AMD Socket-AM2: Same Performance, Faster Memory, Lower Power
by Anand Lal Shimpi on May 23, 2006 12:14 PM EST- Posted in
- CPUs
The Odd Multiplier Issue
Another item that was working against the Athlon 64 X2 5000+ on the previous page was the fact that it used an odd clock multiplier, in this case 13.0, in order to achieve its 2.6GHz clock speed. The problem with odd clock multipliers on AM2 CPUs is that the memory controller actually runs at DDR2-742 instead of DDR2-800. AM2 CPUs with even clock multipliers can run at DDR2-800 without any problems, and the reason why is pretty simple.
Below is the equation for calculating the memory speed of any Athlon 64 processor:
Reference Clock * Clock Multiplier = CPU Frequency
CPU Frequency / Memory Divisor = Memory Frequency
AMD only supports integer memory divisors, but let's start out by looking at how an AM2 CPU with an even clock multiplier fits the equation. For example, an Athlon 64 X2 4800+ runs at 2.4GHz and supports DDR2-800.
200MHz Reference Clock * 12x Clock Multiplier = 2400MHz CPU Frequency
2400MHz CPU Frequency / 6 = 400MHz DDR2-800 Memory Frequency
No problems, right? Now let's see how an odd clock multiplier changes things:
200MHz Reference Clock * 13x Clock Muliplier = 2600MHz CPU Frequency
2600MHz CPU Frequency / 6 = 433MHz DDR2-866 Memory Frequency
2600MHz CPU Frequency / 7 = 371MHz DDR2-742 Memory Frequency
See a problem? Because we can only use integer memory dividers, the only options for memory speed on a CPU with an odd clock multiplier are DDR2-866 or DDR2-742. Since AMD can't run above DDR2-800 spec, the only option is to underclock the memory to DDR2-742. This wasn't a problem on Socket-939 CPUs because DDR-400 ran at a 200MHz frequency, which you could always obtain by dividing the CPU clock frequency by an integer (since AMD never supported half multipliers). In fact, you simply used the same integer as the CPU multiplier. With DDR2-800, you need a 400MHz clock frequency, which you can only generate if you have an even CPU clock multiplier.
The problem gets even more complicated when you take into account the fact that Semprons and single-core Athlon 64s only support DDR2-667, which also has a similar issue.
While we haven't seen any significant downside to only running at DDR2-742 vs. DDR2-800, it is something to keep in mind when deciding what CPU to purchase. If you want your memory controller running at DDR2-800, you may want to stay away from the odd clock multiplier CPUs (X2 5000+, 4400+ and 4200+).
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jmke - Wednesday, May 24, 2006 - link
here ya gohttp://www.digit-life.com/articles2/mainboard/ddr2...">http://www.digit-life.com/articles2/mainboard/ddr2...
Xenoid - Tuesday, May 23, 2006 - link
Ok so AM2 is AMD's offering for 2/4 06.The article title mentions same performance, faster memory, lower power. Wouldn't faster memory (in this case, ddr2) net a higher performance than what we're seeing here? Why is AMD bothering with DDR2 if it's not a significant improvement? If the power usage is so low, does this mean we can overclock a lot easier? I never understood the huge deal behind power usage on a cpu.
coldpower27 - Wednesday, May 24, 2006 - link
DDR production is slowing down, and DDR2 is continuing to mature, AMD does need to change to this memory type now, regardless if they like it or not.DDR2 also allows higher capacities, so you can probably reach 4x2Gb now as 2Gb modles are actually available on DDR2.
Considering DDR2 biggest advanatge bandwidth, is what AMD doesn't really need more fo right now, performance improvements will be negligible.
Furen - Wednesday, May 24, 2006 - link
Also, having all that extra bandwidth available allows AMD to throw quad-core on the same socket without much problem (maybe 1H07... whatever everyone says I doubt AMD will let Intel have the quad-core advantage for a year, I'd say we'll see very low volume quad-cores as close to Intel's Kentsfield/Cloverton as humanly possible). I know we've heard that AM3 is coming next year (from, who else?, The Inquirer) but considering that the DDR3 spec is not finalized quite yet and just how slowly AMD jumped into the DDR2 bandwagon I'd say we won't see it until 2008 at the earliest.Axloth - Wednesday, May 24, 2006 - link
I think there is also marketing side. There are lots of people "unaware" of ddr1-ddr2 comparison. And they probably think that ddr2 "must" be better than ddr1 because of that that "2". Like: ddr2 is upgrade or next generation of ddr1 so its gotta be much faster. Also, they might go for intel because intel uses ddr2 and amd only ddr1... And they think intel's better thanks to ddr2, disregarding cpu qualities of both amd and intel.pzkfwg - Tuesday, May 23, 2006 - link
If DDR2-800 barely beats DDR-400, I was wondering if the AM2 socket could actually be slower than 939 DDR-400 when using DDR2-667 !?! Knowing that a very large amount of people would buy cheaper AM2 system with DDR2-667, that would be ridiculous!mino - Wednesday, May 24, 2006 - link
YES and NO.Remember most people buy generic CL3 or CL2.5 DDR400. IMHO generic DDR2-666 should be ona par with that.
soydios - Tuesday, May 23, 2006 - link
So, the X2 4200+ will not run the memory at full speed. How safe would it be to overclock from 200x11=2200MHz DDR2-733 (2200/6=366x2=733) to 219x11=2200MHz DDR2-803 (2409/6=401.5x2=803) using OCZ DDR2-800 RAM and an Asus Xpress3200 motherboard?Furen - Wednesday, May 24, 2006 - link
I'd say that you can very likely get away with that overclock with pretty much every 4200+ as long as the motherboard allows you to do it.mino - Wednesday, May 24, 2006 - link
Safe as safe. At least from the point it won't blow up :)As for stability it all depends on the motherboard.