CL5 667 MHZ RAM vs. CL6 800 MHz RAM in ThinkPad X61?

Which is capable of 667 MHz (if I recall correctly).

First of all, I don't really understand how this works:
- One RAM is faster in one way (800 MHz) but slower in another way (CL6)
- The other RAM is the other way around

CL6 800 MHz RAM is more prevalent and cheaper.

Do I see an actual difference in performance?

 

First let me start by saying that this is a complicated subject and there is no black and white answer on this topic for all applications.


The 800Mhz is the clock speed of the memory, it will have a very significant impact on the maximum data transfer rate the memory module can achieve. The CL number is the CAS Latency: *Wikipedia* - "is the delay time between the moment a memory controller tells the memory module to access a particular memory column on a RAM module, and the moment the data from the given array location is available on the module's output pins." - *Wikipedia*

As you can see from the above explanation, 800Mhz memory will have a greater maximum transfer rate, while 667Mhz memory with a lower CL value will be able to access a particular area of memory quicker but the maximum data throughput (transfer speed) will be less than the 800Mhz module.

I guess a comparison I could make is to think of CL as the seek speed on a HDD and the memory clock speed as the maximum transfer speed on the HDD.

Both will have an impact on performance but most people will tell you that it is still worth going for the higher clock speed memory even at the cost of higher CAS Latency values.

Most of the time the higher the clock speed of a memory module the higher the CAS Latency values.

The difference between CL5 and CL6 in real world performance is close to insignificant in most applications, it will normally be best to go the higher clock rated memory. In saying that it also depends on the bus speed of your laptop, normally you want to get memory that is going to match the bus speed of your laptop. Seeing that you probably won't be overclocking or can't overclock the laptops bus.

I don't know the bus speed of your ThinkPad X61, but I recommend you look at getting whatever memory module matches with that speed bus and has the lowest CAS value. Say if the X61 is designed to work with 667Mhz memory, then there is probably little point in getting the 800Mhz module.

 

The Intel Core 2 Duo CPU in the X61 supports DDR2 SDRAM at 667 MHz. An installed 800 MHz module will be operated at 667 MHz speed.

(That X61 is a very nice notebook.)

 

Well Kaso has answered what speed memory the X61 supports and being that it's designed to work with 667Mhz RAM there is little point in getting the 800Mhz mem as it won't run any faster than the 667Mhz.

You may find that once the 800Mhz memory is underclocked by the system to rum at 667Mhz, it's CL value may actually be the same as the stock 667Mhz RAM. - Normally when you run RAM at lower clock speeds than it was originally designed for, you can tighten the timings, meaning lower CL values.

 

FWIW, I've used both 667 and 800 RAM on *61 generation, and both work well as long as the DIMMs themselves are error-free.

Personally, I prefer using brand-name RAM (Micron, Mushkin, Elpida, Samsung...) especially nowadays when the price difference when compared to generic offerings is really not that significant at all.

My $0.02 only...

 

Thanks for your interesting insights. Especially if CL6-800MHz RAM goes at CL5-667MHz. Customer services of RAM manufacturers couldn't tell me yet.

 

I don't know where should I ask this, but I asked it here: http://forum.notebookreview.com/har...ile-tweaks-performance-battery-life-heat.html