Good portable assembly would stub the system stuff off, anyway, and once that was done for the cpu class in focus, it was very possible to have a thin HAL and write portable code. A great deal many successful products of the era were written in pure assembly this way.

In any case, you could also get high performance multiplatform video/io assembly libraries on the market, soon enough, back in the day .. it begat a lot of Delphi units too, I seem to recall ..