Explanation of RAM timing

[pell]

On my last machine, I bought Corsair XMS memory because I kept reading that it was faster than anything else (I believe it was PC3200 or something like that)...

Now, it seems like there is timing that needs to be adjusted in the BIOS to take advantage of low latency RAM. I plan on building a new system within the next month, but before I do, could someone please explain how this timing works, and how I would take advantage of higher-end memory?

 

[31computers]

Most computer users don't need to worry about SDRAM latency, because the computer can handle the auto-adjustment to RAM timing based on the Serial Presence Detect (SPD) ROM inside the RAM packaging that defines the four timing values, decided by the RAM manufacturer.

Although the SDRAM latency timing can be adjusted manually, using lower latency settings than the module's rating (overclocking) may cause a computer to crash or fail to boot.

 

[NotAPimecone]

Most computer users don't need to worry about SDRAM latency, because the computer can handle the auto-adjustment to RAM timing based on the Serial Presence Detect (SPD) ROM inside the RAM packaging that defines the four timing values, decided by the RAM manufacturer.

Although the SDRAM latency timing can be adjusted manually, using lower latency settings than the module's rating (overclocking) may cause a computer to crash or fail to boot.

They do if they buy expensive RAM that needs more than 1.8V to run at its specified settings and don't have an EPP-capable motherboard.

My RAM is Corsair PC2-6400 4-4-4-12 @ 2.1V.

When I first installed it, my motherboard set it to 5-5-5-18 @ 1.9V. This (the timings, anyway - I think the mobo's automatic voltage "optimization" set the 1.9V) would be what was on the SPD. However, new high-performance RAM also often has EPP. This can be read by nForce5 and higher motherboards, and auto-set the right settings. My motherboard doesn't know/care about EPP.

In order to take advantage of the specified 4-4-4-12 timings for my RAM, I had to go into the BIOS and set the settings manually. This is not using lower latencies than the module's rating, it's just setting the rated settings that your motherboard may not be smart enough to set for itself.

 

[darkstar782]

SPD should however only give the BIOS timings based at 2.5v for DDR1 and 1.8v for DDR2.

Most high end ram requires much more than this to hit the rated timings, and so the SPD timings are alot looser to enable the system to boot and you set the correct voltage.

This is what nVidia and ATi are doing with "EPP" and "SLi Memory", adding voltage settings to the SPD.

 

[The_OGS]

Hi pell,
Yeah my old Corsair DDR used to run 2,2,2,5 @ 400DDR
Now I'm running 4,4,4,10 @ 800DDR2.
The timings are a delay (in clockcycles) at a given speed in MHz.
Less is better.
Like 31computers says, your PC will read the SPD values so if you run memory rated much faster than you require, you should be rewarded with much tighter memory timings (from SPD, automatically) at the slower memory speed.
Pushing either the timings or the speed in MHz is considered overclocking, and adequate memory voltage will help both noble efforts to succeed.
You can run CPU-Z to quickly see what values the memory has stored in SPD for the different speeds.
Regards

 

[The_OGS]

Yikes! darkstar782, that's a manly 3DMark 06 score...
I thought my 6061 was pretty good, LoL
L8R

 

[darkstar782]

Hehe thanks, but my 8800GTX has to take credit for it, I'm kind of ashamed of the fact that I haven't put in any work to get it to that speed, the GPU is un-volt modded and everything...

I'm more proud of This, 6635 from my old 7900GT 256MB to be honest

 

[Mondoman]

This may help clear up the marketing confusion a bit:
http://forumz.tomshardware.com/hardware/modules.php?name=Forums&file=viewtopic&p=1249881#1249881