Re: Commodore 8296GD

From: Rhialto <>
Date: Thu, 1 Nov 2012 22:34:20 +0100
Message-ID: <>
Hi Michau,

On Thu 01 Nov 2012 at 11:36:42 +0100, Micha? Pleban wrote:
> Hello!
> I am sorry for the delay - the board was already disassembled and packed
> for sending to Steve, so I needed to dig it out. We have a long weekend
> over here so I can play with my machines now :-)

Thanks for your trouble!
Your measurements give interesting results... interesting in the sense
that they don't fit with my theories :-) I was hoping to map bits in
E888 to /RAM_ON, /RAMSEL_9 and /RAMSEL_A, but several of your results
don't fit with that, from the analysis I've done so far.

For instance, if I'm believing that table I posted in the list earlier,
the only case where the I/O area is replaced with RAM also has RAM at
$Fxxx. And you find that there is ROM there all the time.

Also, replacing the I/O by RAM is (in that same table) controlled by the
CR6 bit, and in the schematics I can't really seem to find a jumper that
that signal would go through.

It is also interesting that there are settings where the I/O space reads
as RAM, but the E888 register remains accessible to switch back, AND
also settings where it isn't.

So... this creates more questions really than it answers! I think we'll
need to wait for a schematic to be made, and some answers about what
signals it is actually attaching to...

> Rhialto wrote:
> > - Is the register at E888 write-only, or can you read it as well?
> No, it's write-only. All reads always return $E8.

Ok, this is something I can use in the emulation! (In fact it was
already that way).

> > - Does it seem to appear on multiple addresses, not only E888?
> >From E888 to E88F, then from E898 to E89F, and so on until the end of
> CRTC register space.

And this is also simple - on top of the selection for the CRTC (inside
the I/O space, bit 0080 is high, it simply checks for the 0008 bit to be
high too). Ea

> > - If the memory configuration goes to the setting with even the I/O
> >   mapped out, is the E888 register still writable? It should be, to make
> >   it possible to get ROMs and I/O back.
> Apparently it might be possible to map the $E888 register out with some
> values (see below).

Yes, and the interesting thing is that the area may be RAM, but some
mappings allow to write it again and go back, and others don't...

I'll be looking at your results some more, to see if I can make sense of
them :-)

> Regards,
> Michau.
Thanks a lot!
___ Olaf 'Rhialto' Seibert  -- There's no point being grown-up if you 
\X/ rhialto/at/    -- can't be childish sometimes. -The 4th Doctor

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Received on 2012-11-02 01:00:05

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