Talk:Color Graphics Adapter

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Contents 1 Palettes 2 "Contact Me" is Trixter 3 160x200 composite mode did exist and was possible w/ 16 KB VRAM 4 Severe copyediting 5 CGA trivia 6 Additional palettes 7 "hex values adapted from MC6845 specification" ?? 8 Original name of the card 9 Color 6 on IBM 5153 10 Fonts and the BIOS 11 Pinout? 12 Changes 13 Two details

[edit] Palettes

Hi does anyone know about the signals applied through the pin's to give you the colour and what would they be?

[edit] "Contact Me" is Trixter

Just a quick note that in one of the edits, I said "if you have a problem with the CGA color palette, contact me" but then realized later that I wasn't logged in. I am the one who put the final, correct CGA colors into the palette table -- if you want to change them you'd better have a good reason because I took them from the actual MC6845 specs and verified them with a TTL scan converter. Most people who think they know CGA colors seem to forget that when you toggle the brightness bit, ALL colors go bright, not just the ones that are supposed to go bright. Trixter 15:09, 20 Aug 2004 (UTC)

Got the reference in there? That would be the handiest thing - David Gerard 15:32, 20 Aug 2004 (UTC)

Huh? In graphics mode, yes (IIRC), but in text mode? I seem to recall that there were the full palette of 16 colours available in text mode. Also, all the sources suggest that. Plus the screenshots from the ICONDEMO.exe [1] are 16 colour. In composite mode there also are 16 colours. Surely you meant that only with reference to the 320x200 4 colour mode? Ropers 20:21, 20 Aug 2004 (UTC)

All I'm saying is that people tend to "make up" the 16-color CGA text mode palette with things that seem to make sense, like "FF0000" for "bright red" when in fact that simply wasn't the case. When you enable the "I" bit, ALL components get intensified. Black becomes gray, etc. So "bright red" is really "FF5454". The actual number of "FE" instead of "FF" is me being picky based on the specs, but those are the actual numbers. To further clarify, the graphics mode palette colors are identical to the text mode colors that they are based off of. For example, the "red" in the "red/green/yellow" palette corresponds to the CGA text color "red", etc. It is not a "different" red. Now, composite color mode used completely different colors than CGA text/graphics mode, and one of these days I will hook up my old PC to my broadcast equipment and capture the definitive CGA 16-color composite output color chart. But today is not one of those days :-) Trixter 04:42, 21 Aug 2004 (UTC)

If the hex values presently used in the table are precisely correct ... would it be worth listing them visibly in the table itself? - David Gerard 21:58, 24 Aug 2004 (UTC)

Never mind, I've just done so - noting they're from the MC6845 spec :-) What's the precise name of the spec? It should go in the 'References' section - David Gerard 22:35, 24 Aug 2004 (UTC)

[edit] 160x200 composite mode did exist and was possible w/ 16 KB VRAM

Goplat, I think you're confusing the 160x100 "tweaked text" pseude-graphics mode with the 160x200 composite mode. They were two totally different things:

The 160x100 tweaked text mode was achieved on standard RGB monitors. The 160x200 mode was ONLY available on the CGA card's composite video output (which you could hook up to your telly.) It had nothing to do with text mode. It was a seperate graphics mode in its own right -- only that few folks used it as most CGA boxen were hooked up to RGB monitors permanently.

As regards your video RAM size objection:

• 160x200 equals 32,000 (that's how many pixels we have.
• 16 colours require 4 bits (2x2x2x2=16).
• 4 times 32,000 equals 128,000. That's how many bits we need to encode 32,000 pixels at 16 colours.
• 128,000 bits equal 16,000 bytes (as 1 byte=8 bits, so 128,000 gets divided by 8).
• 1 KB equals 1,024 bytes.
• Thus, 16 KB equal 16,384 bytes.
• 16,000 bytes should thus fit into 16 KB with room to spare.

Fair enough?
Ropers 22:16, 20 Aug 2004 (UTC)

You were right, I screwed it up. Sorry about that. Goplat 22:31, 20 Aug 2004 (UTC)

And I (initially) screwed up as well, forgetting about the fact that 1KB = 1024b (that's now corrected, and it doesn't change my point). Thanks for the quick reply :) -- I even got an edit conflict while I was correcting my own screwup ;-) Ropers 22:38, 20 Aug 2004 (UTC)

No corrections here, you've got it right. But what's interesting to note, and what may be adding to the confusion, is that 320x200x4, 640x200x2, 160x200x16, and 160x100x16(tweaked text) ALL took up EXACTLY 16000 bytes... yet there is 16K available on the adapter, which means you actually have 16384 bytes to work with. What to do with the extra 384 bytes? Not too much, but I once came across a BASIC program that used them for temporary storage because the BASIC program itself was so big it filled all of the RAM available in a 64K PCjr ;-) Trixter 04:47, 21 Aug 2004 (UTC)

I don't know what mode it was, but I know for a fact that Bards Tale ]I[ became 16 color when I discovered that I could plug my TV into my computer.

I had been playing it with just the 4 colors that my Compaq 8088 (or was it 8086?) could produce.

When I discovered the 16 color trick, I played around in BASICA, and discovered that 2 pixels side-by-side were being interpreted as just one color on TV, with an aggregate of the color taken.

So I don't know if it was a different mode or not, or if it was the same mode just being interpreted differently by the two displays, or what. =^_^=

It was quite a shock to see that Bards Tale III was suddenly pretty..!

LionKimbro

[edit] Severe copyediting

I just went through the article again, trying to bring the writing up to FAC standard. I cut a lot of side issues out to try to keep the actual points clear. Also worked over 160x200 16-color severely - it was confusing before, but I think it gets the point across now (and I think I got the point now) - David Gerard 21:47, 24 Aug 2004 (UTC)

I'd like to touch up the first two paragraphs of the article; the wording is somewhat awkward and needs cleanup. Any objections? I'll wait a few weeks for replies before doing it. Trixter 05:12, 31 January 2006 (UTC)

Before I got to cleaning up the first two paragraphs of the article, someone added a very subjective, non-verified, dubious history to the acceptance of CGA. One of the things they claim is that EGA in 1984 meant that CGA finally took off -- not only does the existance of DIP switchins on the EGA card (that let it be used with the CGA monitor) refute this, but MobyGames shows 130+ games that support CGA with publish dates BEFORE 1984, so I have a hard time believing the claim. They also claim that Hercules cards, with their graphics capabilities, drove CGA further into disuse, but MobyGames shows *NO* games that use Hercules graphics under 1985. (Not to mention all of it contradicts my own personal memory of the time period, but I try not to alter articles (any more :-) without proof for discussion.)

So, I officially call "BS" and would like to significantly alter the paragraph to remove the subjective parts (and *still* clean up the original 2). If nobody objects in a few days, that's what I'll do. --Trixter 20:58, 7 February 2006 (UTC)

[edit] CGA trivia

It might be worth mentioning a couple of extra points:

• Original IBM PC motherboards (8088 processors) had a small trimmer capacitor on the 14.318318 quartz oscillator labelled "Color Adjust" - since the IBM CGA card derived its 3.58 MHZ NTSC color subcarrier frequency from the motherboard clock, which was distributed on the original XT expansion bus. All that to save a 39 cent crystal on the CGA card!
• The CGA card had both an RGB connector and a composite video output. The composite video could be connected to a household (NTSC,525-line 60 Hz) TV set, just like a home computer. I did see this used in an industrial context when a PC with a CGA card was hooked up as a text display on a plant-wide closed circuit TV system - an ingenious way of recycling old hardware.
• A common rookie PC user mistake of the early 1980's was trying to plug in a color monitor to a monochrome display adapter, or the reverse - they used the same DE-9 female connector. Legend had it that some monitors were destroyed by this, though I never managed to do it myself.
• A CGA card and IBM monitor produced really terrible-looking text, even using an RGB monitor. Since the 320*200 color mode was worthless for serious graphics, nearly all "business" PCs used either MDA or a Hercules card, both of which produced text that looked even nicer than that on a Kaypro 10. --Wtshymanski 21:11, 23 May 2005 (UTC)

Trimmer: agreed, probably something to mention (but here or on the 5150 page?). Composite: already mentioned in the article. Hookup up monitors: not entirely sure where that would be approprite (btw, not possible to break either monitor on either card this way; the frequencies were different, but not exceedingly high which is what breaks monitors). "Terrible" text: That's quite subjective; I rather like the way text looks on my 5153 :-). I've spend decades looking at 80-col on it and never thought it was terrible. --Trixter 21:05, 7 February 2006 (UTC)

[edit] Additional palettes

In my experience of my first PC I know of additional palettes for the CGA. Now I must disclaim this saying that I had an Amstrad PC1512, which may have been custom-fit to produce unusual features. That said, I saw the following:

• Graphics mode (640×400) with the CGA 16-colour RGBI palette. I know it had that one because GEMPaint wouldn't work without it (used for drawing colour graphics).

• 4-colour palette with #0000A8 (blue) instead of #000000 (black) in addition to magenta, cyan and light grey. Some games (like Alley Cat) would flash between this palette and the normal one for alerting the user.

• 4-colour palette with #A8A800 (yellow) instead of #A85400 (brown) in addition to black, red and green. I saw this at least on one game, Double Dragon, where I remember the game would switch between the normal palette (black, magenta, cyan, light grey) and what I called "Chinese colours".

I repeat, I don't know how much of this is peculiar to the Amstrad PC1512, or could be achieved on any CGA card through hacking. --Shlomital 10:24, 2005 Jun 2 (UTC)

Of these, the 640x200x16 mode is unique to the PC1512, and isn't available on normal CGA. The other two are entirely standard; you get the first one by selecting magenta/cyan/white and then setting the background to blue, and the second by selecting red/green/brown and turning on high intensity --HungryHorace 16:35, 4 Jun 2005 (UTC)

Great! Thanks for educating me on that. I was wondering how much hacking went into the ol' PC1512. --Shlomital 13:15, 2005 Jun 11 (UTC)

I've added a table for the tweaked 3rd palette in the "tweaks" section, but my HTML skills aren't what they used to be and I can't figure out why it is too wide. Can someone fix the table for me? Thanks in advance! Trixter 17:20, 4 January 2006 (UTC)

You added a red-cyan-white palette. What about the red-green-yellow palette mentioned by Shlomital? Another game I know to use this is Legacy of the Ancients. -- 145.254.132.115 14:57, 1 February 2006 (UTC)

That's the same palette as the red-green-brown, just with the high-intensity bit turned on. It's not a fundamentally different third palette. Trixter 21:11, 2 February 2006 (UTC)

[edit] "hex values adapted from MC6845 specification" ??

This claim seems to be nonsense. The MC6845 produced only the address of the current pixel, it was up to other chips to read that address and actually display the color(s) stored there. If anybody can explain how those hex values were derived, please do so. -- 84.58.33.21 14:23, 29 July 2005 (UTC)

I agree. The MC6845's data sheet makes no mention of any color values at all --- the color values mentioned are just what you get with the RGBI color model (with the exception of color 6). Let's remove that table if nobody objects. NewRisingSun 21:12, 16 December 2005 (UTC)

Please don't remove the table; the hex values were created by me based on research from how the TTL actually operates. If you'd like to "fix" the rounding then please do so but do not change the basic color mix which is correct. (For example, most people guess at the "brown" and "intensity" colors and always get them wrong.) Trixter 1:12, 23 December 2005 (UTC)

You claim that these values are "adapted from the MC6845 specification", even though the MC6845 specification contains no such information. Now you say it's based on your research on the "TTL", even though the CGA doesn't use a TTL monitor (that one is used by the MDA/HGC card!)... I guess I'll just call the table "Full CGA 16-color palette" and correct the values. NewRisingSun 20:46, 21 December 2005 (UTC)

Actually, both the CGA and MDA use TTL monitors; TTL in this context just specifies which voltage ranges on the pins mean "on" or "off". From the IBM PC tech ref, page 1-158, describing the IBM Color Display: "Horizontal drive: Positive-level, TTL-compatibility, at a frequency of 15.75 kHz." Identical wording is used on page 1-132 to describe the IBM Monochrome Display except that the frequency is 18.432 KHz. HungryHorace 22:45, 21 December 2005 (UTC)

That would make the EGA's Enhanced Color Display (IBM 5154) a TTL monitor as well.... but if you say so. In any case, I'd still like to get a copy of that ominous "MC6845 specification" that supposedly lists the color values.... the 22-page data sheet from Motorola (plus 2 pages revised addendum) certainly has no such information. NewRisingSun 23:42, 21 December 2005 (UTC)

If you want to remove "MC6845 Specification" then go ahead and do so. I wrote that originally as a deterrent to people guessing at the colors, when you can clearly see that the interface is RGBI not just RGB. But please don't alter the values beyond rounding them upward (ie. changing FE into FF) ... like I wrote earlier, most people think that "light red" is "#FF0000" which is blatantly incorrect because they don't take the "I" component into account. The table has the correct color mix. And yes, the 5154 is also a TTL monitor. Trixter 04:56, 22 December 2005 (UTC)

Well, well, this makes it the fourth version of the CGA colour palette values I've seen. Earliest, the numbers were (hex) 00, 54, B0, FE. Then, it was 00, 54, A8, FC. Later, on a previous version of this article, they were 00, 54, A8, FE. And now it's 00, 55, AA, FF, which I first saw as the "Linux console" colour palette for GNOME Terminal. I still don't know which one is correct (I root for 00, 54, A8, FC, if only because screenshots from the DOS window on Windows 9x, as well as from DOSBox, give out those colours), and I don't know even how to find the answer out, but please, if anyone does, bring it here, with evidence that it's the correct one, because all that juggling doesn't make for a reliable source. --Shlomital 17:14, 27 February 2006 (UTC)

The RGBI color model translates the digital TTL bits to analogue voltage factors of 0, 1/3, 2/3, 3/3. On a digital scale from 0 to 255, that translates to 0, 55, AA, FF. Since the VGA's color registers accept only 6 bits, the scale is from 0 to only 63, which would be 0, 15, 2A, 3F. If you simply multiply these by four (or shift by two bits), which is what most programs do, you'll get 0, 54, A8, FC. In other words, the differences are merely rounding errors. NewRisingSun 20:10, 27 February 2006 (UTC)

The original palette (the one mentioned in "adapted from MC6845 specification") was created by me, using the bit shift method, which is why the values were 54, A8, FC (truncation). I have since re-evaluated that position and now stand by the 55, AA, FF (rounding) numbers because they more closely resemble what they are modeling (ie. full TTL white output = full RGB white output). --Trixter 21:55, 27 February 2006 (UTC)

First, thanks for the convincing explanation. So 54 and A8 and FC are the result of algorithmic upsampling from a lower bit-depth, while 55 and AA and FF are the result of intelligent upsampling according to the specification (the 0/3, 1/3, 2/3, 3/3 factors mentioned above). Second, I've now done one even better and prepared a 48-bit PNG image of the palette, with the values 0000, 5555, AAAA, FFFF. It's on my user page. --Shlomital 18:03, 28 February 2006 (UTC)

[edit] Original name of the card

The original CGA from IBM was actually called "Color/Graphics Monitor Adapter" (C/GMA) (see this auction for pictures: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=8712503810), and the BASICA graphics demos on the DOS 1.x system disks call it that as well. I don't know when exactly the shortened name "CGA" came up (maybe when clones were produced?), but this original name should probably be incorporated in the main article. NewRisingSun 21:12, 16 December 2005 (UTC)

[edit] Color 6 on IBM 5153

It was recently discovered [2] that IBM's original CGA monitor (IBM 5153 Color Display) displays color 6 differently than most compatibles (closer to what you'd expect if you strictly applied the RGBI color model). This should definitely be reflected in the article (as quite a number of games depend on it!); feel free to change my markup to something prettier... :) NewRisingSun 19:50, 17 January 2006 (UTC)

No offense, but this is bullshit. I own a 5153, in fact it is sitting right next to me turned on, and it clearly shows brown. Unless you can come up with something to back up what you wrote, I'm going to revert it. An improperly calibrated montior can be fiddled with to show yellow if you want it to, but that's not original. I'll give you a week to respond. Trixter 04:26, 18 January 2006 (UTC)

(In fact, I've posted to that very forum challenging the photographer's competence. It's not yellow, it's brown, and the poster in that forum is trying to enact revisionist history.) Trixter 04:41, 18 January 2006 (UTC)

Offense taken, due to your pompous attitude. I'll give *YOU* a week to provide a rebuttal to the photographic evidence I linked to, a rebuttal that is more convincing than "bullshit", "the photographer is incompetent", "poster is engaging in a conspiracy to revise history", "trust me", "it's brown because I say so" and "I got the real colors from the 6845 spec". NewRisingSun 14:40, 18 January 2006 (UTC)

Offense not meant, but I am continually exasperated by people who don't actually own CGA hardware and yet keep trying to tell others what the colors are. My first home machine was a model 5150 with CGA in 1983, and I still have it on the table next to me in 2006 and it still works fine.

Okay, I found proof and posted the link in the forum previously referenced [3]. May I please revert the cga palette table changes? If it will help appease you, I fully admit I BS'd on the MC6845 comments -- but check the link to the info, it's the real deal and proves the color is brown. And yes, I have learned the lesson that shortcuts, however well-intentioned, are inappropriate for wikipedia entries. Trixter 05:50, 19 January 2006 (UTC)

We can both agree on that on *most* RGBI monitors, it appears as brown; I have included that in the article. I think we can also agree on that it is relevant information that strictly following the RGBI color model, you'd get dark yellow, and that it requires additional circuity to turn dark yellow into brown. I have updated the article accordingly. I further maintain that on *some* (possibly pre-1983) monitors, that additional circuity is missing, so they display dark yellow, and Great Hierophant's photographic evidence confirms this. If you think this is so irrelevant that the dark yellow should not be in the palette table, then put it somewhere else, but put it somewhere, because people should see what the dark yellow looks like.

Also, I definitely need to rewrite the composite section to present more accurate information, using proper NTSC terminology, especially about the artifacting in the 320x200x4 mode. NewRisingSun 16:26, 19 January 2006 (UTC)

I can concede that a proper "dark yellow" should be visible for the curious; I will move it out of the table, but add it to the Bugs and Errata section so it is not lost. Trixter 01:11, 20 January 2006 (UTC)

[edit] Fonts and the BIOS

I think it needs to be made clear that the font used in graphics modes comes from a completely different source than the font used in text modes - hence my use of "separate". The text-mode fonts are in the character ROM on the card itself; this ROM doesn't appear in the PC's address space, so the BIOS contains its own 128-character font that it uses in graphics modes. On an original IBM PC with an original IBM CGA, the two fonts are the same so the effect isn't noticeable; but putting an IBM CGA card in an Amstrad PC2086, which has a completely different BIOS font, soon shows the difference. This is also the reason why changing the font jumper on the card doesn't change the font used in graphics modes. HungryHorace 10:33, 3 February 2006 (UTC)

I always got the first 128 characters from F000:FA6E (BIOS of course), and the second 128 from where interrupt 1F pointed to (which on my machine eventually leads to C000:2D06). The second location is clearly the CGA, so I'm confused... Trixter 00:22, 6 February 2006 (UTC)

C000:2D06 is not the CGA, but your EGA/VGA. The CGA doesn't come with its own BIOS at C000 ;)NewRisingSun 00:28, 6 February 2006 (UTC)

You are correct, I just (5 minutes ago, damn you're quick) ran it on my EGA/VGA. Let me run the same program on my 5150... --Trixter 00:31, 6 February 2006 (UTC)

Well, that was educational. F000:FA6E is indeed the first 128 chars, and Int 1F points to F000:0000 which (on the 5150/CGA) contains garbage. So I stand corrected, and I'll alter the text in the article to reflect HungryHorace's concerns.--Trixter 02:33, 6 February 2006 (UTC)

[edit] Pinout?

It would be great if some knowledgable person could add the pinout of the CGA video connector. A drawing of the physical connector can be taken from the EGA article. -- 84.130.16.237 16:38, 3 July 2006 (UTC)

[edit] Changes

I made some larger changes to the article, and in order to avoid any needless reverting, I temporarily put them under User:NewRisingSun/cga.

• Rewrote and expanded the "composite" section. I also plan to write a dedicated "Artifact color" article, discussing the matter from a more general technical point of view; after all, the technique is basically the same from CGA to Apple II to Atari to Coco.
• Moved the second and third paragraphs to the end of the article, since the CGA's market penetration goes better along with the mentioning of its competitors.

Please review the changes and discuss. The picture markup is probably suboptimal as well. I'm also a little bemused by the constant past tense of the article; while it is appropriate to describe when it "was" sold, the inner workings of the CGA are still the same. ;) --NewRisingSun

Considering I'm still developing for it as a hobby, I agree :-)

I have no suggestions whatsoever -- as always, you did a fine, thorough job and I am especially looking forward to the composite artifacting article. One of these days I'll re-convert 8088 Corruption to take advantage of composite artifacting. --Trixter 21:49, 2 September 2006 (UTC)

Ok. I have copied over the changes into the article. With regards to my claim "making for a total gamut of well over a hundred colors", I need to calculate how many these are exactly. It should be quite a few though, mostly because of the sixteen different possible settings of color 0. NewRisingSun 23:41, 2 September 2006 (UTC)

When writing how many colors are available, see if you can produce two figures: One for the total number of colors available using all palettes, and another number for how many are distinctly visible (and/or addressable?) on a single screen. I'm assuming the latter will be 16, but I'm hoping it's otherwise...--Trixter 21:41, 3 September 2006 (UTC)

[edit] Two details

I changed the rather vague "this could be changed" about the black and white in hi-res monochrome mode to this text:

By default the colors were black and white, but the foreground color (white) could be changed to any other color of the CGA palette. This could be done at runtime without refreshing the screen.

I know for certain that this is possible, there is a game [4] that allowed the player to change the foreground color with a keystroke. What I don't know is whether it was possible to change the background color too (as could be done in lo-res). I don't think so, since I've never seen it done (though the background color would often display incorrrectly on a VGA display).

Correct, on CGA only the foreground color can be changed. It's the same port write for all three modes, actually: Writing to color select register changes the foreground color in 640x200; changes the background (ie. color index 0) in 320x200; and changes the overscan/border color in text modes. --Trixter 18:05, 15 March 2007 (UTC)

Thanks for clearing that up. Maybe you could put this into the article? -- 213.47.127.75 16:19, 16 March 2007 (UTC)

It's already mentioned in the section "Further RGB graphics modes and tweaks". --Trixter 20:58, 16 March 2007 (UTC)

Another thing that bugs me is the following sentence in the paragraph about text mode:

This mode allowed each character a foreground and a background color, both of which could be freely chosen from the entire CGA palette (see table)—e.g. red on yellow text for one character, white on black for the next and cyan on gray for yet another.

Is this really true? As far as I remember, you can't set the background to a bright color in VGA text mode. If you do, the character will blink instead. Somehow it seems unlikely to me that this is a feature that was introduced later, and I'd expect the CGA to behave the same way. -- 213.47.127.75 20:17, 12 March 2007 (UTC)

You can turn of the blinking, both on CGA cards and on VGA cards, but you have to write to a different port. Calvero2 20:17, 13 March 2007 (UTC)

Was this a documented or an undocumented feature? -- 213.47.127.75 16:19, 16 March 2007 (UTC)

Fully documented, as far back as the CGA tech ref :-) --Trixter 20:58, 16 March 2007 (UTC)