Hallo Andre, > > Marko Mäkelä wrote: > > But I think that $d031-$d03f would work with external decoding logic.... > > IFF the VIC-II does _not_ return _anything_, i.e. does not drive the bus > this _might_ work. If the VIC drives the bus high, you may fry your VIC > when the other chip drives the bus low! For further explanation about my remark using $D031 to DMA the C64: my idea was not using the databus at all. Reading $D031 sets a flipflop which on the end activates the DMA input at the expaqnsion slot. Reading $D030 resets it again. This reading must be done by the host processor/system of course. In this way a lot of simple I/O can be steered by just reading already used addresses. A step further was writing to the read-only registers of the SID. But as you could have read in a previous email, This idea is worthless for I/O attached to the expansion slot because of the CHARROM and RAM under the I/O. Using it INSIDE the C64 it has even advantages above the de-mirroring you and I mentioned. The SCPU uses I/O in the $D000/$D3FF range. So this area cannot be de-mirrored because you run the risk that your own I/O gets in the way of the SCPU. But my idea has one big unsolved problem yet: how can I read data? Maybe one of you knows how the SID behaves when you read a write-only register. > The VIC-II does drive the data bus to high state, but keep in mind that it > is an NMOS device. You can always let the bus perform a wired-AND between > NMOS outputs. If the situation was like you described, then even moving This could be a good answer for this problem..... > the joystick might fry the CIA (remember, one of the joystick ports is > normally an output, outputting high state). I always have wondered about this. One is never to old to learn :-) Other question regarding the 6510: as far as I know it does drive the addresslines and R/W line when readin/writing $0000 and $0001. Correct? But when it writes, does it drive the databus as well? Groetjes, Ruud
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