What is....
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  1. #1
    Join Date
    Jul 1999
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    Inverness, FL, USA
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    What is....

    CAS 2-2-2? I've heard of this term numerous times but I don't understand the terminology (I thnk it has to do with memory but I'm not sure). Thanks for the info!!

  2. #2
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    Yes, it has to do with memory latency timings. Someone gave me a link one time to a page that was extremely helpful in explaining it, but I don't have the link anymore. Maybe someone around here has one.

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  3. #3
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    I cannot remember why there are 3 numbers, but it is the latency of the RAM in clock cycles. ie, 2-2-2 RAM will respond in 2 clock cycles (I think, correct me if I'm wrong please). I have a suspicion that the first number is CAS latancy and the other 2 are something else, as I have seen CAS 3 RAM which is labeled 3-2-2, but I'm not 100% sure of that.

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  4. #4
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    I beleive it was uart who gave the link on the detail explanation about CAS.

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  5. #5
    Join Date
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    CAS means Column Address Strobe. Dynamic memory consists of vast arrays of rows and columns of bits, each row or column containing thousands.

    When you access a memory location, the motherboard sends the memory a row address strobe (RAS) causing the memory chip to dump all the thousands of bits in that row into a column buffer. Next the motherboard changes the address signals for a column address and signals CAS to make the column buffer only send out the desired bits of the column from the memory module to the motherboard.

    The memory module needs time to get the data from the array into the column buffer. CAS latency 2 or CAS latency 3 specifies how many clock cycles the motherboard must wait for the column buffer to get the data, before sending the CAS signal to actually read the data.

    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.

    So, for CAS-2 you wait 2 clock cycles and for CAS-3 you wait 3 clock cycles?

    So, CAS-2 is 33% faster than CAS-3?

    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%.

    So, the bottom line is, moving from CAS-3 to CAS-2 will offer a percentage performance increase in the low single digits for most applications. Programs which are known to be memory intensive i.e. games will see the best improvement.

    The other thing to keep in mind is that CAS-2 memory will run FASTER ( some review sites have taken it to 160MHz!) than CAS-3 memory. So, if you're thinking of overclocking your system (now or in the future), CAS-2 is your best bet for speed and stability.

    So, the Ram Guy sez...

    Buy CAS-2 if [1] you want to wring the last bit of performance out of your system, or [2] you're thinking of overclocking, either now or in the future, or [3] it costs the same as CAS-3, which it sometimes does...


    Hope this helps
    Cool Runnings

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