The Test
Unfortunately the platform that we were testing on would only allow us to run our DDR2-800 at 4-5-4 timings, instead of the 3-3-3 that's possible with this memory on socket 775. That hurts performance a bit, but the real world difference between 4-5-4 and 3-3-3 isn't going to be more than a few percentage points.
The only DDR2-800 we had on hand was in the form of 1GB modules, so we had to use a pair of 1GB DDR-400 which ran at 2-3-2, instead of the 2-2-2 we normally run with our smaller 512MB modules. Once again, the difference in performance isn't tremendous, but we wanted to explain why the timings were different than what we've used in the past.
Both our Socket-AM2 and Socket-939 Athlon 64 X2 processors ran at the same clock speed with the same cache sizes, so the results should give us a clear indication of whether or not AM2 is faster than equivalent 939 configuration.
| CPU: | AMD Athlon 64 X2 Socket-AM2 AMD Athlon 64 X2 Socket-939 |
| Motherboard: | ASUS A8N32-SLI (Socket-939) Unnamed MCP55 Socket-AM2 Motherboard |
| Chipset: | NVIDIA nForce4 SLI x16 NVIDIA MCP55 |
| Chipset Drivers: | nForce4 6.70 |
| Hard Disk: | Seagate 7200.9 300GB SATA |
| Memory: | OCZ PC8000 DDR2-800 4-5-4-15 (1GB x 2) OCZ DDR-400 2-3-2 (1GB x 2) |
| Video Card: | NVIDIA GeForce 7800 GTX |
| Video Drivers: | NVIDIA ForceWare 84.21 |
| Desktop Resolution: | 1280 x 1024 - 32-bit @ 60Hz |
| OS: | Windows XP Professional SP2 |
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Calin - Monday, April 10, 2006 - link
DDR2-800 at 4-4-4 should be equivalent to DDR-400 at 2-2-2. Also, DDR2-800 at 6-6-6 would be the same (latency-wise) as DDR-400 at 3-3-3.Furen - Monday, April 10, 2006 - link
Not quite, DDR2-800 at 4-4-4 is the equivalent of DDR-400 at 4-4-4 because the memory cells run at 200MHz on both modules. Like I said above, though, module latency is not the only factor affecting the total latency, so perhaps DDR2 memory controllers help mitigate this huge latency hit. One of the main reasons why DRAM manufacturers love DDR2 is because their yields are much higher than they are on higher-clocked, aggresively-timed DDR1 due to the higher prefetch (lower operating clock) and the looser timings.defter - Monday, April 10, 2006 - link
That's not true. "Cas latency" values are relative to the 400MHz clock instead of 200MHz base clock that DDR2-800 has.
Furen - Monday, April 10, 2006 - link
The 400MHz clock is the clock the IO operates at, while the memory arrays operate at half the IO clock, so 200MHz in this case (so yes, DDR2 ram operates at a sort of quad data rate). Since a Column Access Strobe is a memory array operation it is, naturally, measured in memory array clocks. The "base clock" for DDR2 is actually 400MHz because it is the external clock.menting - Monday, April 10, 2006 - link
defter is correct,time delay on memory is calculated by the clk speed that the memory takes in * latency
think of it as a black box operation.
MrKaz - Monday, April 10, 2006 - link
OK. Didnt know that.I always tought that DDR1 2-2-2 was always better than higher DDR2 numbers...
Furen - Monday, April 10, 2006 - link
It is, the main factor affecting latency is the memory cell clock, which runs at the same clock on both modules. So you can do a straight comparison between the two latencies to see which will yield you a better MODULE latency. Of course, module latency is just one part of the whole latency equation, the memory controller being the other big part. Perhaps AMD made the controller more latency-friendly by sacrificing maximum bandwidth, which would explain the abnormally-low usable bandwidth.ozzimark - Monday, April 10, 2006 - link
just something to keep in mind. same 1.8ghz cpu clock:200mhz at 2-2-2 = 51.5ns
300mhz at 3-3-3 = 43.8ns
mhz wins over timings when it comes to comparing absolute latency
Furen - Monday, April 10, 2006 - link
That only applies when comparing the same type of memory.DDR2 memory cells run at 1/4 the "Effective clock," so DDR2 800 runs at 200MHz, which is the same as DDR 400.
ozzimark - Monday, April 10, 2006 - link
true, but you notice the latency that is in the review. seems that what i say holds true to an extentbtw, the timings are in signal clocks last i checked, not cell clocks, which runs at 1/2 the speed of the double data rate signal. kinda confusing, but oh well. point of the matter is that ddr400 at 2-3-2 is higher latency than ddr2-800 at 4-5-4