Re: has anybody tried to replace a vic or vic II chip with a board containing a propeller ?

Luke Crook <luke@balooga.com> writes:

> I think a device that only performs VGA output that also plays well with
> existing carts would be quite useful.

There are two ways to generate a VGA output for the C64: You can either
digitize the analog output of the VIC and output an upconverted version
of that or you can read the data the VIC receives from the system and
recreate the output based on that.

The Chameleon uses the second method - you get a higher-quality picture
because you don't need to care about any analog effects, but it needs a
lot more FPGA resources than upscaling the analog signal. In the
Chameleon there is also the additional constraint that it has to work on
the expansion port - I guess Jens didn't want to build an internal
solution that replaces the VIC because on some boards there is a metal
bracket over the VIC and related parts that needs to be desoldered to
get to the chip.

Building a "VGA output" VIC replacement or an adapter board that sits
between the VIC and the rest of the C64 to provide an additional VGA
output would obviously be compatible with any carts on the expansion
port, but to do it from the expansion port like the Chameleon you run
into one big problem: You cannot determine if a CPU access to the
$d000-$d3ff region is meant for the VIC, the character rom or the
underlying RAM just from the expansion port signals - the same problem
also applies to the color ram.

As far as I know the Chameleon solves that problem by basically running
the expansion port in reverse - the internal 6510 of the C64 is stopped,
the memory configuration is set up so the VIC does not see any RAM at
all (except the color ram) and every memory access of the real VIC is
"hand fed" by the cartridge which internally emulates the 6510, VIC and
everything else that is neccessary. Unfortunately this rather clever
trick makes it hard to cooperate with anything but very simple I/O only
cartridges - even ROM-only carts would interfere with the special memory
mapping needed by the Chameleon.

But if you think you can do it better: Go ahead, I'd love to see an
alternative solution! =)

-ik

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