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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  • 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. Reply
  • 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. Reply
  • 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! Reply
  • 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.
    Reply
  • 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? Reply
  • 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. Reply
  • 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.
    Reply
  • peternelson - Tuesday, May 23, 2006 - link


    I see no benchmarking in 64 bit mode.

    This is the future, and for maths, Intel's 64 bit was more of a lame copy of AMD 64 bit performance.

    In future this will be increasingly important so even if 32 bit performances are comparable, I'd want to make sure the picture is the same running 64 bit apps.


    Also you summarise "same performance, faster memory, less power". True, but you FORGET one of the main benefits: Pacifica Virtualisation.

    True hardware virtualisation adds to the actual WORK you can keep that processor busy with. It saves time by letting you switch OS instances without rebooting timewasting.

    As it is hardware based VT you should even be able to virtualise an UNMODIFIED OS like Win XP, maybe even Vista!

    So please play with Xen3.

    As you say virtualisation "works" then it is a BIG factor for me in choosing AM2 over 939, (all other things being equal).

    Also the fastest 939 chips have been produced, and AM2 is reaching higher models now.

    So if you want the VERY fastest, it is only available on AM2.

    Don't forget: Not just performance, but performance PER WATT. For these AM2 chips that is similar to 939.

    However, the announcement of 65W EE and EVEN 35W SFF EE!!! are significant compared to the standard 89W

    Intel seem to be positioning Conroe as being "33% better" on performance per watt. However, Conroe isn't even here but when it is, it may not be able to compete with AMD low power offerings.

    Also consider the whole system for conroe vs AMD. Because that AMD power INCLUDES the memory controller, whereas Intel doesn't. The whole motherboard etc may use less power.

    Also in terms of entire system cost, motherboards for AM2 appear to be a bit cheaper than their Intel equivalents, which may offset the current high prices of AMD processors.
    Reply
  • fitten - Wednesday, May 24, 2006 - link

    You should check out the Woodcrest (server targeted Conroe core) previews for power measurements. Performance per Watt, Woodcrest wins (and will be available in 3 weeks)... Absolute power usage under load, Woodcrest wins... and note that the power measurements are for the complete system (video card and HDDs included). (Deep power conservation couldn't be tested on Woodcrest because the parts didn't have it enabled as they were engineering samples.)

    Also, check out the 64bit vs. 32bit comparisons in programs like Cinebench 9.5. Seems Woodcrest 64bit gives a nice boost there (showing that it isn't just a 'lame copy').

    You also seem to forget that Intel already has virtualization extensions out in currently shipping processors (much less Conroe+).

    As far as price, there have been price lists published already. High end Conroe parts are already listed for 1/2 the price of the high end AMD parts... at $500 that gives another $500 for purchase of a motherboard before it touches just the CPU cost of the AMD... I doubt that the motherboards will be that expensive.

    I have 7 AMD machines (four are Athlon64s or X2s) but right now, it looks like my next machine will be a Core2 one. AMD needs to get an answer out... soon. K8L isn't going to cut it. Sure, it'll be good at FPU but the vast majority of work done by CPUs is integer, which are what the majority of improvements are in Core2 (not that they don't have good FPU improvements). So, if you're in a government lab running FPU intensive simulations, K8L may be for you. If you're anyone else, K8L as it has been described looks kind of anemic and not a match for Core2.

    Maybe the real K8L will surprise us, who knows, but it is at least 6 months away (if not longer). By that time, Intel will already be 25% into it's 2-year cycle for the next Core derivative chip (probably farther, time between releases is set to 2-years). AMD looks to be in a bad situation right now... If they have something they're keeping secret, IMO, they need to at least tease us with it. K8L is not a tease, it's only slightly more than a stifled yawn. The longer they go without giving us something to look forward to, the more it looks like they are in major trouble.
    Reply
  • Accord99 - Wednesday, May 24, 2006 - link

    [quote]Don't forget: Not just performance, but performance PER WATT. For these AM2 chips that is similar to 939.

    However, the announcement of 65W EE and EVEN 35W SFF EE!!! are significant compared to the standard 89W

    Intel seem to be positioning Conroe as being "33% better" on performance per watt. However, Conroe isn't even here but when it is, it may not be able to compete with AMD low power offerings. [/quote]
    Given that Woodcrest 3.0GHz has a TDP of 65W, which is borne out by power measurements conducted by Techreport and 2CPU, it's likely that a Conroe that matches the performance of the 35W X2 will at the very least, also match it in power.
    Reply

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