On Thu, Apr 09, 2009 at 07:28:59AM +0200, ruud.baltissen@apg.nl wrote:
> > How does the VIC refreshs the RAM? According to the VIC article it
> > generates an address and does a write?
> 
> I'm quite sure that this is NOT the case.

The VIC-II can't do a write, because its R/-W line is an input pin.

Some C64s are "$de00 clean", a term that Andreas Boose and I coined back
in 1994.  Reads from the unconnected address space ($de00 to $dfff) will
deliver the data that was read by the VIC-II on its memory cycle.
Andreas wrote a test program that fills the memory with a pattern and
then copies $de00 to a memory buffer, delaying one frame and one cycle
in between.  The memory refresh looks exactly like a read from $3fxx
(IIRC).  I can't remember if the counter was incremented or decremented.
It was 5 successive reads, near the end of the raster line.

Andreas or I wrote a program that runs in $de00..$dfff.  I wrote the program
"dadb" that runs in the 4-bit color memory.  The low nibbles came from the
2114 color RAM and the high nibbles from the main DRAM, fetched by the video
chip.  I must confess that I cheated a little by filling the stack with $d9
and doing RTS most of the time (color memory mostly filled with 0).

	Marko

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