Porting Autodesk Animator Pro to be cross platform

My biggest concern with your plan, in a nutshell: Your approach seems to be to attempt to keep the whole enormous thing working at all times, tweaking the environment ever-further away from DOS. During each tweak to the environment, that means you will have approximately a billion subtle assumptions that might have broken at once, none of which you necessarily understand yet. Untangling them all at once will be incredibly painful.

If I were doing the port, my approach would be to disable as much code as possible to get SOMETHING running in a modern environment, and bring the parts back online, one piece at a time. Write a simple test harness program that loads a display driver and draws some stuff, and compile it for DOS to make sure you understand the interface. Then write some C code that implements the same interface, but with Allegro (or SDL or SFML), and make that program work under Windows or Linux. When the output differs, you have a simple test case to work from.

Your entire job on this port is swapping out implementations of various interfaces and functions with completely new ones. This is a job that unit testing excels at. Don't write any new code without a test of some kind that runs on the old code under DOS! Make your potential problems as small and simple as you possibly can. Port assembly code instead of rewriting it only if you're reasonably confident that it will actually make your job easier (ie, algorithmic stuff that compiles fine with few tweaks under NASM). Don't bite off a bigger piece than you can comfortably fit in your brain at once.

I, for one, look forward to seeing your progress! I think what you're attempting to do is great. Thanks for doing it.

Hmmm - I might approach it by writing an OpenGL video "driver" for it. and todays machines are fast enough with tons of ram that you could do all the pixel specific algorithms on main CPU into a back buffer and it would work. As the "generic" VGA driver just mapped the video buffer to a pointer this would be a place to start. There was a zoom mode in the UI so you can look at the pixels on a high res display.

It is often very difficult to take an existing non-trivial code base that wasn't written with portability in mind - you mention a few - and then try to make it portable. There will be a lot of problems on the way. It is probably a better idea to start from scratch and rewrite the code using the existing code as reference only. If you start from scratch you can leverage existing portable UI solution in your new project like Qt.