DDR2 Ram Question

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What would be the appropriate speed Ram for a X2 3800 or X2 4600? Can you
use 533 or 800? If so is there much difference? What has changed from the
Socket A days? It seems like you used to have to get ram of a speed to match
your CPU or if you did not you would either be really overclocking your Ram
or underclocking your CPU. I have not upgraded in about two years so I am a
bit behind hear please help.


Re: DDR2 Ram Question

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Start with Table 4 on PDF page 33:


200-MHz (DDR2-400) PC2-3200 DIMMs
266-MHz (DDR2-533) PC2-4200 DIMMs
333-MHz (DDR2-667) PC2-5300 DIMMs
400-MHz (DDR2-800) PC2-6400 DIMMs

Multiplier    Core          DRAM Frequency
             Frequency   200MHz  266MHz  333MHz  400MHz
   4          800MHz     160.00  160.00  160.00  160.00
   5         1000MHz     200.00  200.00  200.00  200.00
   6         1200MHz     200.00  240.00  240.00  240.00
   7         1400MHz     200.00  233.33  280.00  280.00
   8         1600MHz     200.00  266.67  320.00  320.00
   9         1800MHz     200.00  257.14  300.00  360.00
  10         2000MHz     200.00  250     333.33  400.00
  11         2200MHz     200.00  244.44  314.29  366.67
  12         2400MHz     200.00  266.67  300.00  400.00
  13         2600MHz     200.00  260.00  325.00  371.43
  14         2800MHz     200.00  254.55  311.11  400.00
  15         3000MHz     200.00  250.00  333.33  375.00

So speeds up to DDR2-800 are supported for any processor,
in the sense that they will run. But notice some of
the limitations, in terms of the clocks provided for
the DIMMs.

First off, the multiplier is multiplied by the nominal
200MHz processor clock. That gives the core freq in the
next column. The core frequency is then divided by a
programmable memory divider. Notice that when the core
runs at 800MHz, the memory divider won't go any lower than
5. The highest memory divider seems to be 15.

You can always get the worth of the memory, by raising the
CPU clock above its 200MHz nominal value. So if your
DDR2-800 memory was receiving a 360MHz clock instead of
the 400MHz it would normally get, you could increase the
FSB clock to 200*(400/360)=222MHz. Of course, your core
is also overclocked by 11% when you do that, but then
the memory is running at full speed.

As for latency, you can look at these articles:

Latency values (table) - DDR2-800 4-4-4 is the same as DDR400 2-2-2
So to win on latency alone, a DDR2-800 3-3-3 type memory would
be needed. In practice that is not necessary (DDR2-800 5-5-5
is good enough).


Latency measured -

The SuperPI results suggest the DDR2-800 5-5-5 is good enough -

Applications results (Photoshop CS 2) - Tomshardware is a bit different
I don't know why the DDR400 2-2-2 is winning here.

Looking at prices, there doesn't appear to be too much of a
premium going from 5-5-5 to 4-4-4 (just stay away from modules
with flashing LEDS on them). So the 4-4-4 are within reach, but do
you get enough benefit to be bothered ? Anything faster than 4-4-4
appears to be madness.

BTW - this is not a comprehensive review of all the AM2 benchmarks
out there, or for that matter, even a correct conclusion. Since
you are buying this thing, it wouldn't hurt to dig up a few more
reviews and come up with your own conclusions.

Note - some of the memories state a voltage requirement for
the stated clock rate and timing values. Make sure the motherboard
has adjustable DIMM voltage, to keep a premium memory happy.
And for some reason, some of the DDR2 state "Intel only", and
I haven't heard a story as to why that would be.


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