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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  • MacGuffin - Tuesday, May 23, 2006 - link

    From Patriot's PDC22G8000+XBLK Rev. 2 review on PCSTATS.
    Rated for DDR2-667 @ 3-3-3-9 (Maintains those timings through DDR2-940!)
    Rated for DDR2-1000 @ 4-4-4-12 (Goes Up to DDR2-1020!)
    Completely stable on the Intel platform they used. It's extremely expensive (saw it for $400+ at NewEgg). But yes, it is possible to run 2GB at these timings already. Its just extremely expensive.
    Reply
  • EdisonStarfire - Tuesday, May 23, 2006 - link

    any opinions on AMD offering a Clearspeed solution as stop-gap in the high end desktop arena ? Reply
  • Griswold - Tuesday, May 23, 2006 - link

    The bottom line is, we now know what we knew last fall, or rather (rightfully) assumed.

    quote:

    AMD does have one last trick up its sleeve before the end of the year, and you will hear about it in June. It's not K8L and it's not going to affect the majority of people, but it is an interesting stop gap solution for the high end in 2006...


    Now you made me curious. Could that be the "noise in june" which Henri Richards mentioned in a Register interview earlier this month?

    Reply
  • smn198 - Wednesday, May 24, 2006 - link

    It is called quad-core. Reply
  • temp2 - Tuesday, May 23, 2006 - link


    The extremetech.com article has a similar teaser at the end, but it is slightly more specific:

    "And given recent discussions with AMD, we can safely say that the company hasn't launched its last FX series CPU for the year quite yet."
    Reply
  • mino - Wednesday, May 24, 2006 - link

    This provided, 3.2 or even 3.4 FX's on 65nm are on the way... Reply
  • Scrogneugneu - Tuesday, May 23, 2006 - link

    Beware the mighty Sempron FX 32 ! Reply
  • mino - Tuesday, May 23, 2006 - link

    2 options:
    1) Quad-core K8 (on 65nm)
    2) High-speed 65nm DC's(improbable)

    BTW what I understand FX-64 is on the way in a few months(july-august). Seems rev. F cores could handle 3.0, just 125W TDP may be the issue.
    Reply
  • peternelson - Tuesday, May 23, 2006 - link


    Well, I don't think it's a QUAD CORE K8 (aka "Deerhound") because that is not due until late 2007.

    And dualcore K8L is not until 1H/2007.

    We need to choose something happening THIS year.
    Reply
  • mino - Wednesday, May 24, 2006 - link

    Actually AMD can made Quad-core CPU's even on 90nm if they need to. The core will be huge, yields poor but IMHO 2.2G Quad at 90nm is possible within 125W TDP.

    Also AFAIK AMD has delayed 65nm at least for a quarter intentionally since what they need now is capacity on 90nm. They could not afford any (even short-term) production reduction at this moment. Provided in 3Q/06 FAB 36 is up and running at 10k starts the could afford to dedicate some of them for some high-end opterons and FX's.
    Reply

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