapter or the IBM Color/Graphics Monitor Adapter , introduced in 1981, was the first graphics card and IBM's first color display card for IBM PCs. For this reason, it also became the first computer color computer display standard.
The standard IBM CGA graphics card comes with 16 kilobytes of video memory and can be connected to direct CRT monitors directly using the RGBI 4-bit digital (TTL) interface, such as the IBM 5153 color display, or to a compatible NTSC Composite TV or video monitor via RCA connectors. The RCA connector only provides baseband video, so to connect the CGA card to a standard television requires a separate RF modulator unless the TV has an RCA jack though with the former combined with the amplifier it is sometimes more practical since it can then connect the antenna to the amplifier and get wireless video.
Built around the Motorola MC6845 display controller, CGA cards feature several graphics and text modes. The highest screen resolution of any mode is 640ÃÆ'â € "200, and the highest color depth supported is 4-bit (16 colors).
Video Color Graphics Adapter
Kemampuan output
CGA supports:
- 320ÃÆ' â € "200 in 4 colors from a 16-color hardware palette. Pixel aspect ratio 1: 1.2.
- 640ÃÆ' â € "200 in 2 colors. Pixel aspect ratio 1: 2,4
(The aspect of the pixel ratio comes from the presentation of the word pixel distribution on the screen in proportion to 4: 3, typical of the monitor at the time.)
Text mode:
- 40ÃÆ'â € "25 with font 8ÃÆ'â €" 8 pixels (effective resolution 320ÃÆ'â € "200)
- 80ÃÆ' â € "25 with font 8ÃÆ'â €" 8 pixels (effective resolution 640ÃÆ'â € "200)
Expanded graphics mode:
- 160ÃÆ' â € "100 16 color modes
- Artificial colors use the NTSC monitor (16 colors of more than 100 possibilities)
IBM bermaksud agar CGA kompatibel dengan satu set televisi rumah. Modus grafis 40ÃÆ'  25 dan 320ÃÆ'  200 dapat digunakan dengan televisi, dan mode grafis 80ÃÆ'  25 dan 640ÃÆ'  200 ditujukan untuk monitor.
Maps Color Graphics Adapter
Palet warna
Although bit depth varies between CGA graphics modes (see below), CGA processes color in its palette in four bits, producing 2 4 = 16 different colors. Four color bits are arranged according to the RGBI color model: the lower three bits represent the red, green, and blue components; bit "intensifier" bits, when set, increases the brightness of the three component colors (red, green, and blue). In graphical mode, color is set per pixel; in text mode, the color is set per character, with an independent foreground and background color for each character.
With RGBI monitor
These four bits are passed on unmodified to the DE-9 connector on 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 will use approximately the following formula to process four-bit digital color numbers to analog voltages ranging from 0.0 to 1.0:
red : = 2/3ÃÆ' - ( colorNumber & amp; 4)/4 1/3ÃÆ' - ( colorNumber & amp; 8 )/8 green : = 2/3ÃÆ' - ( colorNumber & amp; 2)/2 1/3ÃÆ' - ( colorNumber & amp; 8)/8 blue : = 2/3ÃÆ' - ( colorNumber & amp; 1)/1 1/3ÃÆ' - ( colorNumber & amp; 8)/8
Color 6 is treated differently; when using the above formula, color 6 will be dark yellow , as seen on the left but to achieve more pleasing chocolate chocolate, a special circuit in most RGBI monitors , including the IBM 5153 color screen, make exceptions for color 6 and change the tone from dark yellow to brown by halving the analog signal's green amplitude: if colorNumber = 6 then green : = green /2
This is the "RGBI with chocolate tweaks" palette, which is displayed in a full palette on the right, that all PC graphics standards later such as EGA and VGA have been retained for compatibility as default power-on settings from internal palette registers and/or DAC registers.
With a set of composite color monitor/television
For combined output, these four-bit color numbers are encoded by CGA onboard hardware into NTSC-compatible signals fed to the RCA card's output jack. For cost reasons, this is not done using the RGB-to-YIQ converter as referred to by the NTSC standard, but by a series of flip-flops and delay lines. Consequently, the colors are seen less in purity; especially, both cyan and yellow have a greenish color, and the color 6 again looks dark yellow instead of brown. The relative luminance of the colors produced by composite color-generating circuits differs between CGA revisions: they are identical for colors 1-6 and 9-14 with the earliest CGA produced up to 1983, and different for CGAs later due to the addition of additional resistors.
However, as noted, this method works only on the NTSC television set, PAL TV does not display the color as expected when connected to composite output, because superior color separation from PAL prevents artifacting.
Availability of RGBI monitors
When CGA was introduced in 1981, IBM did not offer RGBI monitors. Instead, customers should use RCA output with RF modulator (which they get separately, from third parties) to connect CGA to their television sets. The IBM 5153
Standard text mode
CGA offers four BIOS text modes (called alphanumeric modes in IBM documentation):
- 40ÃÆ'â € "25 characters up to 16 colors. Each character is a 8ÃÆ' â € "8 point pattern. The effective screen resolution in this mode is 320ÃÆ'â € "200 pixels (1: 1 pixel aspect ratio), although individual pixels can not be independently resolved. The choice of pattern for each location is thus limited to one of the 256 characters available, the pattern stored in the ROM chip on the card itself. Display fonts in text mode (hardware code page 437 character set) are therefore fixed and irreversible (though while using the original IBM CGA, it is possible to select one of two different fonts - normal or thin - by changing the jumper. offer this possibility). This card has enough video RAM for eight different text pages in this mode. BIOS 0 & amp; 1 select 40 column text modes. The difference between these two modes can only be seen on a composite monitor; mode 0 disables color bursts, making the colors appear on a gray scale. Mode 1 allows bursts of color, allowing for color. Mode 0 and Mode 1 are functionally identical on RGB monitors and later adapters that mimic CGA without supporting composite color output.
- 80ÃÆ'â € "25 characters in up to 16 colors. Each character again is a pattern of 8ÃÆ' â € "8 dots (the same series of characters used for 40ÃÆ'â €" 25), in pixel aspect ratio of 1: 2,4. The effective screen resolution of this mode is 640ÃÆ'â € "200 pixels. Again, pixels can not be solved individually. Because there are twice as many characters on the screen in this mode, this card has enough video RAM for just four pages of different text. BIOS 2 & amp; 3 select 80 text field modes. Like 40-column text mode, Mode 2 disables color bursts in composite signals and Mode 3 allows it.
In each text mode, each character has a background and a foreground color - ie. red on yellow text for one character, white on black for next, etc. While the same 4-bit nybble value used for the foreground color will normally allow all 16 colors to be used for the background color, the most significant bit of the nybble background is an alternative used to indicate whether or not the character should blink (hardware effect offered by CGA independent of CPU). When a character blinks, the foreground points alternate between the foreground and the background color, so during the flashing period, the character cell is filled with the background color (just like the space character). All characters blinking on the screen blink in sync. The flashing attribute effect is enabled by default and high-intensity background effects are disabled; Disabling blinking is the only way to freely select the last eight color index (8-15) for the background color.
Specifically, GW-BASIC and, later, Microsoft QBASIC (the Microsoft QuickBASIC programming language translator) supplied with MS-DOS (which is the de facto desktop OS while CGA is very popular) supported all text modes from CGA with full color control, but does not provide a normal way through BASIC language to switch CGA from blink mode to 16-color-mode, though it will be possible to directly program hardware registers using OUT statements from BASIC languages. In BASIC, the foreground color 16-31 color number is a flashing 0-15 color version, respectively, but the background color of 8-15 is identical to the color 0-7 each.
Standard graphics mode
CGA offers graphics modes in two resolutions:
- 320ÃÆ'â € "200 pixels , as with text mode 40ÃÆ'â €" 25. However, in graphics mode, each pixel can be separated separately. The tradeoff is that only four colors can be displayed at a time. Also, only one of four colors can be selected freely from 16 CGA colors. Aspects of 1: 1,2 pixel aspect need to be taken into account when drawing large geometric shapes on the screen.
BIOS 4 mode offers two pallets (different in the presence or absence of blue components): green/red/brown and cyan/magenta/white. By setting the high intensity bits, lighter versions of this mode are accessible. The background can be set to one of 16 CGA colors, but black by default.
As in Mode 0 and 2, Mode 5 disables the color burst bit to allow the color to appear on a gray scale on a Composite monitor. In contrast to text mode, disabling composite color composite bits affects the colors displayed on RGB monitors with IBM CGA cards and is properly compatible. Some programmers use Mode 5 as the third unofficial palette on the RGB monitor: cyan/red/white and background color. Intense versions of these colors can also be used and background colors can be changed, but the palette can not be transferred to palette 0 or 1 without enabling composite color signals again. Specifically, it's not mentioned in the IBM Technical Reference manual, and some CGA clones might not support it. pixels
- 640ÃÆ' â € "200 , as with the text mode 80ÃÆ'â €" 25. All pixels can be independently resolved. This mode is monochrome with pixel aspect ratio of 1: 2,4. By default the color is black and bright white, but the foreground color can be changed to another color of the CGA palette. This can be done at runtime without refreshing the screen. The background color can not be changed from black on the correct IBM CGA card. BIOS mode 6 sets graphics mode 640ÃÆ' â € "200. This mode disables composite color surge signals by default. The BIOS does not provide the option to change the color that explodes in 640ÃÆ' â € "200 mode, and the user must write directly to the mode control register to enable it.
In text mode, the bitmap font data comes from the character ROM on the card, which is only available for the card itself. In graphical mode, text output by the BIOS uses two separate tables. The first half of the character set (characters numbered 0 to 127, corresponding to a 7-bit ASCII with some additional graphic symbols) is provided by a table in the BIOS ROM chip on the computer mainboard at a fixed address F000: FA6E (the table still exists in this location even in the BIOS Modern PCs, unlike ROM fonts on the CGA card itself are used for text mode, these tables only provide "bold" instead of "thin" fonts.The second half of the set (characters number 128 to 255, corresponding to the international character , block-graphics and math) provided by the location indicated by the interrupt vector 1F (the vector itself is found in the memory address 0000: 007C; this is actually not a real interrupt vector, since the vector does not point to the executable machine code, as the interrupt vector real on Intel CPU 8086 PC.) Half the second part of the character set usually does not exist (1F vector does not point to the actual font data), and tries to show it will generate trash or empty characters. Character data can be placed into the memory manually by the user, or by utilities such as GRAFTABL.
Graphics mode and further tweaks
A number of official and unofficial features exist that can be utilized to achieve special effects.
- In graphical mode 320ÃÆ' â € "200, the background color (which also affects the border color), which becomes the default in black mode initialization, can be changed to one of 15 other CGA palette colors. This allows for some variations, as well as flashing effects, because the background color can be changed without having to redraw the screen (ie without changing the contents of the video RAM.)
- In text mode, the border color (displayed outside the regular viewing area - to the overscan area) can be changed from the usual black to one of the other 15 colors.
- Through timeliness, you can switch to another palette when the screen is scanned (drawn), allowing the use of one of six palettes per scanline. The best example of this is the California Games game when it runs on a 4.77Ã, MHz 8088 stock. (Running it on a faster computer does not produce any effect, because the method that the programmer uses to replace the palette in a predetermined location is very sensitive to engine speed.) The same can be done with the background color, to create streams and paths in Frogger. Another documented example of this technique is in Jungle Hunt port at Atarisoft to PC.
- The additional colors are often approached using dithering, although low resolution makes it very clear. In particular, many Sierra games use palette 0 at low intensity and dark blue as the background color. This gives him three main RGB colors to work with (as well as chocolate).
Some of the above tweaks can even be merged. Examples can be found in some games. Most software titles do not use this possibility, but there are some impressive exceptions.
160ÃÆ' â €" 100 16 color mode
Technically speaking, this mode is not a graphical mode, but it is tweaked from text mode 80ÃÆ' â € "25. The cell character register register is changed to display only two lines per character cell instead of the eight normal lines. This multiplies the number of lines of text displayed from 25 to 100. This "squeezed" text character is not a full character. The system shows only the top two pixel rows (eight each) before proceeding to the next line.
Character 221 of the CGA character set consists of squares that occupy the entire left half of the character matrix. (Character 222 consists of squares that occupy the right half.)
Since each character can be colored with different foregrounds and backgrounds, colors 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 221 or 222 characters, each half of each truncated character cell can be treated as individual pixels - making 160 horizontal pixels available per line. Thus, 160ÃÆ' â € "100 pixels in 16 colors, with an aspect ratio of 1: 1,2, is possible.
Despite the round way to achieve a 16-color graphic display, it works reasonably well and this mode is even mentioned (though not described) in IBM's official hardware documentation.
More details can be achieved in this mode using other characters, incorporating ASCII art with the above techniques.
Since the CGA has 16Ã, KiB (16,384 bytes) of graphics memory, instead of 16,000, it's just as easy to set the number of lines in this mode to 102 instead of 100 for a resolution of 160ÃÆ' â € "102 (16320 pixels). It uses extra video memory that is not normally used. However, most games do not do this, perhaps out of fear it will only work on multiple monitors but not others - the fears are not unfounded as it is later discovered that certain compatibles have a good card error or ignore any attempt to put the device into this mode.
The same high-text text-cell technique can also be used with text mode 40ÃÆ'â € "25. This only makes sense when using ASCII art, because without it the resulting resolution will only be 80ÃÆ'â €" 100.
Special effects on composite color monitors
Using NTSC TV-out instead of RGBI monitors is 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 "jerk" colors. This is especially a problem with 80 columns text:
For this reason each of the text and graphics modes described above exist twice: once as a normal "color" version and once as a "monochrome" version; the "monochrome" version of each mode will turn off the NTSC color decode in the full-screen monitor, resulting in black-and-white images, but also no color bleeding, hence, sharper images. On RGBI monitors, two versions of each mode are identical, with the exception of 320ÃÆ'â € "200 graphics modes, where the" monochrome "version produces a third palette, as described above.
However, the programmers soon discovered that this defect could be turned into an asset, because different patterns of high resolution dots would "blur" into consistent solid color areas, thus allowing the display of completely new artifact colors. Mode 320ÃÆ' â € "200 four-color and 640ÃÆ'â €" 200 color-on-black modes can be used with this technique.
Internal operation
Live color is 16 normal colors as described above under "CGA color palette".
The color of the artefact is seen because the composite chroma composite NTSC decoder misinterprets some of the lighting information as color, as stated earlier. By placing pixels carefully in an appropriate pattern, a skilled programmer will produce certain cross-colored artifacts that produce the desired color; either from pure black-and-white pixels in the 640ÃÆ'â € "200 mode, or produced from a combination of the colors directly and the artifact in 320ÃÆ'â €" 200 mode, as seen in these photos.
So, with a choice of 320ÃÆ' â € "200 vs 640ÃÆ' â €" 200 modes, a selection of palettes (1 or 2) and freely selectable colors 0 in the 320ÃÆ'â € "200 mode (see above), plus the ability to set colors the foreground in the 640ÃÆ'â € "200 mode freely, each of these parameters produces a set of different artifact colors, making the whole gamut more than 100 colors, 16 of which can be displayed at the same time.
Demonstration later by fans has increased the maximum amount of color CGA generated in one image to about a thousand. In addition to artifacting, this technique involves tweaking text modes that double the line, thereby offering the benefits of 16 foreground and 16 background colors. Certain ASCII characters like U and !! then used to produce the required pattern, which produces a non-hesitate image with an effective resolution of 80ÃÆ'â € "100 on a composite monitor.
Availability and warnings
The 320ÃÆ'-200 variant of this technique (see above) is how the standard BIOS-supported graphics mode is visible on composite color monitors. The 640ÃÆ'-200 variant, however, requires modifying a bit (color burst disable) directly in the CGA hardware register, as a result, commonly referred to as a separate "mode", often just as "composite" color mode, because more color artifacts typical makes it more commonly used than the 320ÃÆ' â € "200 variant.
Being completely dependent on the NTSC encoding/decoding process, composite color artifacts are not available on RGBI monitors, nor are they emulated by EGA, VGA or contemporary graphics adapters.
Modern, game-centric PC DOSBox emulators include CGA modes, which can mimic composite monitors (in graphics mode). In December 2012, the latest official version will mimic the more common 640ÃÆ'-200 composite mode and set 16 color artifacts; support for more complex 320 × 200 variants have been added to the DOSBox code base for the next official build.
Resolution and usage
Composite artifacting, either used intentionally or as an unwanted artefact, reduces effective horizontal resolution to a minimum of 160 pixels, more for black-and-white or black text, without altering vertical resolution. The composite video display generated with "artefact" colors is sometimes described as 160ÃÆ'â € "200/16-color" mode, although technically it is a method, not a mode.
The low resolution of this composite color artifact method causes it to be used almost exclusively in the game. Many of the higher profile profiles are optional, sometimes exclusive, offering graphics that are optimized for composite color monitors. Ultima II, the first game in the game series to be moved to an IBM PC, using CGA composite graphics. King's Quest I'm innovative in the use of 16-color graphics on PC, PCjr and Tandy 1000; even CGA owners can enjoy 16-color graphics using composite color monitors or televisions, thanks to programmers who exploit the incomes of NTSC composite chroma decompression. Selecting 'RGB mode' on the title screen will result in a typical CGA graphical mode limited to 4 colors. In this mode, dithering is used to simulate extra colors.
High color depth
By utilizing color staining, NTSC color clocks and methods similar to those used in 16 color mode, it is possible to display more than 16 colors in a composite monitor.
The NTSC color clock has 160 cycles per scanline, which means that in 40 column modes each pixel occupies half a cycle, while in 80 columns each pixel mode uses a quarter cycle. To limit the display of characters up or two of the upper scan lines, and take advantage of the pixel arrangement on certain characters of the 437 codepage, it is possible to display up to 1024 colors. This technique is used in demo 8088 MPH.
Limitations, bugs and errata
Video time on CGA is provided by the Motorola 6845 video controller. This integrated circuit was originally designed only for character-based alphanumeric (text) views and can only handle a maximum of 128 character lines. To manifest graphics mode with 200 scanlines on CGA, MC6845 is programmed with 100 row characters per image and two scanlines per character line. Since the output of the video memory address by MC6845 is identical for each scanline in character line, CGA must use MC6845 "line line" output (ie scanline in character line) as additional address bit to extract raster data from memory video. This means that unless the size of a single scanline raster data is power of two, the raster data can not be arranged continuously in the video memory. Instead, the first CGA graphics mode just puts the even-numbered scanlines continuously in the first block of video memory, then the second block of odd-numbered scanslines starting at 8.192 video memory positions. This setting generates additional overhead in graphical mode for software that manipulates video memory.
Although the MC6845 video controller can provide time for interlaced video, the CGA circuit aligns the sync signal in such a way that the scan 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 the 80-column text mode produces a random short horizontal line that appears on the screen (known as "snow") if a program writes directly to the video memory. The BIOS avoids problems with only accessing memory during a horizontal retrace, or by temporarily disabling output during scrolling; while causing the screen to flicker, IBM decided that doing it better than the snow. The "snow" issue does not occur on other video adapters, or in most CGA clones.
In 80-column text mode, the pixel clock is duplicated, and all sync signals are issued for twice the number of clock cycles to last for the proper duration. The composite output color decay signal circuit is an exception: since it still outputs the same number of cycles now at twice the clock rate, the resulting burst color signal is too short for most monitors, resulting in unstable or unstable colors. Therefore, IBM documentation lists 80-column text modes as "features" only for RGBI and black-and-white composite monitors. Stable colors can still be achieved by setting the border color to brown, which results in an identical phase with the correct color bursts signal and serves as a substitute for it.
Software support
CGA was widely supported in PC software until the 1990s. Some of the software that support the board are:
- Windows 3.0 (and earlier versions)
- OS/2 1.1 (and earlier versions)
- Graphics Environment Manager (GEM)
Compete adapter
BYTE in January 1982 described the output of the CGA as "very good - slightly better than the color graphics on an existing microcomputer". PC Magazine did not agree, reported in June 1983 that "the monochrome display of IBM is really beautiful for text and is very easy on the eyes, but limited to simple character graphics.The quality of text on the screen is connected to color/Adapter graphic... best medium quality and conducive to tired eyes over the long term ". In retrospective comments, the Next Generation also took a negative view on the CGA, stating, "Even for the time (early 1980s), this graph is pretty bad, at least compared to other color machines available in the market. "
CGA has two main competitors:
- For business use and word processing, IBM announced Monochrome Display Adapter (MDA) at the same time as CGA. MDA is much more popular than CGA at first. The author of IBM's internal publications declared in October 1981 that he had planned to purchase a CGA adapter but changed his mind after seeing it ("rough") and MDA ("beautiful") displaying the text, observing that "you stare at far more text than you look at on color graphics ". MDA produces high-resolution text display in 80ÃÆ'â € "25 mode, displaying each character in a 9ÃÆ'â €" 14 pixel box, where 7ÃÆ'â € "11 is the character itself. This produces sharper and more clearly defined characters than the 8-point 8-point CGA character permissible text matrix. Therefore, MDA is the preferred choice for business use. Also, IBM initially produced MDA cards as MDA printer port/combo cards. This means that users who want to connect the printer to their original IBM PC must pay for MDA cards (initially $ 335), while CGA cards (initially $ 300) can be left to save money. While including a CGA card and connecting an existing TV set to use as a monitor allows the user to cancel the purchase of the monitor, this is not significantly cheaper than buying a monochrome monitor (initially $ 345) and leaving CGA cards. Additionally, 80-column text is virtually unusable on color composite screens, and the IBM 5153 color CGA color display model is required to fully utilize the capabilities of even more expensive CGA cards. Because so many PCs are sold to businesses, high-resolution, sharp monochrome text is more desirable to run applications.
- In 1982, non-IBM Hercules Graphic Card (HGC) was introduced, the first third-party video card for PC. In addition to MDA-compatible text mode, it offers monochrome graphics mode. With a resolution of 720ÃÆ'â € "348 pixels, it has a higher resolution than is generated by CGA. The combination of sharp monochrome text and Hercules graphics capabilities make it ideal for running software like Lotus 1-2-3 that supports business charts. Some games also have Hercules support, and most others can be made to work with HGC via SimCGA, a TSR that will reformat CGA graphics memory into HGC format in the background.
Another alternative: Compatible IBM PCjr (1984) and Tandy 1000 (1985) feature an extended CGA video hardware that extends the video RAM beyond 16 kB, allowing 16 colors at 320ÃÆ'â € "200 and four colors on 640ÃÆ'â € "200 resolution (then Tandys also has a 640ÃÆ'â €" 200 mode with 16Ã, colors). Because the old Tandy 1000 lives longer than PCjr, the video mode is known as "Tandy Graphics Adapter" or "TGA" , and was popular for games during the 1980s. Similar but less widely used is Plantronics Colorplus.
CGA cards succeed in the consumer space by the IBM Enhanced Graphics Adapter (EGA) card, which supports most CGA modes and adds additional resolution (640ÃÆ'â € "350) and a software palette with 16 colors from 64 in both text and text mode graphic. Along with this step, the price of the older CGA card is lowered away; it became an attractive low-cost option and was soon adopted by a new PC cloning company as well. Non-AT entry-level PCs with CGA graphics are being sold very well over the next few years, and consequently there are many games released for the system, despite CGA limitations. The popularity of the CGA began to diminish after VGA became IBM's high-level standard and entry-level EGA standard in 1987. However, most of the software created up to 1990 supported it.
Specifications
Connector
The Color Graphics Adapter uses the standard DE-9 connector for direct-drive video (to RGBI monitors). The connector on the card is female and the one on the monitor cable is male.
Peringatan konektor 12V di dalam
Pin 7 can be used by the card to provide 12V to monitor. It should not connect to most monitors.
Signal
See also
- RGB color model
- Graphics card
- Graphical display resolution
- Graph processing unit
- List of display interfaces
- List of 8-bit computer hardware palettes - CGA section
- Page code 437
- List of graphics chip and company card is off
References
- Notes
External links
- Graphic Adapter Color Note
- Games with CGA Graphics
- Representative screenshots of CGA games
- User Friendly Threads about CGA use
Source of the article : Wikipedia
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