What's CAS and latency?
CAS2 & CAS3
The reason why this stuff is so hyped is because any hardware site you go to,
and some stores as well, have banners that say "Overclock your computer to 133
mhz!! We have the best CAS 2 and CAS 3 modules!!!" Well, time to clarify the
hype.
What CAS 2 and 3 actually denote is the time it takes to transfer data
internally from the memory bank to the output buffer running at 100 MHz. It is
measured in clock cycles (2 or 3). So you can first see that CAS 2 is "faster"
than CAS 3. There is a simple formula to figuring out CAS latency that I
learned somewhere on the web:
CAS latency > Column access time (ns) / System clock cycle (ns)
Okay, this reads CAS latency must be equal or greater than the column access
time divided by the system clock cycle. (This is important for what I'm about
to say, I swear.) So using some easy numbers, let's say your column access
time is 20 ns, and your system clock cycle is 10 ns. (By the way, system clock
cycle is the measurement that we use like 8ns or 10 ns RAM, to put things in
perspective.) So with these numbers, CAS has to be greater than or equal to 2.
So CAS can be 2. If the column access time went up, say to 30 ns, then CAS
would have to be 3.
Now why is this relevant? Because it lets you see how much margin you have
when you are overclocking your bus speed. SDRAM can only be CAS 1, 2, or 3.
Now imagine you have a module that is CAS 2 RAM at 100 MHz, and another one
that is CAS 3 at 100 MHz. The difference is going to be as the system bus MHz
increases. At 112, they may retain their latencies, so you still have 2
modules, CAS 2 and a CAS 3. At 125 MHz, the CAS 2 will become a CAS 3.
Remember the way to figure this out is by the equation above. The number that
changes as we bump up the system bus MHz is the system clock. As bus speed
rises, system clock cycle speed in ns decreases. The CAS 3 module remains at
CAS 3. In this example, then, both modules are CAS 3 at 125 MHz, which means
that spending extra money on a CAS 2 module is not worth it, because there is
no performance benefit. But if you bump up the bus to 133 MHz, then the
original CAS 2 module stays at CAS 3, whereas the CAS 3 module becomes CAS
4...but it can't, since we are limited only to latencies of 1,2, or 3. Thus
the original CAS 3 module can't run at 133 MHz.
Now that was just an example, but it may be true for some RAM types that are
CAS 3 at 100 MHz, and remain CAS 3 at 112 MHz, but cannot go any further
beyond that. I have such a module at home. This is why for extreme
overclocking, CAS 2 is a necessary component, but also remember that
overclocking is really a great deal of luck. You need to have very fast (and
blessed) L2 cache. That is overclocking's main limitation, not the system
memory. For people who only wish to OC to 112 MHz, you probably have a good
deal of flexibility with RAM, as many PC100 modules can handle a 112 MHz
speed. May your L2 chips be blessed!