Software that emulated VGA high res mode on an EGA board

Question

I have this very faint recollection from my late elementary school days back in the end of 1980s of a very specific piece of MS-DOS software that claimed to do what was an incredible thing to an eighth-grader of the day.

The software was marketed with a claim that it would make an EGA graphics board able to emulate the 640x480, 16 color high-res mode of VGA. At our school, we had a computer classroom full of 80286 machines of the day, complete with EGA boards and monitors. I recall trying the software up once in one of those boxes, bringing with me a painting program on a floppy that I had carefully set up to run in VGA mode; the program, named "Dr. HALO" or something like that, was well suited for this testing because it had to be configured for a specific graphics board and would only run at all if the machine could support the chosen mode, not gracefully downgrading if the setup was not met.

To my awe, the painting program did indeed start up and run when the emulation software was active (the emulator deployed as a TSR program, as did a lot of MS-DOS utilities at that time). I had neither the time nor the knowledge to systematically test the performance or reliability aspects, so I can't say how good or usable the emulation would have been in productive use; back in the day it proved its point to me by just showing that such emulation was possible (or, at least, I was reasonably sure that it worked).

Now, decades later, I'd be very interested to find some references as to what was the name of that software, who the developer was, and how the VGA emulation worked. Alternatively, should such emulation not be possible, I'm also interested in that fact and why it is so; I was quite sure that I managed to run the painting program in VGA mode (we had a VGA machine at home, so I could tell the appearance of VGA graphics from EGA), but I do recognize that I was a thirteen-old kid, and it's been a long time.

Answer 1

Sounds quite unlikely it would actually work, but there are a few prerequisites, limitations and assumptions where it might be possible for an EGA card with a TSR to emulate a VGA card for simple purposes.

The only thing directly compatible is the access of video memory via memory segment A000h and setting the read/write bit planes so that definitely is not a problem.

Generally a VGA card would be directly BIOS and register compatible with a program expecting an EGA card in the system, but not vice versa.

There are many things that can be problems.

First of all, EGA card must be expanded to 192k or 256k of memory, in order for the frame buffer to hold data for a 640x480 image. This may not be a huge issue as later EGA cards may have been sold fully populated. It would definitely work with 128k or less. VGA cards always came with full 256k of memory, and later clones with even more than that.

Second thing is, the EGA monitor has no mode with 640 visible lines, and since EGA monitors only support two fixed frequency rates, it is unlikely the horizontal rate could be increased by 37% to show 480 visible lines instead of 350, unless in the unlikely case it was an multisync monitor with EGA interface. As the pixel clocks are also fixed, having more lines would drop the refresh rate. So the assumption might be that you can see the default 350 lines out of 480, and maybe scroll up/down with some keyboard/mouse shorcut if the TSR allows for it.

Third assumption is that the drawing program only uses BIOS functions for controlling the card in order for the TSR to emulate it. While EGA and VGA are quite similar, if not even register compatible in basic functionality, the difference is that in general VGA registers can be read but EGA registers are in general write-only. So the program can't perform read-modify-write operations of VGA registers on an EGA card. The TSR just basically needs to set a 640x350 mode instead of 640x480 if the program requests 480-line mode via BIOS video mode set call.

The VGA registers also can't be emulated by the TSR, as the 286 CPU has no mode where it would support trapping IO port accesses while still being capable of running real mode x86 code. A 386 could do just that in VM86 mode, which is why many virtual hardware emulators required EMM386 in order to run. So the program must either use the BIOS functions which the TSR can emulate, or like what early VGA programs did, just write to registers without reading like it has to be done on EGA cards.

And obviously the EGA card will be limited to the selection of 64 colour default EGA palette, as there is no VGA RAMDAC with a colour look-up table to convert the 6-bit EGA colour values to 18-bit RGB value for analog RGB conversion.

So what you describe might be possible, but it would require that the drawing program is quite simple and does not use VGA hardware directly. It is plausile that the drawing program is very simple and it uses the BIOS for most things and uses EGA compatible IO port writes when necessary to control how to directly access the video RAM.

It also possible that the program simply detects there is no VGA and continues in EGA mode.