[Forum] Try only to realise the truth | ANN.lu |
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Try only to realise the truth : Comment 242 of 335 | ANN.lu |
Posted by Chris Hodges on 06-Jul-2004 11:30 GMT | In reply to Comment 237 (Nate Downes): > I might add that the reason why the AmigaOS required this "software-flag"
> for the classic hardware is that the classic hardware had no concept of
> a northbridge,
Actually, I would consider Buster/Agnus/Alice similar to a northbridge (in parts).
> there was no mannerism for DMA transfer included with any
> hardware back in that day, so the Hi Torro guys made their own. But times
> have changed, there is now a hardware-flag for PCI DMA transfers.
Both Zorro2 and Zorro3 support DMA transfers, unfortunately, the MC680xx before MC68060 (AFAIK) did not have bus snooping.
> Ironically, it is the fact that AmigaOS's core has not had an update
> since 1994 that allows it to work in such an environment.
The update, that allows working in this environment was added with Kick2.0 and CachePre/PostDMA. It was a no-issue before, because chipram was non-cachable by default and only later, CPUs with bigger caches and no snooping for the custom chip stuff and dma bus transfers were added.
So even if CachePre/PostDMA is a NOP under MorphOS, it be required to work for AmigaOS4 drivers. |
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