Compatibility for very old software on a 2014's PC...

[Hakase137]

Mobo: ASUS A55BM-K
dGPU: RX 580 8GB 2048SP
iGPU: HD 8570D
CPU: A8-6600K Quad OC'd @4.2Ghz
RAM: 2*8GB DDR3, can't remember the speed atm...

Hello. A bit of introduction, I'm a low-level programmer, my last work involved making a Sega MegaCD console read files from its dedicated CPU (the only one with access to the CD drive), and execute them from the MegaDrive's CPU (the one with access to I/O), all in Motorola 68000 Assembly. You can tell I love old software, but I'm a complete newbie to PC hardware. I do know though, that I can run MS-DOS 7.10 (CDU version); for this reason and due to new technologies replacing older technologies, I kept this motherboard (and chasis lol) for almost 10 years, and I plan to keep it in good shape even though I have no option but to get a new system...
People love to keep and do maintenance to their old Commodore 64 and such? What I'm doing is just as normal I guess... my main goal is, turn this into my personal old PC, and use it as a "gaming console" of sorts (from Hogwarts Legacy to Digger) on ocassion. The "top cutline" is every non-RTX game and Windows 10, with a couple exceptions (for the good). The "bottom cutline" seems to at least be MS-DOS 7.1.

Is it worth the effort though? The idea is to run as many different games as possible from the different eras, in bare metal. That includes the very old DOS games. I can't really give this a try myself until January, though, hence this post.

According to "some page I can't find anymore" (Could it be the ReactOS wiki?), HD 8570D, the internal GPU, features VESA VBE 3.0. Does it mean that 2.X and some 1.X features are supported?
What about CGA? I don't expect to run the 8088 MPH demo on this system, but think it could keep at least some basic functionality yet? (I found "no MC6845 registers, unless a extra emulation mode is activated via mode utility", what does it mean?; does such an utility exist if needed for something? is it an emulation layer? like, nowadays there's SBEMU, software-based SoundBlaster emulation for AC97...). CPU speed isn't an issue, I can lower it through BIOS and/or a speed limiter for DOS, I'd just like to know if could I run something like AlleyCat here.

[Octocontrabass]

Does it mean that 2.X and some 1.X features are supported?

VBE never really caught on for more than basic mode setting (and bank switching, before linear framebuffers were an option), so I expect most software that uses VBE will work fine with whatever you've got. VBE 2 made some breaking changes to both VBE 1 (removing fixed mode numbers) and VGA itself (removing the 8x14 font), so you may run into some software that gets upset over one of those two things.

What about CGA?

The hardware is only partially compatible at best, but the BIOS calls are nearly identical, so some software will work. Maybe most software?

I found "no MC6845 registers, unless a extra emulation mode is activated via mode utility", what does it mean?; does such an utility exist if needed for something? is it an emulation layer?

Some SVGA cards had a function to raise a NMI any time software tried to access CGA registers. A card-specific utility program would hook the NMI handler and program the SVGA registers to fit the intended CGA configuration. There's no utility program for either of your display adapters, and I doubt they're even capable of generating a NMI.