Memory explained with benchmarks
Tom's Hardware Guide Motherboards & RAM: Ups and Downs: Memory Timings Put to the Test - How Memory Access Works. Another great piece over at Tom's Hardware. A nice refresher for those of us with old computer science degrees. They explain how memory works and what timing means.
There is a sequence to access memore which is:
# row address select (RAS)
# then you wait for column address select (CAS), then you wait
# you get some data on the DQ lines (usually 64 bits worth)
# then you turn off RAS and CAS and do it again.
In short, if you have 2-3-4-6 RAM, this refers to how memory is addressed. If you have typical 200MHz memory, that means every 5ns, you get another cycle.
* 2 (CL) means that you have to wait 2 clock cycles after CAS is asserted in step 2 to actually get data
* 3 (tRCD) means you have to wait 3 cycles after RAS is asserted to assert CAS
* 4 (tRP) means you have to wait 4 cycles to for RAS to charge up (that is after you read the data, you have to wait tRP-CL to deactivate)
* 6 (tRAS) means you have to wait at least 6 cycles for RAS to be active before you can deactivate it.
They also did an incredible set of benchmarks that I'm still digesting. Let's just say, you don't necessarily need to buy super fast RAM anymore, the results seem to vary quite a bit. Although for highly computer bound tasks like video encoding, it is still true, faster is better.