Re: How (does?) SuperCPU 64 control CPU port?

On Sun, Apr 23, 2023 at 12:44 PM <silverdr_at_srebrnysen.com> wrote:

> > This got me thinking: how does a real SCPU handle access to $01 port?
> >
> > I guess that SuperCPU's CPLD can track the writes to $01
>
> Correct me if I am wrong but AFAIR (?) $00 / $01 writes do not show on the
> bus, do they? Although I _think_ there were some tricks to catch them? Can
> anyone remind me how that works(/ed) if at all?
>

I assumed that '816 remains in control all the time. It doesn't have a port
so $00/$01 writes should appear on the bus like any other access - this can
be intercepted and latched somewhere on the SCPU side.

Sure the '816 could run in parallel in order to be able to pick them.
> Having in mind that it has a different timing (and behaviour) of some
> instructions that doesn't seem feasible though. Or am I missing something?
>
> W/O H/W mod controlling tape's not going to happen ;-)
>

That's what I think too. There was no HW mod on C64. Since SCPU had
JiffyDOS built-in, which already drops tape support, maybe it was implied
that it doesn't work. I suppose the kind of C= users who could buy SCPU
didn't bother to play with cassettes anymore.

> Or perhaps SCPU works using a different principle and the onboard 6510 is
> sometimes running?
>
> Tricky, at best… I can imagine that being theoretically possible but to me
> that's just one more reason why it's not so "logical" to put it into
> EXPANSION rather than CPU socket. The only – albeit important one – upside
> of cart-based approach for me is that many boards have CPU soldered-in
>

Yes, a cartridge form would be purely a convenience thing. For now I keep
my C64 case open and unscrewed.

ytm