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+).
Facebook
Twitter
RSS
83 Comments
View All Comments
Griswold - Tuesday, May 23, 2006 - link
Sounds conceiveable indeed. Though, the latter option would probably blow TDP out of proportion on 90nm.
mlittl3 - Tuesday, May 23, 2006 - link
Yeah, that is a problem but Anand did say "trick up its sleeve" so maybe they have one last 90 nm manufacturing process that's better than today's. I've read some articles about L3 cache coming for AMD and one inquirer.net article (take with grain of salt) that says AMD will ramp clock speeds fast. Maybe the trick will have something to do with these factors. Who knows?darkdemyze - Tuesday, May 23, 2006 - link
Whatever it is I'm interested in reading about itRegs - Tuesday, May 23, 2006 - link
Whatever it is, it's going to be expensive.TrogdorJW - Tuesday, May 23, 2006 - link
Actually, I was sort of thinking that the "stopgap solution" might be to cut prices. God only knows that I would love to see a $200 X2 processor!Griswold - Tuesday, May 23, 2006 - link
Well, they will have to drop prices at some point after core 2 is actually available.xFlankerx - Tuesday, May 23, 2006 - link
Indeed, same results as expected. Maybe this will make the AMD fanboys shut up about "waiting to see what the final results are." NOTE: I have a AMD system, I'm simply addressing those that refuse to accept Conroe's superiority.Although...I must say that this "stop gap" solution by AMD has piqued my curiosity.
But I believe that these say it perfectly;
"One of its stipulations for sending out Socket-AM2 review kits was that the CPUs not be compared to Conroe."
"We do get a sense of concern whenever Conroe is brought up around AMD."
"So when Intel first started talking about its new Core architecture, we turned to AMD for a response that it surely must have had in the works for years, but as you all know we came up empty handed."
Those just say it all for me. Seems like AMD's in trouble. From what I've been reading, K8L doesn't bring in architectural changes either. Sure you get Quad Cores, L3 cache, FB-DIMM support, DDR3, and faster HyperTransport, but if AMD doesn't improve on it's performance-per-clock efficiency, then Intel's Quad Cores (due almost 9 months before AMD's) are going to rule supreme yet again.
Griswold - Tuesday, May 23, 2006 - link
Maybe read up on it first.
Memory mirroring, data poisoning, HT retry protocol support, doubled prefetch size (32byte instead of 16), 2x 128bit SSE units (instead of 2x 64bit), out of order load execution, Indirect branch predictors and a handful new instructions sure sounds like a few architectural changes and not just a simple revision stepping.
rADo2 - Tuesday, May 23, 2006 - link
Sorry, links again:Intel Conroe @ 3.9GHz: SuperPI 1M - 12.984s
http://www.xtremesystems.org/forums/showthread.php...">http://www.xtremesystems.org/forums/showthread.php...
AMD FX-57 @ 4.2GHz: SuperPI 1M - 21.992s
http://www.xtremesystems.org/forums/showthread.php...">http://www.xtremesystems.org/forums/showthread.php...
MadAd - Monday, May 29, 2006 - link
Try measuring like for like and then come back with your silly benchmark comparison. EG use a superpi data size that will fit on BOTH cpus caches, not just conroes and then compare performance.With the FX57 having just a 1M cache its bullsht smoke and mirrors saying the 1M superpi is slower, o rly? perhaps thats because it takes more than 1M to hold both the feature and data sets on a 1M superpi.
muppet