Originally Posted By Ensjo
(* Why do 2's appear sometimes, and also consecutive interrupts too close to each other?)

Just for the record, I think I undestood the pattern:
  • The normal behaviour is that the interrupt occurs after every 6 or 5 dots are printed.
  • The long sequences of dots are probably caused by interrupts being disabled somewhere down the ROM printing routine I call; interrupts that occur right in that moment are lost.
  • The 2's and the short sequences of dots appear at the end and at the beginning of the lines. That apparent "agglutination" of interrupts occur because extra processing is needed to SCROLL the screen (copying the contents of the VRAM 32 bytes up, filling the last line with spaces).