> For reading data when the card activates the SBHE-line itself, the above
> mechanism can be used as well. But what about writing data??? In that case
> it seems we need a temp buffer in D0..7.
At first glance I thought I found an even worse scenario: what happens if
one starts a DMA from the C64 towards the ISA-bus startong at on odd addres?
In that case the above temp buffer doesn't contain any valid data.
> SHBE A0 Transfer
> 0 0 word
> 0 1 byte on D8..15
> 1 0 byte on D0..7
> 1 1 will never occur says the doc ***
But then occured to me that there was no problem, see above table. This
triggered the following idea: forget about that buffer. In that case the
following happens:
A0 = 0 : D0..7 is written with data from D0..7 of C64
D8..15 is written with invalid data
A0 = 1 : D0..7 stays the same
D8..15 is written with data from D0..7 of C64
I don't think this will cause problems with memory but what about I/O: could
any harm be done by writing invalid data to D8..15 the first time? Here I
merely have the IDE-interface in mind.
Groetjes, Ruud
http://Ruud.C64.org
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