PC3 == DDR3
Timings.... Copied from Corsair's FAQ...
Q: First of all, what does DDR stand for?
A: "DDR" stands for "Double Data Rate".
Q: So what exactly is being doubled?
A: With DDR, under optimal conditions twice as much data can be obtained from the memory subsystem during the same amount of time.
Q: How does it do this?
A: In most modern PCs, memory data is provided to the processor in "synchronous" fashion. This means that data arrives rhythmically, to the beat of a drum (a tick of a clock, in fact...). The memory clock is actually an electrical signal that bounces between two voltage levels, and shown in the picture to the right.
With standard SDRAM, data is transferred from the memory to the processor when the clock signal bounces from LOW to HIGH. With DDR, data is transferred not only when the clock signal goes from LOW to HIGH, but also when the clock signal goes from HIGH to LOW. Voila! Twice as much data on each tick of the clock!
Q: I'm having a hard time visualizing this...
A: The Ram Guy has been scratching his head for a week trying to think of a real world analogy to make "double data rate" easy to understand. A sharp stone and the resulting flat tire brought this one to light...
The Ram Guy likes road biking and mountain biking, and gets lots of flat tires because he weighs too much. Fortunately, he has a DDR tire pump that makes repairs easier! Where the old technology tire pumps put air in the tire only when you push the pump's plunger DOWN, the Ram Guy's new DDR pump puts air in the tire BOTH when you push the plunger DOWN, and also when you pull the plunger back UP. Twice as much air in each pumping cycle, meeting the inflation demands of today's high tech tires! Now do you get it?
Q: I've heard of PC6400 and PC8500. What does this mean, and what is the difference?
A: Well, this number reflects the memory module bandwidth. Since DDR DIMMs are eight bytes wide, the designation becomes PC800*8 = PC6400 and PC1066*8 = PC8500. So, to repeat, PC6400 uses both edges of a 800MHz clock, and PC8500 uses both edges of a 1066MHz clock.
Q: Everything else the same as SDRAM, i.e. registered vs. un-buffered, ECC vs. non-ECC, etc.?
A: Yup...
CAS Latency: What Is It, and How Does It Impact Performance? This is the question the RAM Guy gets asked more than any other question. So, I figured I'd put together a bulletin containing my $0.02 worth!
Q: First of all, what is CAS?
A: "CAS" is short for "Column Address Strobe". A DRAM memory can be thought of as a matrix, kind of like a spreadsheet with memory cells instead of numbers and formulas. Like the spreadsheet, each cell has a row address and a column address (like "AA57" or "R23C34" in the spreadsheet). As you might have guessed, there is also a RAS signal, which is shorthand for "Row Address Strobe".
Q: And, what do you mean by "latency"?
A: Latency refers to the time that you are waiting to get what you need. Merriam-Webster dictionary defines it as "the interval between stimulus and response".
Q: Now, how does CAS work?
A: To understand this let's walk through a simplified version of how the memory controller actually reads the memory. First, the chip set accesses the ROW of the memory matrix by putting an address on the memory's address pins and activating the RAS signal. Then, we have to wait a few clock cycles (known as RAS-to-CAS Delay). Then, the column address is put on the address pins, and the CAS signal is activated, to access the correct COLUMN of the memory matrix. Then, we wait a few clock cycles -- THIS IS KNOWN AS CAS LATENCY! -- and then the data appears on the pins of the RAM.
Q: So, for CAS-4 you wait 4 clock cycles and for CAS-5 you wait 5 clock cycles?
A: Bingo!
Q: So, CAS-4 is 33% faster than CAS-5?
A: There are a LOT of other factors in the memory performance. Here are a few of the main ones:
Sometimes you have to move to a different row in memory. This means activating RAS, waiting RAS-to-CAS delay, then doing the CAS latency thing.
Other times, you do a "burst" read, when you pull in a lot of data in one big block. In that case, CAS is only activated ONCE, at the beginning of the burst.
Also, don't forget the most important thing: processors have big caches! The cache is where the processor stores recently accessed instructions and data. The cache "hit rate", i.e., the percentage of times the processor finds the information it needs in its own cache, is typically greater than 95%!
OK, OK, so what's the bottom line?
So, the bottom line is, moving from CAS-5 to CAS-4 will offer a percentage performance increase in the low single digits for most applications. Programs which are known to be memory intensive (you gamers might know of some...) will see the best improvement.
ECC: What Is It, and Why Would I Pay Extra For It?
This is the question #2 for the Ram Guy. So, let me try to clear things up a little...
Q: First of all, what does ECC stand for?
A: "ECC" stands for "Error Checking and Correction".
Q: And, what is "Error Checking and Correction"?
A: Error Checking and Correction refers to a technology which allows a computer system to operate even if a memory error occurs.
Q: Why do ECC modules cost more than modules without ECC?
A: In order to check and correct the memory, additional RAMs are required. A non-ECC module which has eight RAMs would need to have a ninth RAM added; a sixteen RAM module would generally need to have TWO additional RAMs added. Obviously, the additional RAMs make the module more expensive.
Q: So it's kind of like the old parity modules, right?
A: Well, kind of, but ECC is a WHOLE LOT more useful. The ECC technology used on most x86-architecture PCs and servers is capable of correcting errors, where parity can only detect errors. If you've ever had an error "detected" on your system, you know the result - the blue screen of DEATH! Really useful, huh... With ECC you would sail right through, without crashing or even interrupting normal operation. Much more useful!
Mobo: Rampage IV BE | CPU:
i7-4930@4.5ghz Hexacore | RAM: 32GB Corsair V-Pro | GPU:
4xR9 290X Quadfire | PSU: eVGA P2 SuperNOVA 1000W\G2 850W | Chassis: NZXT Phantom 820 | Cooling: XSPC Raystorm CPU Block\4xEK 290X Block\2x Laing D5\XSPC Dual Bay Res\Phobya G-Changer 1080 External Rad\Alphacool 140UT60\XSPC AX360 | Fan Con: Lamptron CW611 | Tube: Primochill Adv. LRT Bloodshed Red | 3xCrossover 27" 2560x1440 Monitors |
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