[HP41] mcode question polling point entries

[MeindertKuipers]

Getting a bit closer now.
According to the VASM listings:

Quote:** all subroutine levels are available except in I/O service entry.
** if PKSEQ is set then I/O service routines must either preserve three subroutine
** returns on the subroutine stack or else terminate the partial key sequence.

And with the quoted CAT from OS/X I guess we are in PKSEQ. But my code does not use any extra subroutines, and without using N my code works with OS/X. The moment I change N the PSEQ is disrupted, but I still have to find out how N is used and what to do about it.
Use of any other register is out of the question, these are all in use unfortunately ...