Re: 65816 and I/O

From: Nicolas Welte (
Date: 2001-01-22 17:14:58

MagerValp wrote:
> RB> Thanks for all who answered. Reading the answers I have the
> RB> feeling that my question is misunderstood. Maybe I can ask it in
> RB> this way: How does the Super-CPU manipulate the I/O registers of
> RB> the original 6510 at the addresses $0000 and $0001?
> Dude. Yes, how the hell does it do that? :)

Why should it do this at all? AFAIK, datasette operation is not supported
anyway, so there's not a real problem with these lines. And memory
configuration operation also is not needed, since all programs run internally
in the SCPU. The C64 is just a video buffer and I/O machine, nothing more.
The only problem remains how to switch between I/O and RAM at $d000-$dfff,
but I think you can select this externally by switching Ultimax mode on and
off. I'm not sure, but doesn't the Ultimax mode have the I/O always switched
in, even if the processor port for C64 operation is set to an all-RAM

But I remember that they had a real problem supporting the ASCII/DIN or
ASC/CC key on international C128 machines, because this key can only be read
through the processor port. Don't know how they solved this, because only the
8502 can read that bit. Probably they switch back to the 8502 to read it.

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