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Memory access speeds

I've learned a little about choosing memory over the years, but reviews often bring up advanced topics where my knowledge falls short. I know the rated speeds (e.g., 1600 / PC3 12800) are tied to the bus speed and affect sequential transfer speed; also, the formula (2000*CAS/speed) yields random access latency. The questions that immediately come to mind are:

1. Do non-integer latencies (e.g., 11.25 ns) count, or do they have to be rounded up (like to 12 ns)?

2. If I want 16GB, am I better off with 2x 8GB or 4x 4GB? (For an Intel i5/i7, if that makes a difference.)

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  1. Welcome to Tom's Forum! :)

    Not exactly, it's a Multiplier of the FSB or BCLK equal DRAM Frequency, but the Frequency internally to the RAM is 1/2 relative to its expressed frequency (DDR = Double Data Rate). DDRx strobes twice per cycle on both the top and bottom of it's sine wave.

    ANS (1): Assuming you're referring to the CAS Latency then yes the performance of RAM alone is a function of both Frequency and CAS, and both are important; calculated as: CAS / Frequency MHz * 1000 = X ns. However, rarely expressed as an integer (whole numbers).

    ANS (2): Depends on the Frequency and CAS Timings of the 4GB and 8GB density kits. Often, the loss from using 2xDIMM per channel is compensated or exceeded from the gains in Higher Frequency and Lower CAS of the 4GB density kits.
  2. Thank you, jaquith. :^)

    Sorry I wasn't clear on #2. Assuming you're comparing 4GB and 8GB sticks with identical speeds and CAS latencies (e.g. DDR3 1600 with 8-8-8-8 in both cases). Are you saying one DIMM per channel is better than two ("loss from using 2xDIMM per channel")? I thought doubling up in the same channel was supposed to increase performance.
  3. Best answer
    Correct 2x__GB is slightly faster than 4x__GB in Dual Channel; assuming identical Frequency and CAS Timings.

    Since this is an LGA 1155, I'd look at this Article -

    Ideally, DDR3-1600 CAS 8/9 is best for Sandy Bridge, DIMM @ 1.50v or lower. Faster frequencies and/or lower CAS is nice but the trade-off is system stability.
  4. Thanks, jaquith, that was very helpful (esp. the linked article). The DDR3-1600 CAS 7 looks good in the article; but is it less stable (or does it require higher Vmem)?
  5. For someone like 'me' not so much a problem, I know what voltages to play with and I toughly test and validate.

    Most of the advantages you'll ever see are in synthetic benchmarks, so I'd stick to the 'real world' benchmarks.

    Folks have more issues with 'faster' RAM and say you're gaming and get a shutdown or BSOD from an OC error -- Rendering, 8 hour job gets botched 3/4ths the way through -- for the life of the PC you'll never gain the lost time; what's the good in that; peace has it's value.

    Your call...
  6. Best answer selected by mhill8.
  7. This topic has been closed by Nikorr
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