# What is DDR? What does PC-6400 mean?



## The Duke

DDR = Double Data Rate.
That is because the data can travel both up stream and down stream at the same time, thus doubling the overall rate.
Yes, in general when running the same bus speed ie;DDR2 800, the lower the CAS timing the faster the data is moved in and out of the RAM.

A good read on memory which is DDR based but still on key for DDR2 which is basically DDR X2








http://www.overclock.net/faqs/26706-...s-dummies.html

Also, many take their CAS4 and up it to CAS5 to achieve higher stable bus speeds.
In that methodology, the higher speed basically of sets the slower timings by providing a higher bus speed to move the data more quickly through the system. Otherwise, to maintain the tighter CAS4 timings a Divider is required which not only reduces the RAMs base speed, say from DDR2 800 (200MHz) to DDR2 667 (166MHz) at percentage of about 83% but the RAM only increases @ a relative 83% of the CPU increase.
So if you upped the CPU 100MHz and the RAM is "Divided" from 200MHz down to 166MHz the RAM only increase 83% or 83MHz.
I say about 83% because CPU multiplier dependent it can vary slightly.
The DDR2 RAM divider chart may help to illustrate that effect
http://www.overclock.net/amd-memory/...-dividers.html

Almost forgot, the "PC" rating like in PC6400 is the Peak Transfer Rate.


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## DuckieHo

PC2-*6400*

8 x 800MHz = 6400 bit/s


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## hks85

that all being said, what would you say about this?

lets compare a single stick of ram. its installed at stock FSB (200mhz) with a CL of 4, tested. bios are changed and speed is retested. what kinda speed comparison will we see? something tells me this is an impossible question to answer an that it will vary from mobo/core to mobo/core architecture but i really dont know.

DIMM number 1: DDR800 CL4 @ 200Mhz FSB
DIMM number 2: DDR667 CL5 @ 233Mhz FSB


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## The Duke

A higher raw bus speed with slower timings may compare similarly to a slower bus speed with tighter timings.
Only testing will prove/disprove that... to many variables to prove validity with an answer based on speculation.


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## hks85

when i get back home ill try it out and post results later tonight or perhaps tomorrow. It wont be a 100% valid answer because its using only one cpu architecture bris. anyone else who is interested, please perform test and post you findings using bench software. im using 32bit linix so ill prob be using different software then you which could further hinder the comparison results...

if you do want to join in, do this.

first test (assuming you have DDR2 800)
200Mhz FSB, DDR-800, CL4

test 2,
233Mhz FSB, DDR-667, CL5

please dont change ANY other settings in your bios. use soft bench software and record/post findings on here.

if you've got an older computer with DDR400 you can play along too.

test one = 200FSB DDR400 CL3
test two = 250FSB DDR333 CL4

something tells me no one is actually going to do this and that im sasting my time...


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