PC-card - accessing I/O or C64

From: Ruud Baltissen (Ruud.Baltissen_at_abp.nl)
Date: 2001-05-28 15:42:26

Hallo allemaal,

I need some advise (or a vote).

The 65xx is able to access I/O or memory like a 80x86 does; this CPU is able
to tell wether it is an I/O- or Memory-operation. My first solution to solve
this problem is to create a bit which tells the ISA-bus what kind of
operation it is.
Due to the design of the total system, I need a way to tell the logic what
the source and destination of each DMA-transfer is: the C64 or a ISA-card.
Again I used two bits.

The above system caused one problem: I'm not able to perform a read from I/O
and write it to memory on the ISA-bus without the need to flip the I/O-bit
between every read and write.

I do have an alternative and that is to reserve two 64 KB blocks for telling
the system what type of transfer is meant:
- $FFxxxx: I/O
- $FExxxx: C64
- other address: ISA-bus memory transfer.

FYI: I have two 8-bit latches for addressing A16..23. One is enabled only
during write operations, the other during read operations. This enables me
to perform transfers beyond the 64K-border.
Using again the above sample, the "read-latch" is filled with $FF, the
"write-latch" with (for example) $B8 (videocard) and now the C64 is able to
read data from the RS232-port and to write it directly to the memory of the

The only disadvantage I see for the moment is that we loose 128 KB on a
total of 16 MB.

Some feedback would be appreciated. Thanks.

Groetjes, Ruud


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