Re: PC-card - DMA/SBHE

From: Ruud Baltissen (
Date: 2001-04-17 12:30:41

> 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

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