Light me a pixel

Circa 30 years ago, at the nearer end of the wild 90s, the author of these lines spent quite a significant time optimizing code that was supposed to run as fast as possible, consuming the least amount of resources while showing incredible spinning graphics on a screen (there was also scrolling involved, too, and other not relevant calculations).

These applications were called demos (intros, etc.) and showcased some spectacular graphical effects, backed by a strong mathematical background, and had an in-house developed graphical engine; in those days, there was no DirectX to take all those nasty low-level details off your plate, so all had to be done by hand. Methods for pixel color calculation, color palette setting, vertical retrace of the CRT screen, and flipping of back and front buffers were all coded by hand, using C++ of the 90s and some assembly language routines for the time-critical bits.

One of these methods was putting a pixel on the screen, which...