David Maynard wrote:
BigBadger wrote:

No it's not 333MHz, it's actually a 166 'MHz' FSB processor....333
is just AMD hype to sell the virtues of the DDR bus. Intel do the
same trick but they multiply the real bus speed by 4x.

Double and quad pumping the bus is not "hype." It's an engineering
technique for transferring data twice, or 4 times for quad, per clock
cycle.

333 is the bus cycle rate, e.g. "Bus Speed," and is the relevant
number from a performance standpoint.

Oh dear, oh dear, here we go again ... It depends on whether you measure
the control lines or the data lines for quoting the "bus speed" number.

I actually think a more accurate way of representing it (performance-wise)
is a 128-bit bus (DDR) or 256-bit bus (QDR), both running at 166MHz. A good
example is the latency for main memory. Say you have a 166MHz DDR system
(aka DDR333), and a 100MHz QDR system (aka QDR400), and the CPU runs at 1GHz
(6.0x for DDR, 10.0x for QDR). Excluding memory latencies, to fill a
randomly-accessed 64-byte cache line would take:
Waiting for bus strobe: 3.0 cycles (DDR system), 5.0 cycles (QDR system)
Transferring data: 24 cycles (DDR), 20 cycles (QDR)
Total: 27 cycles (DDR), 25 cycles (QDR)
So DDR333 is, under random access conditions, only marginally slower than
QDR400. The actual break-even point is 180MHz (actually slightly above due
to memory latencies), but hopefully you get the idea. Of course, the QDR
system will perform better under "streaming" type conditions, where the
higher latency won't matter so much.

Incidentally, this issue is exasparated by the P4's 128-byte cache line, as
opposed to the 64-byte cache line of the K7.

Finally, using the non-multiplied speed would stop people complaining that
their motherboard only goes up to 250MHz

[...]

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Michael Brown
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