Well, extremely portable, I wouldn't say so.

If you look at the ancestry of SDL2-Doom, for example, there are three ancestors:

- doomgeneric, which seems not simple (https://github.com/maximevince/fbDOOM/compare/master...ozkl:...)

- the above is based on fbDOOM, which is quite simple (https://github.com/maximevince/fbDOOM)

- the above is based on insane-adding-machines/DOOM, which is also not simple (https://github.com/id-Software/DOOM/compare/master...insane-...)

The SDL port itself (https://github.com/AlexOberhofer/sdl2-doom) is definitely not simple.

Porting something that it's not based on a multiplatform library (which is commonly SDL for old game ports) is not an easy task in general.

On SDL, there's an SDL2+minimal ANSI C environment for plan9/9front called NPE.

The SDL2 audio and video libraries are just shims against 9front drawing/audio functions, and often they just work.

Plugging a DOS game to an SDL(2) library won't "just work".

Some concepts are inherent to the hardware and the O/S.

Hardware example: EGA (not the case of doom, but of other, slightly older games) uses bitplanes, it's not just an array of byes; audio cards used specific drivers.

O/S: timers may need to be emulated, as the game may not have a straightforward game loop; due to this, even exiting cleanly may not be trivial to implement.

Not an issue for Q1, it wasn't for neither EGA or VGA for DOS exclusivelly, they had DOS, Windows and Unix versions since day 1.