Praetor

Section 5 -- Addendum
Number of Sticks?
Whether to get 1x1024 or 2x512 ... the choice is dependent on the platform. Assuming the sticks are all comparable, if you are using a dual-channel capable system the 2x512 allows the system to actually use the second memory controller so it would be adviseable there. If on the other hand you are using a single channel system (i.e., an older system or a S754 setup) then a 1x1024 will give you slightly (albeit almost nonnoticeable) performance improvement (because the command rate, CR, will be 1 instead of 2) and allows for future upgrading.

What Happens When I Run a System with 4 DIMMs Filled?
What happens when you have all 4 DIMMS populated with RAM? In many cases, your memory clock will drop to the second highest clock speed (provided the memory was able to run at the highest supported clock speed natively). Just look at any older motherboard manual for evidence of this.

With respect to the Athlon64 with on-die memory controlers, those running with "Venice/SanDiego" cores (or newer) will be able to run with all DIMM slots populated at the full speed. With the exception of Athlon64-based processors (i.e., on-die memory controllers), this isnt a concrete rule in that some memory controllers will only drop the speed depending on the type of DDR (and i would imagine DDR2) memory installed (i.e., double sided or single sided).

Generally double-sided (DS) memory is a bit more finicky than single-sided (SS) meaning DS memory will tend to downclock (i.e., 4 sticks of PC3200 will run at PC2700). How do servers, which require craploads of memory ... deal with this? They [generally] use something called Registered Memory which, to simply state, improves signal stability (thus allowing the memory to run at the higher clock)

Timings Examined in Actual Time
As RAM speeds increase so do the timings. DDR-400 can be readily found with a CAS latency of 2, and many DDR2 modules can run a CAS latency of 4 at DDR2-800. And now with the release of DDR3-1333, the common CAS latency is 9 (Kingston has released some that can run CAS7 at DDR3-1333). The question is, how much slower is it really? To answer it we will do some math:

First, a definition: Hertz - A measurement of frequency in cycles per second. One Hertz is one cycle per second.

With that in mind:
DDR-400 (or 200 MHz) completes 1 cycle in 5 nanoseconds.

To actually do the math:
1 second / 200,000,000 Hertz = 0.000000005 seconds / hertz (5 nanoseonds per cycle)

DDR2-800 (400 MHz) completes 1 cycle in 2.5ns.
DDR3-1333 (667MHz) completes 1 cycle in 1.5ns.

DDR-400 @ CAS2 the latency is 10ns.
DDR2-800 @ CAS4 the latency is still 10ns.
DDR3-1333 @ CAS9 is 13.5ns.
DDR3-1333 @ CAS7 is 10.5ns.

This is the same thing that happened with DDR2 when it was first released. The initial speeds have higher real/relative latencies but as it matures the relative latency and the real latency will come down as the clock speed can be raised high enough to cover the gap.

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