> If you need more detail, I have the definitive ISA > bus reference book ;) If it is not too much trouble, please do. I thought I had some edocs on DMA. Only to find out yesterday that the I didn't > > Of course, the 8237 provides the addresslines. I completely forgot. But if > > so, why "tristating" the 6522 ? > > Am I misunderstanding? I thought the 6522 was used to recieve and send the > data bus information to the expansion bus. (?) My apologies. I was thinking about using a 6522 for the data- AND addressbus. I had the one for the addressbus in mind when reading your comment. > > But I'm still thinking about adding an 8237 (optional) as I can imagine that > > there are datatransfers which need a bigger throughput the a 6510/8502 can > > provide. > > The trusty 6510 can access all 64k of its ram. ;) What else would you want > to transfer? Or, perhaps, are you going fo r more speed? <grin> Instead of "bigger" I meant "faster" indeed. Regardless of any method, I still have a problem. The idea for the card is to use both free I/O areas in $DE00/DFFF: one as data-window, the other for adressing the needed I/O. This I/O is needed for a) reading bits 8..15, b) supplying addresslines A8..23 and c) telling the card wether it is an I/O- or memory-operation. The question I have is: when performing a DMA, let's say reading data from an attached 1.44 MB floppy, should the data transfer from the floppy to the C64 or from the floppy to the memory attached to the PC-card (like video-memory)? Ideal would be to be able to use both methods but that involves some more hardware. I have an idea but it certainly would rule out the 6522's. Another idea: the PC-card needs 256+ bytes. This means that it hardly can be combined with another cart. Maybe it can be used together with the REU, but I'm not sure. My idea is to add a 74LS139 to the original C= which enables you to reduce the area occupied by the VIC-II from $D000/D3FF to $D000/D0FF. The other 3*256 bytes now can be used for your own purposes. With jumpers or a 2-pin connector one can choose which configuration is to be used for the PC-card. About compatibility: I don't know any valid reason why a good programmer would prefer the mirror addresses to program the VIC-II. So I presume all well written software should run on such an altered C=. IMHO the change can be performed on all C64's and C128's. Groetjes, Ruud http://Ruud.C64.org - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail email@example.com.
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