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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  • 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.
    Reply
  • Questar - Tuesday, May 23, 2006 - link

    SuperPi is an outlier in Conroe benchmarks. Reply
  • Griswold - Tuesday, May 23, 2006 - link

    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
  • Puddyglum - Tuesday, May 23, 2006 - link

    Do these results really warrant a change of architecture? I wonder if there is some bottleneck keeping the performance stuck where it was with the 939's. Reply
  • Questar - Tuesday, May 23, 2006 - link

    It's not an architecture change.

    It's a new socket to support DDR2 memory, that's it.
    Reply
  • xFlankerx - Tuesday, May 23, 2006 - link

    Athlon 64s simply don't need the additional bandwidth provided by DDR2. They aren't as starved for it as Pentium 4s. Reply

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