Ahmon Dancy wrote:
> So, this differs from regular DMA in that:
> It can do scatter-gather.
Wether or not scatter-gather is supported depends on the chip. But yes,
bus mastering PCI cards can often do scatter-gather.
> The normal dma controller is not involved (?)
Yes.
The "PC" architecture (you have a IBM PC compatible computer, right?)
specifies some ancient Intel chip as a DMA controller. Nowadays that
thing is really, really outdated. At top speed it can do something
like 100 or 300kb per second. Your average new harddisk now does 100
times as much!!!
This Intel chip by the way is from the 16-bit era. So it cannot
address more than 64k. There is a hack that allows you to access 1M,
and maybe 16M. But by teh time that people would have more than 16M
RAM in their PC, this thing was no longer used.
If you'd do DMA on an IBM PC, the DMA chip would allow you to miss out
on for example the DMA "address" and "count" registers. This was an
interesting savings back in the early eighties, but nowadays the
100-fold performance improvement seems to motivate designers to take
the two registers into the chips....
Oh, I don't think that PCI specifies the old "PC-DMA" anymore, so any
DMA on PCI is PCI-busmastering.
Roger.
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