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+).

Athlon 64 X2 5000+: A Cheap FX or Overpriced 4800+? Power Consumption
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• rADo2 - Tuesday, May 23, 2006 - link

K8L is just a marketing, nothing else. Have you seen K8L CPU? No? AMD is about 2x slower than upcomming Conroe, so they have to spread some fud, to keep their fanboys happy...

This is the magical performance I am speaking about, AMD cannot come even close:

Intel Conroe @ 3.9GHz: SuperPI 1M - 12.984s
http://www.xtremesystems.org/forums/...ad.php?t=99...">http://www.xtremesystems.org/forums/...ad.php?t=99...

AMD FX-57 @ 4.2GHz: SuperPI 1M - 21.992s
http://www.xtremesystems.org/forums/...d.php?t=100...">http://www.xtremesystems.org/forums/...d.php?t=100...

I own X2 4400+ myself (it was a good choice in 6/2005), but within last few months AMD is a very bad choice, as for the price of quite obsolete singelcore AMD you can buy dualcore Intel D930 @ 65nm, and later Conroe. I think only AMD fanboys are buying AMD now, AMD has the worst price/value ratio, and Conroe will only make this gap wider.
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• Griswold - Tuesday, May 23, 2006 - link

Oh and yea, I run superpi all day long because its such a valuable application that earns me money! :P Reply
• rADo2 - Tuesday, May 23, 2006 - link

SuperPI tells A LOT about gaming performance ;) Reply
• mesyn191 - Wednesday, May 24, 2006 - link

SuperPi tells you nothing except how well a CPU runs SuperPi, its not a benchmark.

Its also about as in cache and branchless as your gonna get BTW so the performance increases you can get on it by simply scaling clockspeed are impossible as well.
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• Questar - Tuesday, May 23, 2006 - link

SuperPi is an outlier in Conroe benchmarks. Reply

O rly? Reply
• Griswold - Tuesday, May 23, 2006 - link

Blablabla... Reply
• absolsp - Tuesday, May 23, 2006 - link

As suspected, not much of performance gain. Happy with my existing AMD setup. Reply
• tony215 - Wednesday, May 24, 2006 - link

likewise, I will be sticking with my 939 venice set-up until conroe is released. Even then, I will wait for some more independent conroe test/reviews before going with Intel. Reply
• Locutus465 - Tuesday, May 23, 2006 - link

After reading reviews of the new chipset offerings from nVidia and ATI, personally I'm glad I'm running an nForce 4 s393 board. Seems to me the new AM2 chipsets and boards are going to need some maturing before they get good. The new solutions were *not* deffinitivly better than what is out there for s939. In fact, nVidia's offering in my opinion was particularly lack luster in terms of actual performance (compared to the older nForce 4 platform). Reply