Dual channel DDR and the E7500 chipset

[Mark Hahn]

here's a somewhat obsessive investigation of how much prefetch distance
is useful for stream on my E7500:

	http://sharcnet.mcmaster.ca/~hahn/stream.prefetch.png

that's a version of stream_d.c to which I've added simple uses of
__builtin_prefetch.  best point is between 232 and 239 where scores 
are 1381/1380/1610/1626 versus 1179/1177/1419/1418 for no prefetch.
the specific distance would necessarily depend on HW, clock,
memory latency, compiler, phase-of-moon, etc.

regards, mark hahn.