CAS/RAS Settings, how do they affect performance?

[earthman]

I am having an argument with someone. He claims that if your CAS setting is 3 instead of 2, for example, then your memory is 50 percent slower. I know this is not true, but I wonder if someone knows a little more on this subject and how much it really affects your system.

 

[CTho9305]

Not sure on the % difference... it would actually vary with the applicatoin. But basically CAS latency is the result of the structure of SDRAM - the memory is laid out in rows/columns. To read a certain location, you have to precharge a line, and once it is charged, read from the cell you want. Now, the benefit of SDRAM is that once you charge a column and read, it will also give you the rest of the column (Or something like that) since there is a high probability that you will need that data soon also (due to "spatial locality" - BurntKooshie can explain that better).

So... I would imagine that if you wrote something that only used one byte from each column, then the next cycle read from a different column, there might be a 50% difference. However, I would also imagine that this would be pretty difficult to pull off

edit: i'm sure BK's article mentions something about spatial locality

 

[Tweakmeister]

Check out the memory article over at www.arstechnica.com >> it's older but the technology has stayed the same.

 

[earthman]

According to Crucial, the extra clock cycle only comes into play the first time the register is addressed, therefore the actual performance hit is very small.