Dual channel support for Sempron series socket A's. Anyone know?

By Vigilante
Feb 27, 2006
  1. Specifically, does anyone know if there is any benefit to running dual channel RAM with a socket A Sempron 2500+ or faster/slower Semprons?

    Because I know for the Athlons, the CPU needs to have dual pipes or dual something, in order for dual channel to be effective. So I'm wondering if running D/C on a Sempron gains any performance at all. I did some googling but couldn't find a definitive source for this info.

  2. DonNagual

    DonNagual TechSpot Ambassador Posts: 2,404

    Well, I don't have any definitive links I can give you on whether or not there would be any performance gains, but I believe only the socket 939 Athlons utilize dual channel ram.

    And I do not believe there is any performance difference between "ram A" and "dual channal ram A" except for the fact that the dual channel ram has been tested and pre-approved that the two sticks of ram will work together. I do not believe you'd get any performance increase in using "dual channel" ram on a socket A motherboard.

    But I am simply going on my understanding of dual channel, and have no proof to back this up....

    I'm going to watch this thread to as a learning experience ;)
  3. Vigilante

    Vigilante TechSpot Paladin Topic Starter Posts: 1,666

    I found out a little bit more info.

    Namely that, if what I read is true:

    A) For Socket A, the dual channel support is ONLY motherboard chipset dependent, so you reap the benefits no matter the CPU. However this seems odd because the MAX CPU external clock is 400mhz, which two DDR3200 in D.C. should technically be 800, the cpu FSB wouldn't handle 800. So still not sure if there is a benefit. But I am pretty sure that D.C. mode has naught to do with the CPU itself.

    B) For socket 754 boards, D.C. is completely null and means nothing. You get no benefit. Or maybe 754 boards don't have that configuration anyway?

    C) For 64bit Athlons, the memory controller is built into the CPU, and NOT the chipset, so D.C. DOES depend on the CPU being used.

    Anyone care to validate and clarify?

    Also note that this info is only for AMD. Intel is an entirely different issue. As I've learned, Intel CPUs multiply the FSB by FOUR. I didn't know that. So on AMD when the FSB is 200, which is doubled, you get a rated 400FSB. But Intel's 400FSB is actually 100mhz that is quadrupled. Dual Channel support on Intel's are fishy, because if you have their fastest 1066fsb, the RAM doesn't even exists now that runs that fast. Which would be DDR2-400 quadrupled. Unless this RAM is already out.

    Anyway, I'm still pretty vague on these points.
  4. DonNagual

    DonNagual TechSpot Ambassador Posts: 2,404

    I just did some research myself, and learned that the 939 AMDs are the exception. I knew the 754s couldn't support dual channel, but I didn't know that before the memory controllers where moved to the CPU, there were some mobos supporting dual channel configs.

    For AMD, the nVidia nForce 2 and VIA KT880 chipset apparently do support dual channel ram. Learn something new every day!
  5. DonNagual

    DonNagual TechSpot Ambassador Posts: 2,404

    I could be wrong, but it sounds like you are mixing up "DDR" and "dual channel". If your system's FSB is 200Mhz and you are using DDR, your ram runs at 400Mhz. But dual channel is something completely different. It gives a nice performance leap, but is not dependant or related to the FSB.

    Sorry if I am stating something you already know. Just slap me. Every once and a while, I need a good slap.
  6. Didou

    Didou Bowtie extraordinair! Posts: 4,274

    It is very dependent on the FSB, at least on CPUs that still use a traditional FSB (which means all Intel chips & all AMD chips prior to Athlon64s).

    The FSB is what links the CPU to the Northbridge & the rest of the system including memory. Let's take the example of an AthlonXP with a 166MHz FSB (333 DDR). Such an FSB is capable of transfering about 2.7GB/s of data which is perfect for PC2700 Ram as it can also transfer such an amount of data.

    When you run the PC2700 ram in dual channel, it will be able to transfer 2.7 x 2 ~= 5.4GB/s. Half of this memory bandwith goes to waste as the CPU cannot use more then 2.7GB/s.

    So to answer your question Vigilante, dual channel for such a system is overkill. The performance incerase is close to nonexistent.
  7. Vigilante

    Vigilante TechSpot Paladin Topic Starter Posts: 1,666

    That's what I thought. But it gets more interresting.

    If you have a socket A board and a CPU that supports 400fsb (200mhs * 2). The the fastest RAM you can use would be DDR400. Right. But if you put that DDR400 in D.C. mode, it can technically transfer at 6400MB/s, or twice the 3200MB/s. However, since the CPU is still limited to 400, this gives you no gain.

    But then lets say you're on a budget, and you have 2 sticks of PC2100, or DDR266, this is only running half the speed your CPU supports, but if you put 2 sticks of pc2100 in D.C. mode, it would raise the speed back up to the 400 speed, thus running 2 sticks of PC2100 in D.C. gives you the same performance as running a single PC3200. Unless I'm wrong.

    Lastly, as pertaining to my original question. Athlons are just to expensive anymore to buy, but the problem is that all these Semprons use 333fsb for some reason. Only the 754 versions seem to have 400fsb support. So then I am thinking, if I buy a DDR400 stick in single channel, my RAM is already going faster then the 333 CPU can support, thus the speed is already wasted, regardless of D.C., in which it would be faster, but have no benefit at all.

    Oh ya, and I wasn't mixing up D.C. with DDR. Just saying that, your CPU's fsb speed, such as 100mhz, is doubled out the door to make a 200mhz FSB. Thus matching perfectly to DDR266 (PC2100) RAM. BUT, D.C. doubles that rate yet again, thus giving your RAM the speed of a 400fsb CPU. However, since the CPU fsb is limited to 200 anyway, the D.C. would not give any gain. Which makes me wonder if running D.C. at all on AMD chips ever did have any benefit? Assuming your already had RAM whos speed matched the CPU's fsb.
    This all comes to nothing anyway when I realize the Sempron has a 333 bus, so D.C. would mean nothing if I have DDR400 RAM.

    But it's to late! I already bought a 1GB Corsair kit last night. Doesn't matter anyway, it's 1gb either way I configure it. And it was a dollar or two cheaper then the single 1gb stick.
  8. Didou

    Didou Bowtie extraordinair! Posts: 4,274

    PC2100 runs at 133MHz (266 DDR) for a peak bandwidth of 2.1 GB/s (thus the name PC2100). If you run it in dual channel, at its peak it can transfer 2.1 x 2 ~= 4.2 GB/s. Quite a bit more then PC3200.
    A 100MHz (200 DDR) FSB matches perfectly with PC1600, not PC2100.
    It's worth it on socket 939/940 parts, close to useless on previous AMD parts.
  9. Vigilante

    Vigilante TechSpot Paladin Topic Starter Posts: 1,666

    Yes but I don't think you get the gain because CPU can't talk that fast. Or maybe just some CPUs can't. Not positive.

    That's what I meant :blush:
    Luckily I never had to use PC1600. We don't even sell it. I meant 133. Or maybe I meant PC1600. You'll never know :)

    Oh well, I'm off for a frappuccino.
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