Re: VIC-registers

From: Marko Mäkelä (
Date: 1998-03-01 15:24:25

Ruud wrote:

> > The $d700-$d7ff area is quite safe, it's free on the C128 as well.
> What about the SID in C64 ????? I have to disagree in this matter.

I thought that you were talking about internal modifications.  I didn't
realize that you were going to tap these addresses with external decoding.

But I think that $d031-$d03f would work with external decoding logic.  The
VIC-II will return $FF when these addresses are read or written, and the
data on the bus will simply be ANDed with $FF, i.e. no effect.  The only
problem is that you need quite much decoding logic (to tap all 16 address

> Anyway this means there are only two solutions left. The first one is to
> use dipswitches so you can use your own HW with as much as cartridges as
> possible. But this means that you cannot use it with cartidges like FC3
> who uses both IO-lines without any further decoding. 

Well, you can modify e.g. the Action Replay hardware so that it will leave
$de01-$deff free, and that it won't react on $de00 on read access.  The
cartridge has quite funny design: the control register is write-only, but
it will react to any (read or write) access to $de00-$deff.

I patched one Action Replay or a REU so that it'll work simultaneously
with the REU connected.  I think that the modification made the REU
registers write-only.  Of course this doesn't work with any software that
tries to read the REU registers, but with prlink it does.


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