reenigne wrote:I have actually played around with this, and there are two problems. One is that the CGA card only has 16kB of VRAM, which is only enough to cover half of a 640x200x4 screen. This by itself would be a huge problem - you could have a reduced viewport rectangle or use CRTC trickery to make a non-rectangular viewport as long as it only covers about half of the screen area.
The more serious problem is that the snow suppression circuitry on the CGA card is always active in graphics modes, which means that the data bytes are latched on the wrong cycles to display correctly. That means that if you write bytes ABCDEFGH to VRAM it won't display ABCDEFGH but ABCBEFGF (every fourth byte will be a repeat of the byte two bytes before it). So you don't get a true all-pixels-addressable display. You can see the effect of this in CGAArt (which emulates this behavior of an IBM CGA card correctly) by choosing "high-res 2bpp graphics" in Video card->Registers->Mode. I hope one to do something with this "improper mode" in a demo.
Ah, I see. Thanks a lot for explaining this, too! 😀 I always wondered if that "mode" could be ever used something for.
Personally, I'm looking forward a demo, as well. Maybe it could be used for some special effects (moving stars, abstract objects, etc).
Anyway, please don't feel rushed. I'm glad CGAArt's emulation is somewhat complete already, so we can experiment a little bit
with these kind of things without the need for having original CGA hardware that normally would be required for that.
(I and I suppose many others from PAL lands had/have less of a chance to get hands on real CGA cards, so this is really fascinating.
For example, back in time. clone cards and Mono/Color combo cards were seemingly more common in Germany than IBM CGA cards, I believe.
Not sure about the UK, though or why it was that way at all (if it was, my memories are a bit sketchy).
Maybe it was like this because printer ports and hi-res graphics (Hercules) were considered to be more relevant for business.
Also, the home computer market and console market of the 80s in Europe was healthier than in NA, or so I heard.
No Atari VCS desaster here. At least not in that dimension. The VCS was sold well intol the early 90s, I heard.
In contrast, PC games in north america were much more popular, so CGA graphics got tuned very well. Esp. Composite CGA.
The best we got in terms of CGA was CGA in 4 colours via RGB. On the bright side, this allowed many SCART-equipped TVs to be used as CGA monitors.
The RCA outputs of the clone cards and internal CGAs did read "monochrome" more than often. I still wonder why the NTSC facility got artificially
removed or disabled. And how it was done. Was only the colour-burst signal surpressed or was the removal more complex ?)
Edit: I apologize for the long post. Just realized how long it really is. 😅
I hope you don't mind. I didn't mean to overrun your thread.
I'm too chatty sometimes (a bad habit of mine, my sister says).
"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel
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