dilemma of EGA & VGA combo

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Re: dilemma of EGA & VGA combo

Postby Jo22 » 2018-11-10 @ 06:29

Thanks for your reply. No offense, I don't dislike CGA. I wrote a few utilties in my freetime for it, after all. :)
What makes me sad is that there was apparently little need or incentive to support 640x350 mode.

What I meant to say: It makes me (personally) sad that something advanced as EGA got misused as "a CGA card on sugar" so often.
- Way back in the 1980s it could have us brought amazing graphics, comparable to that of the PC-98,
if users or developers only had fully working monitors for it in order to get into full EGA experience (chicken-egg problem).

Speaking of games, I remember CD-Man also was among one of the 80s games with normal EGA support.
And it ran fine on a slow 286, by the way. In Japan, they used XT class machines (PC-98; V20 or 8086 CPUs) even, to display 640x400 modes.
Performance wise, I had barely issues playing 640x480 VGA games on my 286es, either (one was the SLT/286 laptop).
Shareware games like Asteroid 2.0 or Digger from the early 90s ran just fine on a 286-12.

Since I mentioned EGATrek before, here's what the author of EGATrek wrote in '89:
"This game requires a PCompatible with an EGA card/monitor.
There seems to be a real shortage of games that take advantage of the full EGA capabilities (640x350, 16 colors).
This is my second EGA game; the first one, known as "Mah Jongg" has been out since mid-1987 and has received a gratifying response.
(If you don't have "Mah Jongg" and would like a copy, I'll be happy to mail one in exchange for the $15 registration fee.)
If you're still running an older display adapter that doesn't support EGA I'm afraid you're out of luck running this particular game.
I'm sure you can find a version that supports your system though.
"

Years later, Win 3.1 was about the only platform I had got access to which featured games in standard resolution. :(

Edit: This is by no means meant as critism; I understand that you people like your CGA monitors.
So do I, when it comes to mono TTL monitors or B/W TV sets. :)

Edit: Posting re-written, since I was seeing what it else was leading to.
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Re: dilemma of EGA & VGA combo

Postby digger » 2018-11-10 @ 19:35

Don't worry! I know you didn't dislike CGA, and that's not what I tried to say. By the way, I myself *do* despise CGA, not because of the 320x200 resolution, but because of the ugly and limited 4-color palettes that made Tandy and EGA graphics look so wonderfully lush and colorful in comparison.

Anyway, I agree that 640x350 high-res EGA modes allowed for some impressive graphics back in the day. I remember visiting a fellow primary school student's house, seeing that Mah Jongg EGA 640x350 game on his Dad's Tulip Compact 2 XT machine and being blown away by the crisp and lush graphics. Those Mah Jongg pieces almost seemed to leap out of the screen. Also notable was the fact that there were no visible scanlines in that mode. The same monitor would show scanlines in 320x200 modes. I guess that was a standard trait of EGA monitors. Ot was it? It was the only actual EGA monitor I remember having seen up close.

Nevertheless, I still believe that 640x350 would not have been commonly used in games even if EGA cards had not been downwards compatible with CGA monitors. It was simply not a common resolution outside of EGA. Older systems didn't support it, nor did the popular home computers of the time, and it didn't have a standard or easy-to-use aspect ratio. Also, I still believe that performance was a factor. Certain kinds of games may have worked with it fine, but for fast-paced games with a lot of things moving around the screen, a beefy system would be required. Such systems often already had VGA anyway.
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Re: dilemma of EGA & VGA combo

Postby Jo22 » 2018-11-11 @ 16:28

Thanks for your understanding! It's been nice talking to you.

EGA and CGA are both really interesting, I think, so that's why I'm a bit sad about the 320x200 focus (160x100/160x200 are neat, though).
CGA not only had 640x200 mono, but also a 640x200 4c mode that never made it due to incomplete decoding.
On top of that, there was Plantronics and a nice 640x400 mode that was featured by several clones (ex. AT&T, M24, 82c426)
and emulator boards (AT Speed/PC Speed on Atari ?).

That being said, I've got nothing against the art of creating low-res sprites (in pixel-art), either.
On a 640 by xxx screen these sprites had an even larger view-port (just think of Microman for Win 3.x).

By the way, I just tried VGA2EGA picture viewer and found out that there was a 640x200 mode, too! :)
Do you know if that mode would work on a real CGA monitor, too, if an EGA card was in place ?
It would be the same resolution as for CGA's Hi-res mode (640x200 B/W).
The program is available here (there's also a CGA version).
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Re: dilemma of EGA & VGA combo

Postby digger » 2018-11-16 @ 01:28

Jo22 wrote:Thanks for your understanding! It's been nice talking to you.


Likewise! :)

CGA not only had 640x200 mono, but also a 640x200 4c mode that never made it due to incomplete decoding.


I had no idea! It's so amazing how I keep learning new things about vintage computers here almost every day. :lol: But wouldn't a 640x200 4-color mode have required twice the video RAM? And with twice the video RAM, wouldn't CGA have been able to support 16-color graphics in 320x200 mode as well?

On top of that, there was Plantronics and a nice 640x400 mode that was featured by several clones (ex. AT&T, M24, 82c426)
and emulator boards (AT Speed/PC Speed on Atari ?).


Yes, I know that variant well. My Dad's Olivetti M24 was the first computer we had at home. That's the one I grew up with. :) By the way, did you know that the CGA compatibility of the Olivetti/AT&T graphics card was incomplete? I my experience, it was damn near perfect, playing pretty much every CGA-compatible game I threw at it. But it did lack the unofficial cyan-red-white palette. It would display games that try to use that palette in the official cyan-magenta-white palette instead. I remember playing the CGA version of Outrun at that fellow student's house I talked about earlier, the one with the EGA system. I don't remember why the game was running in CGA mode even though they had EGA, but I do remember being surprised that the color red was used instead of magenta. Back then I shrugged it off as a glitch, an imperfection in the downward CGA compatibility of EGA. It wasn't until many years later that I found out that the imperfection lied with the M24's CGA compatibility instead. I'm still not convinced I missed out on much by not having that third 3-palette CGA mode on my Dad's computer. After having seen EGA in all its 16-color glory, CGA looked dreadful to me regardless. :lol: You're right about those hacked 160x100 pseudo-graphics modes, though. The relatively few games that supported it looked so bright and colorful, particularly Moon Bugs. :) It's a shame there weren't more games that took advantage of that mode. Although Paku Paku didn't come out until much more recently, I'm sure I would have been blown away by it back in the day.

I've made it my quest to this day to get EGA graphics working on that same M24, using its original monitor. It has proven quite challenging, since that monitor works at a different frequency than an actual CGA monitor, due to its support for a twice as high vertical resolution (640x400 as opposed to 640x200, as you mentioned yourself). I had to track down an EGA card that would specifically support that mode. I ended up getting an EGA Wonder 800 through eBay tha thas a jumper for that mode (25kHz instead of 15kHz, if I recall correctly).

The M24's monitor also lacked another CGA-specific feature, namely the ability to display the color brown. The original IBM CGA monitor had hardware circuitry that would show color 6 as brown instead of dark yellow. The M24's monitor lacked that circuitry and would therefore display color 6 as a somewhat ugly mustard-like dark yellow instead of brown. This means that even if I finally ever get that EGA Wonder card to work with the M24's monitor, EGA games will look different, due to brown being displayed as dark yellow. I wonder how that will look. In many adventure games showing sceneries with a lot of trees and wooden objects, such as the King's Quest games and Monkey Island, I expect this difference to be quite noticeable. Writing all this, the motivation to fix the apparently defective power supply in that M24 is coming back to me. Perhaps it will be a nice project to work on with my Dad in the coming holidays. :)

By the way, I just tried VGA2EGA picture viewer and found out that there was a 640x200 mode, too! :)
Do you know if that mode would work on a real CGA monitor, too, if an EGA card was in place ?
It would be the same resolution as for CGA's Hi-res mode (640x200 B/W).


I've always been pretty sure that would be possible. After all, the monitor never cared how many of the maximum 16 colors the graphics card to display on the screen at once. In 640x200 2-color CGA mode, you could freely pick any of the 16 colors as the foreground color, so it was not like CGA monitors were limited to monochrome graphics in that mode or anything.

Some 256-color adventure games by both Sierra and LucasArts supported 16-color 640x200 EGA mode and used the higher resolution of that mode to emulate 320x200 256-color graphics through dithering. The results were surprisingly impressive, although of course they would never reach the same quality as actual 256-color graphics. Also, effects like fade-in/fade-out and color cycling would of course still not be possible on EGA, even at the higher resolution. But still, the idea of playing a game with graphics that were designed for 256-color VGA on a CGA monitor with an EGA card at 640x200 at 16 colors is something that has particularly fascinated me. I think the biggest appeal of that would be the satisfaction of truly seeing a CGA monitor in use at its maximum capability (the highest resolution it supported combined with all the colors it supported being shown on the screen at once).

By the way, this brings me to the one real downside of IBM guaranteeing full downward CGA monitor compatibility in EGA cards, namely the fact that support for custom 16-color palettes out of 64 possible colors would only be activated in 640x350 mode. Support for more than the standard 16-color palette required support for advanced signaling on both the card-side and the monitor-side, which would have broken compatibility with CGA monitors. Had IBM allowed 64-color support in 320x200 and 640x200 modes as well, even at the cost of sacrificing full downward compatibility with CGA monitors, I wonder how far EGA could have been pushed in many games. Especially later on, as EGA cards gained more video RAM, perhaps it would have been possible to support 64 colors at once (not just custom 16-color palettes out of 64 colors) at 320x200 or even 640x200 modes. I believe this would have closed the quality gap with VGA considerably. Perhaps even some innovative company would have eventually released a graphics card that would have been hardware-compatible with EGA monitors, yet software-compatible with 320x200 256-color VGA, by implementing dithering in hardware using a 64-color 640x200 graphics mode. We'll never know what the possibilities would have been. Well, except if someone were to make a variant of the VGA2EGA utility that would assume such a hypothetical 64-color 640x200 EGA-on-steroids mode. ;)

The program is available here (there's also a CGA version).


Thanks for sharing these links here. These should be fun to play with and the VGA2EGA viewer in particular should give a nice idea how well 256-color games could have been made to look in 640x200 16-color mode with dithering. The dithering algorithms have improved a lot since those days, so an even better emulation effect could be achieved these days using the same hardware, methinks.

I'm glad you enjoy discussing these obscure retro PC graphics hardware details as much as I do. ;)
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Re: dilemma of EGA & VGA combo

Postby Jo22 » 2018-11-16 @ 09:15

Wow, thanks a lot for the long reply! :D
It also was an interesting read about you dad's M24. Some Turbo Pascal programs shipped with an ATT.BGI,
which is a graphics driver for the hi-res mode. StarView, which I came across some time ago, supported it way back in the 1980s.

digger wrote:
CGA not only had 640x200 mono, but also a 640x200 4c mode that never made it due to incomplete decoding.

I had no idea! It's so amazing how I keep learning new things about vintage computers here almost every day. :lol:
But wouldn't a 640x200 4-color mode have required twice the video RAM? And with twice the video RAM, wouldn't CGA
have been able to support 16-color graphics in 320x200 mode as well?

You're on spot with that! :) reenigne and me discussed this a bit in an earlier thread.
Your M24 has got 32KiB of CGA RAM, while the original IBM card only has 16KiB. Never the less, IBM reserved 32KiB as CGA or "video" memory. :)
That'll be the range from B8000-BFFFF, I recall. If memory serves, programs could also use the other 16KiB as an alternate CGA page frame.
I'm now highly speaking under correction, but I believe that there was something about an alternate control register, too.
If my memory only wasn't so sketchy at the moment (can't concentrate very good since a few days, maybe personal stress, dunno.)
If I had to make a wild guess, then I would guess that CGA's max limit of 640x200 in monochrome in its final form wasn't intended right from the start.
Rather, it seems like the designers had to make a compromise because of technical difficulties they could not solve, at least not in time.
That would, perhaps, also explain other oddities, like why some early IBM MDA cards being able to produce colour, but later models not anymore.
And in some point of view, your M24 completed IBM's CGA. Its custom 640x400 mode uses a memory layout that is so close to CGA,
as if it was born in the minds of IBM engineers. The documentation of it are available in M24 manuals at http://olivettim24.hadesnet.org/
Some very intresting CGA information is also available at https://www.seasip.info/VintagePC/cga.html

That being said, I'm just tellinngyou what I remember from my memory (some of which I read many years ago).
It's better to double-check what I stated before, maybe I mixed somehting up (to err is human :blush: ).
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Re: dilemma of EGA & VGA combo

Postby Jo22 » 2018-11-18 @ 07:34

Okay, so I took heart and did some testing with my CGA monitor on an EGA card.
I used one of these EGA/VGA dilemma cards for that test.

Setting CGA mode worked fine, after I found out
that the DIP switches of my O37c had to be set inverted:
http://www.vcfed.org/forum/showthread.p ... A-VGA-card
Before that, I got a mixed up screen as you can see in pic no.1.

Switching the monitor type for EGA caused a standard EGA signal,
640x350/21khz, I guess, since the sync was way off.

On the other had, setting an EGA mode from CGA mode didn't work either.
The mode utility didn't accept anything besides CGA.

In CGA mode, in CGA Compatibility Tester, I ran M6845 tests
and they turned out ok. The monitor is a ~59 Hz.

Just video speed is better:
483 KB/s vs IBM CGA 291 KB/s (block read)
512 KB/s vs IBM CGA 340 KB/s (block write)
413 KB/s vs IBM CGA 199 KB/s (interleaved read)
488 KB/s vs IBM CGA 192 KB/s (interleaved write)
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Re: dilemma of EGA & VGA combo

Postby Jo22 » 2018-11-19 @ 07:55

Update. Also tried to hook up the CGA monitor to my Schneider Tower AT's internal EGA chip.
Unfortunately, it did not work. The setting is there (CGA monitor on EGA), but the built-in chip fails to work. :(
Anyway, maybe it is better that way. The 037c's TTL port sure works fine for CGA/EGA if native monitors are used.
But who nneds that, anyway. VGA is awesome and CGA/EGA emulation can be manually selected from there as well.
Speaking of EGA and VGA, I uploaded a video about EGA dual-page mode, to give an impression how seamless motion looks like.
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Re: dilemma of EGA & VGA combo

Postby digger » 2018-11-24 @ 14:04

Wow, that's really disappointing! :( I was honestly of the opinion that using EGA cards with CGA monitors for 320x200x16 and 640x200x16 modes (anything other than 640x350 modes) was a very common and viable upgrade path for gamers back in the day. Perhaps only certain EGA cards offered this feature? That would still be weird, considering how IBM went out of its way to ensure downwards compatibility with CGA monitors on their original EGA card.

Does anybody else have any experience with this?
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Re: dilemma of EGA & VGA combo

Postby Jo22 » 2018-11-24 @ 19:29

Well, yes, I was a bit disappointed, too. Maybe it was also related to my EGA and EGA/VGA chips only, not sure. Sorry. :(
The Schneider's chip is/was a socalled Super EGA chip, and can do 640x480 or 800x600 on a multisync monitor.
Anyway, I don't know much about it. dr.zeissler or keropi may know more about these kind of chips, I believe.
Enhanced EGAs were also known as LEGA and SEGA. They appeared shortly before (or after ?) VGA became popular.
Here's an example thread I do remember - viewtopic.php?f=63&t=55188

Edit: I also like to hear other's opinions, as well! :)
I admit that I know only a little about real EGA hardware, since I rather grew up with it as a VGA mode.. :sweatdrop:
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Re: dilemma of EGA & VGA combo

Postby Scali » 2018-11-24 @ 20:53

The original IBM EGA had a dip switch to select 200-line mode for CGA monitors.
If your clone doesn't have this dip switch, you could still use a CGA monitor, since the 200-line modes are compatible with CGA by definition. The dip switch only makes the EGA card boot up in a 200-line textmode as well, instead of the default 350-line mode.
However, I'm pretty sure that if you would put a program in your autoexec.bat that switches to the 200-line textmode on boot, it will work.
See here: http://www.delorie.com/djgpp/doc/rbinter/it/10/0.html
You'd normally start in mode 03h, but mode 0Eh would work on EGA with a CGA monitor. It's a graphics mode and it will be slower, but it should work in DOS.
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Re: dilemma of EGA & VGA combo

Postby Jo22 » 2018-12-01 @ 21:05

Thanks a lot for this information! So far, I tried "Color-Monitor on EGA-Adapter primary" for internal videoconfig.
But there are lots of other options (Multisync(70hz) w/ CGA-Emulation; Multisync w/ CGA-Emulation; MDA; Mono on EGA-Adapter, etc.)

That being said, I'm looking forward to the results of other testers, too, since unfortunately, this Super EGA chip of my PC has no jumpers.
It's all software controlled and a part of the -um- "logic board" (I apologize for using an Apple term here).
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