while neither Super Chase H.Q. nor the circuit preview in F-1 GP 3 do benefit from my change, it is nice to see that sharing registers as per Anomie's research fixes the in-game graphics in F-1 GP 3. the game is now playable!

[Linked Image from mamedev.emulab.it]
(notice I reached the end of the first lap, something not possible in current svn)

As a drawback, few glitches has appeared in Super Chase H.Q. in-game road, but for sure emulation is more accurate now than it was.

Now I'm cleaning up a bit the code and then I commit the fix. I also noticed that STA77/STAT78 support seems incomplete and I will investigate it afterwards