aha... I now see what my problem is.. I thought INCFSZ worked in a slightly
different way. I thought that it did the increment, then tested the zero
flag (a reasonable assumption, don't you think?).  Now I see that it does
not test the zero flag, it is to do with the result from the increment
operation (which does not modify the flags anyway)

This controller is weird, what with this and the carry flag working
backwards for subtraction, I am getting confused :-)