RE: 65GS10

From: Marko Mäkelä (Marko.Makela_at_HUT.FI)
Date: 2002-04-18 09:56:39

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    On Thu, 18 Apr 2002, Gideon Zweijtzer wrote:
    > So the original CPU writes random data to $0 and $1, but still does a write,
    > right?
    The data it writes is not random.  On the C128, if you enable the 2 MHz
    mode and make sure that the video chip is not refreshing the memory at the
    critical moment, you can write a desired value there, I think it was with
    ldx #1:sta $ff,x (which will copy the value from $ff to $0 in the RAM).
    On the C64, the data to be written has to be fetched by the video chip.
    > So there should be no problem in just writing the actual contents of
    > $0 and $1 also into memory.
    It depends on what level of compatibility you want to achieve.  What about
    the built-in temperature sensor of the 6510, are you going to support
    that? :-)
    > Yes, I agree; it will break a lot of video timing stuff. I could put some
    > effort in making my 6510 implementation completely 6510 cycle-compliant, but
    > I want to focus on the faster CPU.
    I hope that you consider releasing your work under an open source license,
    so that others can concentrate on producing a cycle compliant
    implementation.  Maybe it would be possible to integrate a NMOS 6502
    compatible processor, a video chip and some audio stuff on the same FPGA,
    if there are so many spare gates on the chip?  I guess the I/O chips
    should be placed on a separate FPGA, as otherwise you'd run out of pins.
    > This implies that the program counter is always one cycle ahead
    > already, which introduces a problem for a few instructions like a
    > branch when it is taken. (Also for the implied instructions, since the
    > PC is already fetching the second byte of the next instruction while
    > executing the implied instruction! Of course I could stall the CPU for
    > one cycle in this case, so the timing would look like 2 cycles again.)
    If you simply stalled the processor there, I believe you would be
    closer to the real 6510.  Have you read the document
    > Besides; I still have to look into the possibility of using the
    > internal block RAM for the registers. I am just afraid that I can't
    > write two registers in one cycle and this might be needed in some
    > cases.
    Maybe you should keep some of the special registers outside the block
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