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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  • mino - Wednesday, May 24, 2006 - link

    Well, IMHO the point is AMD has used exclusive 3-level cache structure in the past so they have som experience with thi arrangement. Reply
  • Questar - Tuesday, May 23, 2006 - link

    No flame here, look it up for yourself.

    Z-RAM has high capacitive loading, which results in slow speed.

    At 4MB it'll run half the speed of SRAM.
    Reply
  • Questar - Tuesday, May 23, 2006 - link

    Large amounts of Z-RAM are pig slow. Reply
  • Ecmaster76 - Tuesday, May 23, 2006 - link

    Seriously, the one area an Athlon X2 would be bandwidth starved and does it get tested in the preview? NO
    In the review? NO

    How long ago did we know that the K8 was not bandwidth limited in single application usage? YEARS


    So yeah, DDR2 din't increase the 3dMark, big surprise
    Reply
  • mlittl3 - Tuesday, May 23, 2006 - link

    I think 3dMark06 is multithreaded now so all available cores and bandwidth should be used within the limits of the program. I could be wrong about this however. Reply
  • Ecmaster76 - Tuesday, May 23, 2006 - link

    3Dmark06 is almost completely GPU limited. The 3Dmark CPU score did increase a bit, but I really was referring to graphics benchmarks in general. Reply
  • cscpianoman - Tuesday, May 23, 2006 - link

    I was just noticing the performance differences between the FX and the EE. In some cases the FX tromps the EE by "gasp" 30%! In other cases the EE makes it's mark. This is part of the reason I am skeptic on Conroe. Yeah it's good. But I always take what Intel, or AMD for that matter, with a grain of salt. Just today we saw the 30% advantage translate down to about 15%. This seems just like any other generation change where 15% is to be expected. The current hype for the Conroe is a product of Intel's excellent marketing dept. Reply
  • mlittl3 - Tuesday, May 23, 2006 - link

    Until K8L (which will have microarchitectural improvements), there are a few things that could allow AMD to look good against Core 2.

    1) Price drops so that comparable K8 and Core 2 processors are the same price giving the same performance/$ ratio (this metric is important for the budget constrained)

    2) Nov. '06 release of 65 nm AM2 processors so that K8 and Core 2 processors will have the same performance/watt ratio (65 nm could give K8 a 20% drop in power and with Core 2 being 20% faster, they will have the same performance/watt ratio which is popular now). Also, everytime AMD transitions to a new die process they add some minor bug fixes and minor memory/microarchitectural enhancements which could also boost performance by a few percent.

    3) Continual improvements to DDR2 latency might yield a 2-2-2 DDR2 800 memory module which will probably benefit K8 more (maybe ~5% improvement) than Core 2 but this is a wild guess here and I don't know if it is even possible. However, DDR400 latency started around 4-4-4 and dropped to 2-2-2 so it could happen.

    With the same performance/price and performance/watt as Core 2, K8 could stay competitive and OEMS and users decide on which company (if not both) they would like to do business with. This is all speculation and of course everyone is more than welcome to rip my reasoning to death.
    Reply
  • JarredWalton - Tuesday, May 23, 2006 - link

    I seriously doubt we will ever see DDR2-800 running at 2-2-2 timings. (Feel free to quote me on this in the future and make fun of me if I'm proven wrong. :-)) Just think how long we had DDR memory around, and no one ever managed to create 1-1-1 DDR-400 memory. I do think we will see 3-3-3 DDR2-800, and possibly even higher bandwidth with those timings. In fact, we almost have that already judging from my experiences so far with socket AM2. (I can post and run benchmarks, but I wouldn't call the system 100% stable.) Reply
  • mlittl3 - Tuesday, May 23, 2006 - link

    LOL! I will! Reply

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