Aces High Bulletin Board

General Forums => Hardware and Software => Topic started by: Getback on March 28, 2009, 05:08:29 AM

Title: DDR3 CAS Latency
Post by: Getback on March 28, 2009, 05:08:29 AM

I've noticed that the Cas Latency of DDR is around 7-9 while DDR CAS Latency is around 4-5. Anyone have an insight why this is?


Addendum: CAS Latency is the time it takes for the memory controller to access a certain column in a row and have it available at the pins. So on the surface it appears this setup is slower than DDR2. Now given that you now have sets of 3 does this mean that data now has a extra output path. In other words say you have 3 out put paths instead of 2? So if CAS Latency was 5 / 2 paths would equal 2.5 and 7/3 paths would 2.3333. Thus it would be almost equal to ddr2. It gets dizzying. Is there any advantage to DDR3? I'm sure I'm off somewhere here.

Let's take it a step further. Windows Xp 32 bit will only read 3+ gig of ram and 64 bit will read considerably more. Also many applications such as AH only allow 2 gigs of memroy to be used. So how does this all work with 6 gigs of ram?

Title: Re: DDR3 CAS Latency
Post by: MrRiplEy[H] on March 28, 2009, 05:32:48 AM

It's because DDR is architecturally different from DDR.  :rofl

Title: Re: DDR3 CAS Latency
Post by: Bino on March 28, 2009, 10:13:46 AM

Getback, here is a pretty good RAM-101 (http://www.overclock.net/intel-memory/1807-ram-101-intel-systems-particular.html) post about how RAM works.

Title: Re: DDR3 CAS Latency
Post by: Getback on March 28, 2009, 11:30:20 AM

Quote from: Bino on March 28, 2009, 10:13:46 AM

Getback, here is a pretty good RAM-101 (http://www.overclock.net/intel-memory/1807-ram-101-intel-systems-particular.html) post about how RAM works.

Just excellent Bino. I think many folks should read this. I bookmarked the page.

Thanks for the link.

Title: Re: DDR3 CAS Latency
Post by: Bino on March 29, 2009, 08:07:34 AM

Glad it was helpful.   :salute

Here is the next installment, all about RAM Timing (http://www.overclock.net/intel-memory/9981-ram-timing-explained.html).

BTW, your first question...

Quote

I've noticed that the Cas Latency of DDR is around 7-9 while DDR CAS Latency is around 4-5. Anyone have an insight why this is?

I presume you're comparing DDR2 & DDR3, right?  Well, remember the numbers are ticks of the clock, and at higher rates the clock simply ticks faster.

Title: Re: DDR3 CAS Latency
Post by: BaldEagl on March 29, 2009, 07:03:47 PM

Generally, the faster the memory bus the looser the timings have to be.

My old Dell runs PC100 SDRAM @ CL2.  That doesn't mean I want to somehow try to convert my gaming rig to PC100 SDRAM.   ;)