CAS rating for RAM and overclocking

[Tro]

<p>I haven't messed with over clocking in like 5 years or so and not that computer savvy but back then the lower the CAS latency rating the better so a CAS rating of 3 was considered to be a good over clocking rating and a rating of lets say 6 would be a bad rating for memory.When I search for CAS-3, I get like 256MB or 512 MB or 1GB when I am looking for 4GB (2x2GB Dual channel);All I see now for 4GB DC kits is CAS ratings of 7, 8 and 9 on what seems to me good quality RAM. So my questions are:1. If I am looking to over clock now, what CAS rating ram should I get? Most I see is all CAS 7, 8 or 9 typically in the 4GB size options. I would like to OC the E8400 to 4.0GHZ.. Just not sure what RAM CAS rating would provide a stable 4.0GHZ.  -and-2. If my MB has a FSB speed of 1333 and the RAM I want is listed as "SDRAM DDR3 1600 (PC3 12800)", will it work with that MB? or does the RAM have to match exactly the MB FSB Speed? I haven't found any info indicating whether the RAM would be backwards compatible to 1333 if it has a rating of 1600.Any info would be greatly appreciated.Thanks</p>

[Despak]

Never worry about CAS rating tbh. If your mb has a rating of 1333, and you want to overclock it; I woul dget the 1600 speed ram and overclock the cpu. Then adjust the divider for the ram to get as close to 1600 as possible.

[Tro]

<p>Sounds good..</p><p>Thanks for the reply ..</p>

[MurFalad]

<p><cite>Talsmar@Crushbone wrote:</cite></p><blockquote><p>I haven't messed with over clocking in like 5 years or so and not that computer savvy but back then the lower the CAS latency rating the better so a CAS rating of 3 was considered to be a good over clocking rating and a rating of lets say 6 would be a bad rating for memory.When I search for CAS-3, I get like 256MB or 512 MB or 1GB when I am looking for 4GB (2x2GB Dual channel);All I see now for 4GB DC kits is CAS ratings of 7, 8 and 9 on what seems to me good quality RAM. So my questions are:1. If I am looking to over clock now, what CAS rating ram should I get? Most I see is all CAS 7, 8 or 9 typically in the 4GB size options. I would like to OC the E8400 to 4.0GHZ.. Just not sure what RAM CAS rating would provide a stable 4.0GHZ.  -and-2. If my MB has a FSB speed of 1333 and the RAM I want is listed as "SDRAM DDR3 1600 (PC3 12800)", will it work with that MB? or does the RAM have to match exactly the MB FSB Speed? I haven't found any info indicating whether the RAM would be backwards compatible to 1333 if it has a rating of 1600.Any info would be greatly appreciated.Thanks</p></blockquote><p>One thing here is the CAS number is only meaningful when compared to rams of the same reference clock speed, comparing say DDR 333 memory with a CAS 2 to DDR3 1600 with a CAS of 9 the DDR memory would be</p><p>1 clock = 6 nano seconds as its double data rate (we use both edges for 333Mhz), therefore CAS 2 is two clocks = 12 ns</p><p>For DDR3 1600 CAS 9</p><p>1 clock = 1.25ns, therefore CAS 9 is 9 clocks = 11.25 ns</p><p>So in this case CAS 9 is actually a lesser delay, basically the simple way to make sense of the CAS figures is only to compare like to like, ie DDR3 12800 for DDR3 12800.  Basically the latencies haven't changed all that much over the past few years while the speeds have really gone up.</p><p>1) The lower the numbers for that type of RAM the better, a wooly answer, but if you look through a couple of retailers selections you'll find the lowest CAS value ram for the rated speed (1600 1333 etc).</p><p>Its always good to have a slightly lower latency, but I'd not lose any sleep over it since a lot of other factors will come into play for the eventual speed/response of your memory system such as the size of your Cache1, 2, and maybe 3 if you have a PhenonII/I7 etc, saying that I do tend to try and get the lowest latency myself as its always handy.</p><p>2) The ram is definitely backwards compatible when it comes to speed, having ram that can run at higher speeds then you need is helpful for overclocking of course since that's one component that shouldn't limit your eventual maximum frequency.</p>

[TSR-DanielH]

<p><cite>MurFalad wrote:</cite></p><blockquote><p>One thing here is the CAS number is only meaningful when compared to rams of the same reference clock speed, comparing say DDR 333 memory with a CAS 2 to DDR3 1600 with a CAS of 9 the DDR memory would be</p><p>1 clock = 6 nano seconds as its double data rate (we use both edges for 333Mhz), therefore CAS 2 is two clocks = 12 ns</p><p>For DDR3 1600 CAS 9</p><p>1 clock = 1.25ns, therefore CAS 9 is 9 clocks = 11.25 ns</p><p>So in this case CAS 9 is actually a lesser delay, basically the simple way to make sense of the CAS figures is only to compare like to like, ie DDR3 12800 for DDR3 12800.  Basically the latencies haven't changed all that much over the past few years while the speeds have really gone up.</p><p>1) The lower the numbers for that type of RAM the better, a wooly answer, but if you look through a couple of retailers selections you'll find the lowest CAS value ram for the rated speed (1600 1333 etc).</p><p>Its always good to have a slightly lower latency, but I'd not lose any sleep over it since a lot of other factors will come into play for the eventual speed/response of your memory system such as the size of your Cache1, 2, and maybe 3 if you have a PhenonII/I7 etc, saying that I do tend to try and get the lowest latency myself as its always handy.</p><p>2) The ram is definitely backwards compatible when it comes to speed, having ram that can run at higher speeds then you need is helpful for overclocking of course since that's one component that shouldn't limit your eventual maximum frequency.</p></blockquote><p>This is pretty much what I was going to post.</p><p>Lower latencies are better, but RAM doesn't follow a good price vs performance curve when you move up to more expensive and lower latency sticks.  RAM is also somewhat variable in production quality, so you may be able to get a lower stable latency than advertised in many cases.</p><p>Still, as mentioned previously, its not something you need to lose sleep over.  I doubt your memory will bottleneck your system either way.  The most important thing for RAM is getting the memory divider close to or at 1:1.</p>

[Tro]

<p>Thanks all for the excellent info.. pretty much answered what I was looking for.</p><p>Appreciate it..</p>