Re: 6702 chip

From: Rob Clarke <>
Date: Tue, 29 May 2012 08:26:34 +0200
Message-ID: <>
Spectacular effort guys. Well done.


On 28/05/2012 21:49, Rhialto wrote:
> On Mon 28 May 2012 at 10:36:55 +0000, William Levak wrote:
>> This looks like it could reproduce the output with one addition: a
>> control register for the shift registers that turns individual shift
>> registers on or off.  We know this control register exists because
>> bits 3 and 5 are initially off and are turned on if the correct
>> input is received.
> Ah, but the "trick" here is that the shift registers for those output
> bits initially have only zeros in them. So they'll never change the
> output bit until a  1 is put in them. Which is done by providing a
> changed input bit. (It turns out that setting "previous input bits" to
> all zeros works fine.) The bit that is changed if the input changes is
> the leftmost bit in the shift register (before it is shifted).
>> In the case where the output for a bit is all 0 or all 1, the shift
>> register is turned off.
> because all bits in it are 0.
>> This occurs when the corresponding bit of the input is changed and the
>> current output of the shift register is high.
> The output of the shift is wrapped around to the left, so if the input
> changes at the right moment, the 1 which was wrapped around will be
> toggled to a 0.
> The same method can turn it on again later. And of course there can be
> multiple 1s. If the number is odd, the second cycle of N bits will be
> the inverse of the first one.
> I've put the code into VICE and it works with the Waterloo software, so
> I checked it in. As of revision 25635 you can try it out.
> Tarball of the full VICE source:
> or
> svn co vice-emu
> -Olaf.

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Received on 2012-05-29 07:00:05

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