On Thu, Apr 09, 2009 at 07:28:59AM +0200, email@example.com 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 Message was sent through the cbm-hackers mailing listReceived on 2009-04-09 09:29:34
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