How to choose RAM: Part 1 – Understanding RAM terms

| May 13, 2009 | (20)

Hey everyone. Today we’re having our first guest post, by Eric Odgen of SimplePCs.net.

Here’s a short Bio of him:
Hi, my name is Eric Ogden, I’m from Washington State and am currently a student working towards a bachelor’s degree in Computer Science. I have a love for computers and I feel that as I love them and understand them, I should share my knowledge with people who want to understand them better. Feel free to visit my site for more helpful information on computers.

Here’s his post, I hope that you enjoy it:

This is Part 1 of my super-mega-ultra-crazy-RAM guide. In Part 1 I’m going to cover the major terms that you might come across when shopping for memory, and in Part 2 I’ll cover how to go about shopping for RAM. (Mathieu: Link for part two at the end of this post)

RAM – Random Access Memory

How can you shop for RAM if you don’t even know what it is? Random Access Memory is a form of computer storage. It’s like your hard drive in your computer, but smaller and much, much faster. The part that makes it “random” is the fact that any data stored at any location can be accessed in constant time, meaning you don’t have to wait for the data to be found or for it to travel some distance to be read.

Think of it like this, you live in New York, but your friend Steve lives in L.A. You don’t want to fly all the way to L.A. because you just don’t have time in your busy schedule. If our world were like the world of computers, you could walk out your front door and instantly be in L.A. No travel time.

That’s what RAM is like, if your computer needs some piece of information it doesn’t waste time traveling to the address and getting the information, it can just get it. And no address in RAM is “closer” to its destination than another, for instance, the left side of the RAM chip isn’t read or accessed any faster than the right.

Also, your computer needs RAM to be able to even boot. Don’t think you can cut a corner here and not buy RAM, it doesn’t work like that.

DDR (1,2, & 3) SDRAM – Double Data Rate Synchronous Dynamic Random Access Memory

All you need to know about the term DDR is that this is probably the type of RAM you’ll be looking to buy for your desktop, and that it’s faster then just regular SDRAM. As for the 1,2, & 3 aspect of DDR, here’s what you need to know.

  • DDR 1
  • Standard Name

    Memory Clock Data Transfers/s

    Module Name Peak Transfer Rate
    DDR-200 100 MHz 200 Million PC-1600 1600 MB/s
    DDR-266 133 MHz 266 Million PC-2100 2100 MB/s
    DDR-333 166 MHz 333 Million PC-2700 2700 MB/s
    DDR-400 200 MHz 400 Million PC-3200 3200 MB/s
  • DDR 2
  • Standard Name Memory Clock Data Transfer/s Module Name Peak Transfer Rate
    DDR2-400 100 MHz 400 Million PC2-3200 3200 MB/s
    DDR2-533 133 MHz 533 Million PC2-4200

    PC2-4300

    4266 MB/s
    DDR2-667 166 MHz 667 Million PC2-5300
    PC2-5400
    5333 MB/s
    DDR2-800 200 MHz 800 Million PC2-6400 6400 MB/s
    DDR2-1066 266 MHz 1066 Million PC2-8500
    PC2-8600
    8533 MB/s

    Fun fact: DDR2-667 was actually supposed to be DDR2-666 but because someone freaked out over 666, they ended raising the speed by 1 to 667.

    Note: DDR2-533/667/1066 have two Module Names listed, this is because the PC2-xxxx denotes theoretical bandwidth, usually being rounded up or down. This doesn’t really matter, though some manufacturers have used the higher bandwidth to achieve slightly higher data rates.

  • DDR 3
  • Standard Name Memory Clock Data Transfers/s Module Name Peak Transfer Rate
    DDR3-800 100 MHz 800 Million PC3-6400 6400 MB/s
    DDR3-1066 133 MHz 1066 Million PC3-8500 8533 MB/s
    DDR3-1333 166 MHz 667 Million PC3-10667 10667 MB/s
    DDR3-1600 200 MHz 1600 Million PC3-12800 12800 MB/s

    As you can see, the memory clock of the RAM doesn’t change from DDR1 to DDR3, the speed increase comes from the input/output bus clock speed being doubled. This isn’t really relevant to shopping for RAM but it does help you understand the difference between the three speeds.

  • SO-Dimm (aka, Laptop Memory)
  • SO-DIMM memory has the same specifications as regular DDR desktop memory except for the fact that SO-DIMM is smaller. Because of it’s smaller size it’s used in laptops and net-tops (like the Mac Mini). DDR1 and DDR2 SO-DIMM modules have 200 pins, whereas DDR3 SO-DIMM has 204.
    NOTE
    : DDR1 and DDR2 SO-DIMM modules are not interchangeable, and the layout of the pins on the module prevent you from installing it into the wrong system.

  • Speed (MHz)
  • I won’t go into details here, I’ll just say that the faster the RAM, the faster your computer will be. It really is that simple. The higher the number, the faster the memory.

  • CAS/Timings/Latency
  • This is a little complex but I found a nice little chart on wikipedia that explains this all and makes it very simple to understand why the normal computing individual can ignore it.

    NOTE: Ignore Bit time and Command Rate if you don’t understand what they mean. Just note that as the technology improves, the data rate increases (good) while the bit time decreases (also good).

    Type Data rate Bit time Cmd rate Cycle time CL 1st word 4th w. 8th w.
    PC100 100 MT/s 10 ns 100 MHz 10 ns 2 20 ns 50 ns 90 ns
    PC133 133 MT/s 7.5 ns 133 MHz 7.5 ns 3 22.5 ns 45 ns 75 ns
    DDR-333 333 MT/s 3 ns 166 MHz 6 ns 2.5 15 ns 24 ns 36 ns
    DDR-400 400 MT/s 2.5 ns 200 MHz 5 ns 3 15 ns 22.5 ns 32.5 ns
    2.5 12.5 ns 20 ns 30 ns
    2 10 ns 17.5 ns 27.5 ns
    DDR2-667 667 MT/s 1.5 ns 333 MHz 3 ns 5 15 ns 19.5 ns 25.5 ns
    DDR2-800 800 MT/s 1.25 ns 400 MHz 2.5 ns 6 15 ns 18.75 ns 23.75 ns
    5 12.5 ns 16.25 ns 21.25 ns
    DDR3-1066 1066 MT/s 0.94 ns 533 MHz 1.88 ns 7 13.13 ns 15.95 ns 19.7 ns
    DDR3-1333 1333 MT/s 0.75 ns 666 MHz 1.5 ns 9 13.5 ns 15.75 ns 18.75 ns
    6 9 ns 11.25 ns 14.25 ns
    DDR3-1375 1375 MT/s 0.73 ns 687 MHz 1.5 ns 5 7.27 ns 9.45 ns 12.36 ns
    DDR3-1600 1600 MT/s 0.625 ns 800 MHz 1.25 ns 9 11.25 ns 13.125 ns 15.625 ns
    8 10 ns 11.875 ns 14.375 ns
    7 8.75 ns 10.625 ns 13.125 ns

    The thing to really note here is:

    1. The original seek time for the eighth word was 90 ns (nanoseconds) and now it’s down to 12.36. This is substantial.
    2. In the CS (CAS Latency) column you’ll see that for some speeds there are different values. The lower the value, the faster the seek times. That’s all you need to know.

    The thing to consider here is that with higher speed RAM (like DDR3) the lower the CAS Latency, the more you pay, and sometimes this can be quite a large sum. I would say that most people would be perfectly fine buying the higher latency RAM.

    Most of the time the lower latencies will be listed as “Performance” or “Enthusiast” RAM, so if you don’t feel too enthusiastic about your RAM performance (down to the nanosecond that is) then you probably won’t feel much of a difference with the higher latency.

  • Heat Spreaders
  • Heat spreaders are a fairly large scam in the memory world. Most computer users don’t overclock their RAM and thus, do not need heat spreaders. Many high performance RAM modules (designed for overclocking) already have at least a minor form of heat spreaders on them and anything past that isn’t necessary. If your RAM comes with them, great, but if they don’t, I wouldn’t worry too much about not purchasing them.

  • 4GB Limit
  • Windows XP (and other 32-bit operating systems) can only use 4GB’s of memory. This means 4GB’s of memory in the entire system. This includes your graphics card and any other peripherals that use even the slightest bit of memory.

    This becomes a problem if you’re still hanging on to Windows XP but want a super-duper fast graphics card. Some graphics cards now have 1GB or 1.5 GB’s of video RAM, which means that will be taken out of the total 4GB’s that XP can address.

    The answer: get a 64 bit operating system like Windows Vista x64 or Windows 7. Keep this in mind if you’re wanting lots of RAM but also wanting XP; there’s a tradeoff.

    That’s it for Part 1 of my super-mega-ultra-crazy-RAM guide, please visit my site, SimplePCs, for RAM Guide Part 2 – The Buyer’s Guide

    Mathieu: Thank you Eric for writing this guest post. Everyone, what did you think of this guest post? Give Eric some feedback and tell me: Would you like to see more in the future? Let me know.

    Category: Choose PC Parts, How to

    About Mathieu Bourgie: HR Founder - Computer expert with 13 years of experience in building, fixing and modifying PCs. Over the years, I’ve developed a passion for PC hardware and now I enjoy helping others build their own PCs! In April 2008, I launched Hardware Revolution and ... Read more at my about page .

    • Pat

      This is truly helpful, and so clearly presented. I thank Eric profusely and hope to see more.

    • Pingback: RAM Guide Part 2 – The Buyer’s Guide « Simple PCs

    • anon

      great article!

    • iz funny

      An okay article. Would have been better with pictures and the differences in those pictures.

    • Aleksander

      I KNOW THAT THE LIMIT OF RAM UNDER XP IS 3 GB

    • Gric7

      Mathieu thanks for all your great articles.
      I would like to read part 2 of this ram article, but it is a dead link. I can't seem to find it by way of google either.
      Is the article gone?
      Thanks

      • http://www.hardware-revolution.com/ MathieuB

        Gric7,

        That article was a guest post, with the second part linking to the author's website where the 2nd part used to be. Looks like he did a redesign of his website and that the 2nd part got lost in the process. I unfortunately don't have any control over external content, but I'll see if I can publish some kind of all-in-one updated RAM guide some time soon.

        Take care,
        Mathieu

    • William

      Hi, im william. I’m using windows XP with a DDR1 512MB RAM. Then i bought a DDR1 1GB RAM. I installed that to the second slot. But my PC seems running just the same…slow..What should i do..? I’m thinking of reverse the RAM (1gb on 1st slot , and half gb on the 2nd) is that going to work…?
      Or maybe i would just use only 1gb as my RAM…? Please help me..

      • Anonymous

        Try just using the 1GB stick in the first slot, see how it works. If its all the same, your bottleneck is somewhere else. Cheers.

    • Anonymous

      whenwe are updating our machine   we should know how to choose a good memory…

      that’s very important

    • Anonymous

      Haha, I choose DDR3. Most of motherboard only support DDR3 now. DDR2 has been the past.

    • wansaby

      so what if my cpu supports up to 1333 ram, my motherbord 1333and 1600 ram
      can i then use 1600 ram and if i need to overclock my cpu to mach the ram what needs to be overclocked?

    • Par5Eagles

      One thought- i’ve had good experiences with windows xp 64 bit… definitely dated now but it is an option for someone who wants to stick with xp.

    • Anonimo Anonimo

      I don’t know if you where just trying to be funny with the “someone freaked out with the 666″ because that was not the reason why it’s 667 and not 666. Pretty much all speeds in the computer world since the ‘ye olde days’ are divisions of round numbers, but having a 1666.(insert infinite number of 6′s here) mhz cpu, for example, is highly impractical so the number is rounded to the closest number that drops the dot which is 1667. Of couse sometimes there may be marketing reasons but 667 is right by the math and 666 is not.

    • ps4

      can i use 666 ram on a mother board that says it uses 533/667/800mhz sdram?

    • SHIV SHARMA

      wow what a great info about rams. thanx a lot Mathieu Bourgie. well done.

      shiv sharma (india, rajasthan)

    • Mr L

      Hey Mathieu i still don,t undstand how memorys work pls pls help me?

    • omprakash

      my pc is duo core, may be 1.4 GHz. which ram will suit my computer?ddr2?ddr3?
      i wanna buy a 2 gb ram

    • bogdan

      hey i have 512 mb ram ddr2 and i wanna know what ram i need i wanna buy another 512mbram or 1 gb ram but i wanna know everything about ram here is my Y!M bog_boog@yahoo.com

    • yea yea

      great job! I’ve been having a hard time searching for ram and what the values stand for. this really helped.