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DDR2 Explanation

Discussion in 'Hardware' started by Lasereth, 21 Feb 2007.

  1. Lasereth

    Lasereth What's a Dremel?

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    DDR memory was easy to follow. My Athlon XP 3200 had a 200 MHz system clock and a 400 MHz FSB. So I used DDR 400 PC3200 ram, which was 200 MHz clocked and matched with 400 MHz FSB CPUs.

    Now I have an Athlon 64 X2 4200+ with DDR2 800 memory. But I don't get why I had to buy DDR2 800 memory. Everywhere I read suggested that if you buy a dual core Athlon 64 you NEED DDR2 800 memory, but why?? Isn't DDR2 800 actually 400 MHz system clocked? My Athlon 64 X2 has a system clock of 200 MHz, just the same as my old Athlon XP. So why the need for DDR2 800? Why shouldn't I have bought DDR2 400, which is 200 MHz and would have matched my CPU system clock?

    I've read a dozen guides on the Internet about it but none of them explain what I'm looking for. Thanks in advance!!
     
  2. Guest-16

    Guest-16 Guest

    No you don't need it, but it's the maximum standard compatible AM2 systems will use, so surely you want the most performance possible?

    DDR2 800 is based from the 200MHz system clock, but AMD Althon 64 CPUs don't have a typical "front side bus".

    Why the need for DDR2? Why don't we still use EDO SIMMs? Same reason: performance and progress. Obviously 800MHz is twice as much as 400MHz, so there are twice as many clock cycles in the same amount of time.
     
  3. Lasereth

    Lasereth What's a Dremel?

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    I appreciate the reply but I'm still confused. In an older DDR system, buying PC3200 ram for a 166 system clock CPU didn't make it any faster, because the ram was faster than the CPU (200 MHz ram on a 166 MHz system clock). Now you're saying I can run 400 MHz ram on a 200 MHz system clock and the PC actually benefits from it now?
     
  4. Ramble

    Ramble Ginger Nut

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    DDR worked on teh fact that it could double the clock rate, using the FSB rate as a reference. That's why a 200MHz FSB would give you 400MHz RAM (DDR 400).

    DDR2 is the same basic theory but it quadruples the rate, therefore giving you 800MHz (sorta).

    I think I've explained it badly, but I'm pretty sure that's the basic theory.
     
  5. Lasereth

    Lasereth What's a Dremel?

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    That makes sense. But aren't the Core 2 Duos running at 266 MHz system clock? 266x4 = 1064, not 800. Would you need DDR2 1066 to run a Core 2 Duo system??
     
  6. Ramble

    Ramble Ginger Nut

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    No, but it would be optimal.
    I'm pretty sure that this time around the silly FSB/RAM speed thing has been gotten rid of as all C2D mobos should run at least DDR2-800, but I think most should run DDR2-1066.
     
  7. thestig198

    thestig198 Artificially Intelligent

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    http://en.wikipedia.org/wiki/DDR2

    "The key difference between DDR and DDR2 is that in DDR2 the bus is clocked at twice the speed of the memory cells, allowing transfers from two different cells to occur in the same memory cell cycle."
     
  8. Lasereth

    Lasereth What's a Dremel?

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    Are you saying matching your ram to the system clock on your CPU doesn't matter with DDR2? I see a lot of people telling me to use DDR800 ram with my athlon 64 x2 but no one can tell me why. They just say it will run better, despite the ram being 400 MHz and my CPU being 200 MHz system clock. I don't get how it can improve performance if my CPU system clock isn't nearly as fast as the ram.
     
  9. Ramble

    Ramble Ginger Nut

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    RAM operations take something like 3-20 clock sycles depending on what you're doing.

    Plus, in C2D the FSB is quad-pumped to 1066MHz, the same thing exists in AMD64 with hypertransport. The 200MHz thing is really only used a reference clock.
     
  10. Lasereth

    Lasereth What's a Dremel?

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    So my DDR2 800 ram isn't a waste of money?
     
  11. teamtd11

    teamtd11 *Custom User Title*

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    Not all all, you will have more memory bandwidth than you can shake a stick at :hehe:

    I spent £180 on my 2GB of pc6400, and i dont regret it at all, the power :baby:
     
  12. Supershanks

    Supershanks What's a Dremel?

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