Memory Latency Explained

Discussion in 'CPU, Motherboards and Memory' started by zRoCkIsAdDiCtInG, Jul 8, 2005.

  1. zRoCkIsAdDiCtInG

    zRoCkIsAdDiCtInG HWF Guitar Freak

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    I've noticed really that little people actually know that there is more to memory settings than just CAS Latency, and it's actually quite annoying, so I figured out that maybe I should just post a few bits of info to start them out, as well as inform them how to raise speed of their system through lowering or raising these settings, though I recommend not to toy unless you have experience or research the topic further. :good:

    So onto the basic settings:
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    CAS Latency: This stands for Column-Access-Strobe
    -The CAS Latency manages how fast memory units return a response to a data request. the settings range from 2 to 3, and lower settings provide speed gains, though the results make a small performance difference. The best performance gains are with SDR-RAM
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    TRP: AKA RAS Precharge, has settings similar to TRCD, but has little significance on performance. It ranges from 2 to 4 and small performance gains are recieved from lower settings.
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    tRAS: All Right, Bank cycle time is another name for tRAS, its the number of clock cycles needed to make the complete difference in charge between the bit and lines of referenceto restore data in the cells of memory. Settings range from 5 to 7, and with lower numbers, you recieve small performance gains.
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    TRCD: This is known as RAS to CAS Delay, being numbered by clock cycles, and it possibly can limit the memory performance. It's measured from 2 to 4, and lower settings raise performance markedly.
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    Bank Interleave: The setting that determines if memory is laced or not, it ranges from 0 to 2B to 4B and higher settings improve performance.
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    CMD: One of the most powerful memory settings known as Command Rate, which ranges from 1T to 3T, lower settings increase performance noticeably.
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    These are but mere basics if you want to know more about memory, but that drags you into reading about memory controllers and sequencing, which I won't get into (though I have so little of a life, I actually read a bit on these topics)

    And now while others brag about low cas and we have a T1 command rate, we can secretly laugh at them. Then again, I recommend not to toy with these settings unless you know what consequences it may have on your PC

    Welcome to the CAS Latency Rebellion :good:

    Please keep this thread on topic! anything off-topic will be deleted. Also Anti-Trend stuck the thread not me :) Sniper
  2. Exfoliate

    Exfoliate Geek Trainee

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    Theres more to memory than CAS Latency!!!

    Top notch post dude, that was very useful stuff as I practically fell into the party this post is directed at. I think this should be a sticky myself, if there was a pure memory thead you should be concidered as mod:good:
  3. zRoCkIsAdDiCtInG

    zRoCkIsAdDiCtInG HWF Guitar Freak

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    Theres more to memory than CAS Latency!!!

    I just hope that more people will understand more and stop ripping themselves off sometimes with the memory, and understand how it is,

    expect more things like this, i'm writing a GPU one soon

    "zRock's study guides" haha
  4. max12590

    max12590 Masterful Geek

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    Well I also do some posting over at Hackthissite and there is this one guy named Pilot, he wrote like 30 page report on CPU's. It's really good. You can probably go over there and pm him for it. But yea, good work, the basics of RAM. The other thing that people don't realize is that overclocking RAM lessens your ability to overcolck their CPU. So they say I can only overclock 400 MHz, but if they run the RAM at 233 MHz with Hyper-Threading/Transport off or low they can get much higher.
  5. zRoCkIsAdDiCtInG

    zRoCkIsAdDiCtInG HWF Guitar Freak

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    Theres more to memory than CAS Latency!!!

    yeah in to depth things will follow, im just writing the basics first

    I bet I'll spend a good time on the following GPU guide
  6. Exfoliate

    Exfoliate Geek Trainee

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    Theres more to memory than CAS Latency!!!

    Good, I'll be looking foward to that, I'll, uh, do an article on...pimp my rig:)
  7. zRoCkIsAdDiCtInG

    zRoCkIsAdDiCtInG HWF Guitar Freak

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    Theres more to memory than CAS Latency!!!

    I finished the basics GPU guide, its mostly for newbs but I added in the pixel pipeline details of its process and will add in more, still newb or specialist, its worth the read.
  8. zRoCkIsAdDiCtInG

    zRoCkIsAdDiCtInG HWF Guitar Freak

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    Theres more to memory than CAS Latency!!!

    Anti Trend stuck it? wow, I'm honored haha, hes like amazing (faints)
  9. Exfoliate

    Exfoliate Geek Trainee

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    Theres more to memory than CAS Latency!!!

    Woah, sorry Sniper, and good work AT, I forgot Mod's had that kind of power. I'll try to keep things on target, it's just so hard...
    Do you have any other guides planned Z?
  10. zRoCkIsAdDiCtInG

    zRoCkIsAdDiCtInG HWF Guitar Freak

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    Theres more to memory than CAS Latency!!!

    the video one came out kind of cruddy, i'm working on fixing it up, so everyone expect a complete makeover

    yeah, probably ever major pc component

    basics
    and optimization probably
  11. vol7ron

    vol7ron Guest

    Nice, but you have the order mixed up. As well as a few other things.

    Its normally:
    CL-tRCD-tRP-tRAS-CMD

    Written out:
    CAS Latency-RAS to CAS Delay-RAS Precharge-Active to Precharge Delay-Command Rate

    Defined:
    CL-time between a command sent to memory from the proc and when it is replied to
    tRCD-time between accessing row and column in memory (addresses are stored in a table kinda like an Excel Spreadsheet)
    tRP- time between disabling a line of data and accessing another line
    tRAS- how long memory has to wait until next access to memory can be started
    CMD- time it takes between the mem chip being activated and when the first command may be sent to memory (memory is activated first and then a command is sent to it) Unlike the above this is normally in just T1 or T2 where the numbers are defined in clock cycles (T1 = 1 clock cycle; T2 = 2 clock cycles)


    I have notice a considerable difference in CAS latency (as most people do) just from going from 3 to 2.5. All the other timings DO matter. A general rule I use when just glancing is to add up the 4 numbers and get the lowest one (like a golf score). The middle 2 numbers you can generally use interchangeably 2-4-3-7 = 2-3-4-7 in my opinion.



    THE MOST IMPORTANT THING: Test your memory. Using Memtest or SiSoftware Sandra2007 (my pick). People are usually concerned with overclocking their memory. But I've seen better improvement in going for the lowest timing and then raising the clock speed (like around 350MB/s improvement in bandwidth with Kingston ValueRAM). In my opinion, Never turn off HyperThreading just to raise your clock speed or lower your memory timings, I've tested it - it sucks (lowered bandwidth by like 2GB/s)


    Didn't mean to step on anyones toes, but just wanted to clarify and correct some things.
  12. T Vlasceanu

    T Vlasceanu Geek Trainee

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    Thank you kindly. This information is very usefull for me. Great site! Thanks again!
  13. Ashton

    Ashton Geek Trainee

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    Thanks, this is pretty useful. Would buying low-latency ram increase performance much?
  14. Big B

    Big B HWF Godfather

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    Depends on the CPU. These days, timings don't seem to be as important as much as the performance of the CPU. However, if you're into overclocking, lower latencies may translate into higher clocking RAM than something with a slower timing.

    For DDR2, 4-4-4 timings are pretty low. I'm not sure if you'll see faster than 3-3-3 timings, but that wasn't the goal of DDR2. Higher speed was, and while lower latencies could probably be achieved, I'm not sure it'd be at a price people would want. For example, I have 2GB of OCZ DDR2 PC6400 (DDR2-800) coming in a few days that cost me less than $50. It's timings are fairly slow at 5-5-5, but moving to CAS 3 would either raise the price roughly $25 or force me to go with some PC2-5300 (DDR2-667). There's also some Corsair DHX that's running over $200, but not everybody wan'ts to pay that kind of cash. I can't imagine DDR2-1066 CAS3 coming under $100 anytime soon. Granted, memory prices have dropped (I pad almost $100 for my Crucial Ballistix PC3200 1GB kit over a year ago).
  15. Ashton

    Ashton Geek Trainee

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    Well I found a cheap 2GB Corsair DHX for $70, it has 4 4 4 12 timings, runs at 800mhz and includes heatsinks.

    So I'll probably order that, thanks for the help
  16. PCRecycle11

    PCRecycle11 Geek Trainee

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    This really great info about CAS latency. I'm planning to buy a memory tomorrow but I just wondering can we really see the difference between CAS latency 9 vs. 7. Is it really worth the extra dollars?

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