Re: PCcard (2)

From: Ruud Baltissen (
Date: 2001-04-10 08:48:31

> 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
> > so, why "tristating" the 6522 ?
> Am I misunderstanding?  I thought the 6522 was used to recieve and send
> 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

> > But I'm still thinking about adding an 8237 (optional) as I can imagine
> > there are datatransfers which need a bigger throughput the a 6510/8502
> > 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

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
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

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