advice on sdram cas settings?

[Verruckt]

first off, is the correct terminology "cas timing" or "cas latency"?

Anyways, I have PC133 CAS2 SDRAM, but it is set to cas3 through the bios and the mobo is running at 143 FSB.

Why does some RAM need to be set to CAS3 when overclocking? What is this whole CAS business, and what does changing it do?

If I run the mobo at 138 FSB might the RAM work at CAS2?

Which is better - CAS3 RAM, 143FSB and a 933 P3
or
CAS2 RAM, 138 FSB and a 900 P3?

specs:
128MB Mushkin "high performance" PC133 DIMM (http://www.pcnut.com/parts/memory.htm)
P3 650 cB0 stepping @933
SOYO SY-6VCA (via 133a) mobo

...thanks

 

[dunkster]

CAS#-rating actually refers to memory latency.

Think of the CAS number as wait-states between data transactions.

If you monitored memory-clock-cycle activity for CAS2 and CAS3 memory, you'd see something like this:
CAS2: wait/wait/data.
CAS3: wait/wait/wait/data.


The impact in benchmarks will show up as higher memory throughput for CAS2 than CAS3.

CAS#-rating has very little to do with how fast a cpu will overclock in cpu speed. However, you may find that CAS2-rated memory may revert to CAS3 performance as memory bus speed increases (Example: CAS2 performance at 133MHz fsb, but only cas3 performance at 150MHz fsb).

 

[Jhhnn]

As I understand it, the actual amount of time in each wait state gets shorter as the fsb increases,as each clock tick is closer together. This often necessitates raising the number of waits from 2 to 3 to allow for enough total time for the memory to perform properly. Benchmarking your particular system is the only way to tell which of the configurations you mention will actually perform better, probably the lower cas timing and lower fsb. You pay a big penalty in memory performance for the stated gain in fsb. The whole subject of memory timings is very complex, and several good in- depth articles can be found in Anandtech archives, and in the archives of other hardware sites. Add the peculiarities of different chipsets and bioses, and well, you might be able to do the subject justice with a doctoral dissertation, if you wrote it really fast before it all changes again!