[jacquesm]

And I wrote that stuff in the 80's, when your typical 'OS' on a computer was much closer to what a BIOS is than an actual operating system. Usually the games were far more sophisticated in terms of what their internal services provided than the equivalent OS services. The most the BIOS usually did for you was to boot your game code, change graphics modes, handle the keyboard and read and write raw media. Everything else you had to do for yourself.

Today there are far more levels of abstraction, especially when you use the various 3D engines or other large frameworks. But essentially those have taken over the role of a lot of that OS-like code.