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Re: What to do with slow Chip/ST RAM
On Wed, 11 Mar 2009, Frank Wille wrote:
Would anyone be interested in adding support to use chip/ST
RAM on the amiga/atari in a similar fashion (obviously some of
it is likely to be reserved for device DMA).
I doubt that it would work very well for Amiga, because (I think) the amount
of available Chip RAM is not constant. There may be allocations for floppy
disk, audio or video DMA buffers at any time.
The altmem device works with a fixed contiguous memory region?
It would probably be best to allocate most of the slow RAM to the
altmem device on boot and leave just enough to work as DMA space
for the devices. The important question is how *much* slower is
the slow ram. tsutsui@ commented that mvme86k already handles
split between fast and *very* slow (uncached!) ram by just
assigning priorities in uvm() so the fast ram is all used first.
Maybe that is a better approach for amiga and atari (maybe they
are already doing that?)
An interesting benchmark would be to run a system with the
memory bank priorities set appropriately, and then run it
again with the slow ram unsued apart from dma allocations
and see if having more ram (albeit slower) offsets the
speed penalty of accessing it.
In an ideal world the system could be tunable, so those with
large quantities for fast ram would not use slow at all, and
those with very little fast ram would...
David/absolute -- www.NetBSD.org: No hype required --
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