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Using multiple video cards (ISA + PCI) in a single DOS build -- possible or not?

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First post, by Der Kuenringer

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I know that VGA ISA cards would always take precedence over PCI & AGP cards. But having two video cards would supposedly work, if the ISA card isn't VGA (or if not initially recognized as such?). At least I've read as much here in the forums.

So would a system with both an EGA (ISA) card and a VGA (PCI) card prioritize the latter? Better yet, there are several ISA cards that sport both TTL and VGA video outputs. Some of these cards let you configure them to power-up in either VGA, EGA, CGA or MDA mode, and store that configuration in EEPROM. The ATi VGA Wonder cards for example. Other cards probably do the same with jumper settings.
Does anyone of you have a dual card like that to test? Will it recognize a PCI VGA card as the primary video card under such condition?

Just to clarify, I'm not interested in multi-monitor operation, just about having both ISA & PCI cards in the system, and using them in turns.

Reply 1 of 5, by Tiido

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Vast majority of cards map their video BIOS into C0000 range and you usually have no sort of control over that, this is going to make having multiple ISA cards coexist not gonna work and in worst case actually damage something due to bus contention that happens. (PCI allows remapping and multiple PCI cards can work).

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Reply 3 of 5, by Grzyb

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Since the very beginning, IBM PC supported dual video cards setup: one of the cards has to be monochrome (MDA), and the other - color (CGA/EGA/VGA).
So, yes, it should work: ISA MDA + PCI VGA.

Also, there's many cards designed to be installed together with others:
- 8514/A, originally for MCA, but there were also clones for ISA, some of them without a VGA chip - see ATi 8514/Ultra
- TIGA, again - some without a VGA chip, or with a jumper to disable the VGA
- Number Nine Imagine 128, it came with a separate VGA chip which can be disabled by jumper
- Voodoo 1 and 2

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Reply 4 of 5, by Der Kuenringer

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Grzyb wrote on 2020-12-14, 00:07:
Since the very beginning, IBM PC supported dual video cards setup: one of the cards has to be monochrome (MDA), and the other - […]
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Since the very beginning, IBM PC supported dual video cards setup: one of the cards has to be monochrome (MDA), and the other - color (CGA/EGA/VGA).
So, yes, it should work: ISA MDA + PCI VGA.

Also, there's many cards designed to be installed together with others:
- 8514/A, originally for MCA, but there were also clones for ISA, some of them without a VGA chip - see ATi 8514/Ultra
- TIGA, again - some without a VGA chip, or with a jumper to disable the VGA
- Number Nine Imagine 128, it came with a separate VGA chip which can be disabled by jumper
- Voodoo 1 and 2

I am aware multiple cards will work in principle, but will only the combination of monochrome + EGA/VGA work, or would it also work to have both an EGA + VGA card present in the same system? I was under the impression that this monochrome + EGA/VGA pairing was a specific limitation only with early multi-monitor use? Since I'm not using them both at the same time, would any one of these qualifications even apply?

A further, unrelated complication is the ISA vs. PCI conflict, and whether it would still hold true under the special circumstance of one card being ISA and the other PCI. The examples you mentioned merely extend the video processing capabilities of the base video adapter (except maybe the TIGA card, not familiar with it), so obviously they can't conflict with one another.
I was raising this question in the context of what has been discussed in this thread here: 486 Mobos: Can you install multiple VGA cards - like AGP/ PCI switching?

Reply 5 of 5, by watlers_world

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Chips&Technologies F82C451:"A Lowend VGA solution. Support up to 16 cards in one system."
http://vgamuseum.info/index.php/companies/ite … ologies-f82c451

However... with each added VGA display CPU performance is divided between the displays.
It would seem that an effectual video accelerator would cost more than a second computer.
Perhaps it would be better to use two computers for one display to double visible CPU power.

Video overlay and video annotation controllers are possible.
https://www.prosoft.ru/cms/f/448752.pdf

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