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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  • soydios - Tuesday, May 23, 2006 - link

    AMD motherboards are less expensive because they don't have to put in a memory controller.

    AMD processors are more expensive for 2 reasons:
    - integrated memory controller takes up more die space (offset by cheaper motherboard)
    - AMD is still using 90nm on 200mm wafers, while Intel is using 65nm on 300mm wafers (Intel gets more CPUs per wafer bigtime)
    Reply
  • peternelson - Tuesday, May 23, 2006 - link


    Sempron AM2 can do memory up to DDR2-667

    Dualcore AM2 can do memory up to DDR2-800

    However, PLEASE CHECK SINGLE CORE MEMORY SPEED (multiplier issues aside) which you say limited to 667 whereas I got the impression they can also do 800 like dualcores. Correct as necessary.
    Reply
  • smitty3268 - Tuesday, May 23, 2006 - link

    I accidentally hit the "not worth reading" button, so I'm writing this comment to undo it :) Reply
  • fikimiki - Tuesday, May 23, 2006 - link

    There are a couple of reasons for that:
    - K8L photo had a Z-RAM implemented, so they are using this kind of cache for a quite long time.
    - Shared L3 should help Athlon64 in matching Super-Pi and overall performance.
    - Usage of Z-RAM will reduce cache die size by 75% with no architectural changes.

    So FX-64 to beat fastest Core 2 just needs 4MB of cache...
    Easy trick but can be useful to survive till 65nm production...
    Reply
  • Questar - Tuesday, May 23, 2006 - link

    quote:

    K8L photo had a Z-RAM implemented, so they are using this kind of cache for a quite long time.


    It's not going to be Z-RAM. Z-RAM won't even be in K8L.

    “We’ve looked at data from Innovative Silicon and it looks very promising. We still need to assure ourselves that this will work in our own application. We need to see how it scales and we need to make our own test vehicles,”

    Jones, an executive experienced in intellectual property licensing, also declined to comment on AMD’s timetable for introduction of Z-RAM but offered a more general perspective. “In the past it has been two years from when you sign a deal to when it is in production.”


    http://www.eetimes.com/news/latest/showArticle.jht...">http://www.eetimes.com/news/latest/showArticle.jht...
    Reply
  • munky - Tuesday, May 23, 2006 - link

    I think the June trick AMD will pull out is the Clearspeed coprocessor. It definitely won't affect many users, but for those who do invest in the technology, it could provide a decent boost in number crunching power. Reply
  • peternelson - Tuesday, May 23, 2006 - link


    Clearspeed are working on being one acceleration solution, yes, but the already launched acceleration on socket 940 is companies offering plug in Xilinx4 FPGA on hypertransport.

    I hope that gets re-engineered onto socket F pretty quickly. We may see announcements once socket F is actually launched in July.
    Reply
  • darkdemyze - Tuesday, May 23, 2006 - link

    z-ram isn't due for AMD procs for quite some time, I doubt this is their plan for June.. Reply
  • mlittl3 - Tuesday, May 23, 2006 - link

    Basically this is what I said above for my guess of the "trick" AMD will use. Anand said it will only affect some high-end users, read FX series so it can't be price cuts as some have suggested (that would effect everyone). Adding L3 cache is the only performance improvement I can think of that doesn't require changing the microarchitecture of the cores (well at least not a big change).

    However, TDP is still an issue here as someone above suggested. I don't know how much more power it takes to run L3 cache. Last time AMD did it was on K6 and power wasn't really measured back then.

    By the way, please ignore Questar's comment below about z-ram being pig slow. I really don't think he knows what he is talking about. /shields eyes from incoming Questar flame
    Reply
  • johnsonx - Wednesday, May 24, 2006 - link

    K6-III did not have L3 cache. It had L2 cache, making the cache that all socket-7 boards had then an L3 cache.

    So, let's stop saying things like 'AMD hasn't done L3 cache since K6-III', etc.
    Reply

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