Ask Root: Memory

mterickson82 Asks:

Explain memory to me, please. What's this DDR2 800 mean? And how does it relate to my motherboard? 

Root: Memory can sound complex at first. But thankfully, I brought two friends with me today that will help address the common questions about memory. Say hello.

Sparker366: You owe me 10 bucks.

TheLarrikin: Who's root?

Root: Thanks guys.
TheLarrikin:
Sparker366: haha
Root: No, really. Thank's for ya'lls time. I know you guys are busy. So I hear this a lot from the new builders out there. Let's take this G.SKILL memory for instance. What can you guys tell me about this Spec's page?
Sparker366: You mind?
TheLarrikin: After you.
Sparker366:
Sparker366: From a compatibility standpoint, you have to look at two things when matching it up to your mobo. Speed and Voltage. In your example, it would be the DDR2 800 PC2 6400, and 1.8V-2.0V
Root: And for those who don't understand that?
Sparker366: Well DDR2 is part of the SDRAM family. It uses a 240pin module design with a notch in a different location then the older DDR so it can't be used in the older memory slots.
Sparker366:The 800 refers to the transfer rate. 800 has a memory clock of 200MHz at a data transfer rate of 800 Million per second (that's where the 800 comes from). Your motherboard will state what memory it can take speed wise.
Root: And voltage?
TheLarrikin: The motherboard can sometimes be flaky when it doesn't support a certain voltage. Safest bet is to check out the motherboard's compatible memory. Voltage matters most to Overclockers. The lower the voltage is, the more you can raise it, whilst maintaining stability, and increase the speed of your RAM.
TheLarrikin: Due to manufacturing revisions, DDR2 memory runs at a much lower voltage than the older DDR memory. Of course, if you have a laptop, you could undervolt your memory to save battery life (with the trade-off of having a slower machine).
Root: Speed and voltage check. What about Timings? We hear that term alot.
TheLarrikin:'Timing' is exactly that, time. Well, specifically, how much it takes for your RAM to do anything. The given 4-4-4-12 is how much time it takes for your RAM to do everything.
Root: What do the numbers mean?
TheLarrikin: I thought you might ask that. The first number, CAS Latency, is how long your memory takes (in clock cycles) to find a specified column of data. The second number is your RCD Latency. It stands for 'Row to Column Delay', and is how long between the Memory getting the Row/Column, and being able to write to it. The third number is RP Latency, and is how long your memory takes to close one Row of memory in order to be able to open another. The fourth and final number is RAS Latency, which is how long your memory takes to find a specified row in your previously specified column of data.
Root: Say I just skimmed over that, what's this translated into for the beginner builder?
Sparker366: Lower the numbers, the better. For the one you picked, that's a good baseline for DDR2 800. This is where the price comes into play, and one of the biggest reasons why it jumps significantly between two memories sticks at the same speed.
Root: And this PC2 6400 number?
Sparker366: I actually don't know that one lol
TheLarrikin: PC2 6400 is an approximate number (because it's rounded to end in nice, pretty zeros) that indicates it's theoretical bandwidth. It's easily calculated. Let's look at your example: DDR2 800 = PC2 6400. Hmm... 6400 / 800 = 8. That's all you do - take the DDR2 number, and multiply it by 8. Why, you ask? DDR2 RAM transfers data on a 64-bit wide bus (or 64 bits at a time). One byte is 8 bits, and therefore, the bus is 8 bytes wide. That's where the 8 comes from!
Root: Say I skimmed....
TheLarrikin: Higher the PC2 number, the faster the memory.
Root: haha thank you.
Sparker366: So about that money.
Root: Thank you so much guys.
Sparker366: You owe me 10 dollars.
Root: I appreciate ya'lls time. If anyone has any questions regarding this subject, feel free to leave a comment below.
Sparker366: You're ignoring me now aren't you...
Root: And now a few tips from my side kick, and partner in crime, PROACEX1

PROACEX1: 

• Don't use "Value RAM" when overclocking (cheap memory normally has a very low tolerance)
• You can lower your voltages yet still retain the original clock speed.
• Don't combine cheap memory with a good mobo (or visa verasa)... Your mobo may auto-clock your memory beyond what it should be (Thanks to EPP, most EPP compatible memory and motherboards won't have this issue)
• 2GB is the pretty much the standard now, get at least that.
• Try to get 2 sticks (aka kits, which are matched memory modules that allow you to run the memory in Dual Channel mode). SLI is a slightly improved version of Dual Channel
• How much the memory can be overclocked is based upon the EPP values.
  

Until next time users,

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