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
POST A COMMENT

80 Comments

View All Comments

  • Furen - Wednesday, May 24, 2006 - link

    http://img.clubic.com/photo/00119525.jpg">http://img.clubic.com/photo/00119525.jpg

    Look at that and tell me how you can possibly fit twice that (90nm dual-core) in one package. Dual-core CPUs are huge to begin with, doubling the number of cores would probably require a pretty big drop in L2 sizes (think 256KB per core...). AMD still is production limited and designing a quad-core chip without going to 65nm would pretty much doom it to being a VERY low-volume part. Heck, Intel's Conroe is huge as well, it's just on a smaller process (the 160sq. mm die size would correspond to something like 300+sq. mm on the 90nm process).
    Reply
  • jones377 - Tuesday, May 23, 2006 - link

    It's called Socket F Reply
  • Griswold - Tuesday, May 23, 2006 - link

    I dont think so. Socket F isnt really a "secret" nor a stopgap solution. Reply
  • peternelson - Tuesday, May 23, 2006 - link

    I think we hear more about socket F in June and it launches July.

    But that's not what this is alluding to.

    There was an announcement of a roadmap change from Q1/2007 to DECEMBER 2006.

    If I remember right it was two AM2 processors on 65 nanometre process.
    Reply
  • jones377 - Tuesday, May 23, 2006 - link

    Perhaps not, but it's coming out at around that timeframe. Anything else and we would have gotten wind of it long ago. Reply
  • mlittl3 - Tuesday, May 23, 2006 - link

    A couple of things before I give my guess about the stopgap solution...

    1) K8L as state above WILL HAVE microarchitectural improvements. This has been all over the internet.

    2) AMD's processor pricing page states that the X2 5000+ and FX-62 will be available for both 939 and AM2. I don't know if they messed up but if not, it looks like 939 users can upgrade yet again.

    Okay, here's my guess for the stopgap solution...drum roll...L3 cache. I think AMD will release a 2.8 revised FX-62 with L3 cache or an ahead of schedule 3.0 GHz FX-64 with L3 cache. Just my guess.
    Reply
  • AllYourBaseAreBelong2Us - Tuesday, May 23, 2006 - link

    The stopgap solution is the 65nm process that will allow AMD to ramp up the speed a bit more and get better TDP ratings.
    Reply
  • Griswold - Tuesday, May 23, 2006 - link

    quote:

    Okay, here's my guess for the stopgap solution...drum roll...L3 cache. I think AMD will release a 2.8 revised FX-62 with L3 cache or an ahead of schedule 3.0 GHz FX-64 with L3 cache. Just my guess.


    Sounds conceiveable indeed. Though, the latter option would probably blow TDP out of proportion on 90nm.
    Reply
  • mlittl3 - Tuesday, May 23, 2006 - link

    Yeah, that is a problem but Anand did say "trick up its sleeve" so maybe they have one last 90 nm manufacturing process that's better than today's. I've read some articles about L3 cache coming for AMD and one inquirer.net article (take with grain of salt) that says AMD will ramp clock speeds fast. Maybe the trick will have something to do with these factors. Who knows? Reply
  • darkdemyze - Tuesday, May 23, 2006 - link

    Whatever it is I'm interested in reading about it Reply

Log in

Don't have an account? Sign up now