There is game OIDS, where programmer limited framerate to 10 fps using 5 Vbl waits in every screen generating cycle. However, it needs only 2 Vbl time on simpler planets, and 3 on complexer ones. So, framerate could be 25-17.
I'm thinking about best way to achieve it, maintaining constant movement speed of objects on screen, without that not wise framerate limiting,
I think that dml could say something here considering his experience.
Idea at moment is to draw next frame at begin calculating with 2 Vbl (1/25 sec on PAL) needed for it - so doing movements for 40mS time period. That's the start Vbl count. Then, looking how much it really took - what depends mostly from scene complexity. If it is 3 or 4 Vbls, then calculating next frame for that time period (because scene certainly will not change much in so short time). So, calculating bigger movement for next frame. And so on.
That should result in relative constant speed of movement on screen, even if framerate changes in meantime.
I will go on 2 video buffers (as it is already) - so it will run with 512KB RAM for sure. 3 video buffers are better (can get little higher framerate, since can render ahead during wait on Vbl), but I don't think that it will be worth to go on it
English language is like bad boss on workplace: it expecting from you to strictly follow all, numerous rules, but self bending rules as much likes