Keep in mind that the G5's use PC3200 memory, which has a
theoretical limit of 3.2GB/sec.
However, there are two channels of it. So the theoretical peak
memory throughput from the memory controller's perspective is 6.4 GB/s.
So 3.1 GB/sec is pretty good. The 6.4GB/sec is just the cpu
interface. And I'm not sure, but it might even be that this is
3.2GB/sec in and 3.2GB/sec out (the G5 has two 32 bit busses which
can be used simultaneously, one for incoming and one for outgoing
data -- but I don't know whether they're both rated at 6.4GB/sec or
whether that's the combined bandwidth).
6.4GB/s is combined. As you say, the FSB channels are
unidirectional, and 4 bytes wide. The throughput of one channel is 4
bytes * clock speed / 1.125. (The 1/1.125 factor accounts for
address/command traffic overhead.)
The bus clock speed is 1/2 the CPU core speed on most PowerMac G5s.
So for a dual 2.5 GHz G5, you get 4 * 1.25 / 1.125 = 4.44GB/s in one
direction, 8.88GB/s total.
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