Could the stack be made 16bits deep? I've been thinking about it lately,
and I'm not sure. The question is that *maybe* the S register could be
seen as 8 bits (the lsb 8 bits of a 16bit S register) by the 6502 code
(so as to remain compatible with existing code), but be in fact a 16bit
register, so that when you PHA beyond the 256th byte, even though the S
register "rolls", in fact you'll get back the correct data when you PLA
back. The problem here arises when the stack is manipulated "by hand",
not by PHA/PLA. Well, this is something to think about. If such a thing
can be done, it should.