latency, pc133 vs ddr

[74merc]

ok, we have CAS 2 pc133 and CAS 2 PC2100.
the latency is in actual clock cycles, correct? as in, it takes the EXACT same amount of time for the request to be... answered, but with DDR, the request is fufilled faster due to the higher bandwidth, right?
also, I thought I had it figured out, a latency of 2 is about as fast as its gonna get, seeing as how it takes 1 clock to send the request and one clock to answer.
right?

 

[Guest]

Actually, (somebody spank me if I'm wrong) all DDRs are CAS 2.5.
With this response time they lag behind the CAS 2 PC133/PC100 but their double memory bandwidth provides faster data transfer. I guess that's why DDR is fast but not 2X as fast as PC133/PC100.

Better go Green than Blue!

 

[74merc]

well, theoretical for the purpose of the discussion... CAS 2 on both memory types.
the latency is based on the physical clock cycle and not the DDR... stuff, right?

 

[jg38141]

Since the way the cas number is arrived at is a ratio rounded off, then wouldn't cas 2.5 be cas 3? Like 2,3,2 is cas 2, but (not that it's common) 2,3,3 would be cas 3 and so on.

HACK THE PLANET!!!

 

[Guest]

well well well, I think there are CAS2 DDR's also. I bought two CAS2 PC1600 DDR from crucial...

 

[74merc]

anyway, anyone want to tackle this one? hmm?

 

[yoda271828]

CAS latency is in actual clock cycles, even for DDR. DDR with CAS 2.5 rating uses the <i>equivelent</i> of 5 clock cycles (since it can transfer 2 bits per clock). DDR does come in CAS 2 also.

So CAS 2 SDRAM does have a lower latency than CAS 2.5 DDR-SDRAM, but the same as CAS 2 DDR-SDRAM. The CAS 2.5 DDR-SDRAM will most likely outperform CAS 2 SDRAM though due to the higher bandwidth.

 

[smn198]

So will real CAS2 DDR ever be made (called CAS 1?). I think that this will explain some of the unfavourable benchmarks for DDR especially in SYSmark and other ones which rely more on latency and less on bandwidth than 3Dmark, etc.

<A HREF="http://www.anandtech.com/mysystemrig.html?id=1686" target="_new">System spec.</A> Ideas appreciated.

 

[mpjesse]

Yes- and it is being made. Granted it's a little hard to come by compared to CAS2.5 DDR SDRAM- but it is obtainable. And there is CAS1 RAM out there- believe it or not. Though, it's not made for PC's. The only machines I know of that have CAS1 RAM are SGI machines.

-MP Jesse

 

[tfbww]

mjesse, You've mentioned that you are using the Magik1 DDR mobo. What is the CAS latency of your memory? Did you try other settings to see the diff?

 

[mpjesse]

The ram is Crucial's CAS2.5 PC2100 DDR Memory... my current BIOS revision doesn't allow a lot of tuning to the RAM... there is a new version out- i just haven't upgraded to it yet.
-MP Jesse

 

[tfbww]

I'd be very interested to hear about the performance delta if and when you upgrade to the CAS2 memory.