Terminal-BASIC becomes stable and functional enough to run the program like classic tetris game. It's quite compact and runnable not only on Arduino MEGA microcomputer but even on Single chip BASIC computer. But it simply is TOO slow!.. As shown on the videos below, SDL TVout emulation is ok, but on real device with TVoutEx output it's fail.

Arduino MEGA with 32 Kb shield:

SDL emulation:

Very disappointing. I will try output through the dedicated VDC, such as NGT20. After all, TVout library forms interrupt overloaded environment.