memory frequency

[madcow]

I am using cpuz and it said RAM frequency is 333 Mhz. I thought the speed of the RAM that comes with the t60 is 667 Mhz. Can some clear this up for me?

http://www.cpuid.com/cpuz.php

[hoplite]

DDR so 333 X 2 = 667MHz.

I believe...

[rkawakami]

Memory (RAM) clock is actually 333Mhz on the motherboard but since the T60 appears to use DDR (double-data rate) modules, the effective speed is double.

With SDR (Synchronous Dynamic RAM), you get one piece of data with every clock "tick". DDR gives you two pieces of data with each "tick". Think of it as at the "tick" and half-way to the next "tick".

Here's a crude drawing:

Code: Select all

SDR (Synchronous Dynamic RAM)
  _   _   _   _   _   _   _
_| |_| |_| |_| |_| |_| |_|   System clock

     ^   ^   ^   ^   ^   ^   Data access

DDR (Double Data Rate Synchronous Dynamic RAM)
  _   _   _   _   _   _   _
_| |_| |_| |_| |_| |_| |_|   System clock

     ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^   Data access

The system clock is represented by a digital signal rising and falling between two different voltage levels. For the SDR, data is available at each rising edge. For DDR, data is available at both the rising and falling edges. Thus, DDR has twice the effective "speed".

[madcow]

Thanks guys. I feel much better now.

[bontistic]

Ray,

This explanation probably works with the 533MHz PC2-4200 clock as well right? (266 x 2 = 532)

[rkawakami]

Yes, the PC2-4200 is a DDR2 specification. It is a slightly different version of a double data rate memory but the clock usage is the same. So the "533Mhz" speed memory is using a 266Mhz system clock. Here's a resource I found that touches a little bit upon the DDR vs. DDR2 differences:

http://www.hardwaresecrets.com/article/167