What does PC1066 mean, and and what advantage does a 1:1 r..

What does PC1066 mean, and and what advantage does a 1:1 ratio confer?

I consider the question definitively setteled. The CPU clock : memory clock
ratio is identical to the FSB : memory bus ratio. The nomenclature is
murky, but DDR2 PC1066 memory is qualified to run with a memory bus of 1066
MHz. The CPU clock : memory clock ratio as it appears on nVidia 680i SLI
motherboards represents the FSB : memory bus ratio. DDR2 PC1066 memory is
required to operate at a 1:1 FSB: memory bus ratio (unless lower rated
memory is overclocked.)

For this system
E4300/ EVGA 680i / Patriot SLI-Ready DDR2 PC1066
FSB at 1200 MHz for CPU speed of 2.7 GHz

Three memory benchmarks in SiSoft Sandra 2007 ver 2007.4.11.22
(Memory Latency, Cache and Memory, Memory Bandwidth)
with memory timing held constant for all memory bus speeds

(Memory timing settings in EVGA 680i BIOS)
SLI Memory [Disabled]
tCL: 5
tRCD: 5
tRP: 5
tRAS: 16
CMD: 2T
tRRD: 3
tRC: 21
tWR: 9
tREF: 7.8 ns

Gave the following results with memory bus speeds of 400 MHz, 600 MHz, 800
MHz, 1200 MHz -
__________
Memory bus = 400 MHz

**Memory Latency**
Random 16 MByte 126.6 ns / 341.7 clocks
Linear 16 MByte 15.4 ns / 41.6 clocks

**Cache and Memory**
Combined Index 12548
Speed factor 104.6

**Memory Bandwidth**
Int. Buffered 4401
Float Buffered 4368
Est. Efficiency 46%
____________
Memory bus = 600 MHz

**Memory Latency**
Random 16 MByte 91.8 ns / 247.8 clocks
Linear 16 MBytes 11.7 ns / 31.6 clocks

**Cache and Memory**
Combined Index 15075
Speed factor 68.7

**Memory Bandwidth**
Int. Buffered 5567
Float Buffered 5091
Est. Efficiency 58%
__________
Memory bus = 800 MHz

**Memory Latency**
Random 16 MByte 81.9 ns / 221.2 clocks
Linear 16 MByte 11.1 ns / 29.9 clocks

**Cache and Memory**
Combined Index 166384
Speed factor 53.4

**Memory Bandwidth**
Int. Buffered 6042
Float Buffered 6021
Est. Efficiency 63%
__________
Memory bus = 1200 MHz

**Memory Latency**
Random 16 MByte 63.5 ns / 171.3 clocks
Linear 16 MByte 9.3 ns / 25.4 clocks

**Cache and Memory**
Combined Index 19725
Speed Factor 36.9

**Memory Bandwidth**
Int. Buffered: 6438
Float Buffered 6442
Est. Efficiency 67%
__________

Hope this helps.

Phil Weldon

Phil Weldon wrote:
> What does PC1066 mean, and and what advantage does a 1:1 ratio confer?
>
> I consider the question definitively setteled.

The processor FSB is 64 bits wide. If operating at FSB1066, data transfer
rate is a maximum of 1066 * 8 bytes = 8528MB/sec.

In a dual channel setup, you have DDR2-1066 (PC2-8500) on each channel.
As the number implies, that means each channel transfers at 8500MB/sec,
and two channels transfer at 17000MB/sec. That is twice the rate that
the FSB can handle.

So, what of it ? The Intel architecture features an external memory
controller. The memory controller is located on the Northbridge
chip. In addition to the connection of the processor FSB and the
memory channels, there are also the PCI Express lanes for the video
card. This could be, for example, PCI Express x16, at 4000MB/sec
transmit and 4000MB/sec receive. So you could have the processor doing
a burst, and the video card doing a bidirectional burst (if such a
thing is possible), and that would more or less fill the memory bus.
The Northbridge also has the DMI interface (hub bus), which could be
another 4 PCI Express x1 lanes worth.

So, in all of that, is there something magic about the clocks on
the memory and FSB ?

Actually, due to the strap in the Northbridge, there is a bit of
unpredictability, about what will happen to performance as you
overclock. In fact, there is a difference in overclock results,
between "nominal BIOS/clockgen overclock" versus "overclock via BIOS".
And that is due to how the Northbridge strap is set up by the BIOS.
Since I like to back up these enthusiast concepts, with a trip to
the datasheet, I was disappointed to find no mention of any of the
details of any "Strap" in the Intel docs. Nor of any "latency setting"
in the Northbridge, that apparently the BIOS sets up. But people
did do enough testing and presentation of their results, to show
there is an appreciable difference between the two overclock methods,
which lends credibility to the strap concept. Even if the proponent of
the strap theory is not able to explain it very well (i.e. in a way
that a hardware designer would understand).

So there are days of reading material ahead of you, if you wish
to learn the details of Core2 overclocking. You have to slog
through a lot of enthusiast chatter, to get nuggets of information.

In case you missed the point of the above two paragraphs, it is
this. You should *benchmark* your overclocking setup, and not
stare at the clocks. The memory and core clock on a Core2 Duo setup,
don't tell the whole story. In fact, you may find a counterintuitive
result, where a setup with a lower set of clock values, is giving
a higher benchmark like SuperPI. Thus, on Core2 Duo, you don't stop
and crack open a beer, after just cranking the clock. There is more
to it than that. And kudos to the guys who took the time to test
and figure it out. I doubt I would have bothered.

Anandtech did some testing here, and in these results, the biggest
"jump" might be at DDR2-533. I believe the top five results are
with a constant core clock, while the bottom three are different.

http://www.anandtech.com/memory/showdoc.aspx?i=2732&p=4

I would say, rather than "the question definitively setteled", you
are now on a "journey of discovery".

Very little of this is explained in datasheets, which annoys me
greatly. I expected better of Intel. I'm not even sure there
is a nice tutorial anywhere, that sums up all the results
collected so far.

Paul