View Full Version : Getting 1GB Ram for iBook, does CAS matter?

21st October 2005, 08:12 PM
I'm going to get a 1GB stick for my iBook. I'm wondering if there's any noticeable speed difference between CAS 2.5 and CAS 3 RAM.... is it worth the extra 20 or so bucks for the 2.5?

21st October 2005, 08:17 PM
"High performance" CAS 2.5 RAM runs at CAS3 on such a machine, so I'd just go the the cheaper stuff.


21st October 2005, 08:24 PM
Do you know this for a fact? I currently have a 512MB PC2100 stick... and System Profiler says it has 25330 speed. Which number denotes CAS latency? I hope in a way that it is true, so I can get the cheaper one and not worry about it.

21st October 2005, 10:01 PM
im also interested in shoving a 1gb or another 512 stick in my ibook, once again, i'd be interested to know of any performance increase?

22nd October 2005, 09:38 AM
"25330" = 2.5. Considering the iBook has RAM soldered in, installing anything faster than CAS 3 probably wont make any useful difference. That being said, there's a lot of inexpensive and good quality CAS 2.5 RAM around.

22nd October 2005, 02:18 PM
Originally posted by plunkotica@Oct 21 2005, 10:01 PM
im also interested in shoving a 1gb or another 512 stick in my ibook, once again, i'd be interested to know of any performance increase?
If you mean how much performance you will get with 1-1.5GB vs 512MB, I can tell you it will be noticably better. Go with the extra GB if you can :)

kim jong il
22nd October 2005, 10:38 PM
CAS latency refers to the speed OR number of CPU clock cycles that pass following a RAM data request at which data appears at the RAM output pins. While Macs are notoriously fussy about RAM specs and RAM with a CAS latency of 3-3-3 generally works just fine and will be no faster than the superior 2-2-2 or 2.5-2-2. However slightly out of tolerance/spec RAM (slower 3+ and some generic) may not work in your Mac and result in unstable performance or kernel panics.

I'd spend the extra $10-$20 if it's on a 1GB chip.


23rd October 2005, 01:09 PM
Provided there's some way to control ram timings 2.5 (or better yet, 2) will perform much better than a CAS latency of 3.

Physical memory is addressed in terms of rows and columns. CAS refers to the Column Access Strobe latency or, the amount of time (cycles) between when a request to retrieve memory from a particular location and when it's acted upon it. In this case, half the time spent retrieving data is dependent on the CAS latency of the memory module.

Most memory should be guaranteed to CAS2.5 for it's stated speed -- if it's not, I'd be angry.
You can also buy higher rated stuff that runs at CAS2 but as I said, you need to be able to set these more aggressive timings to get any benefit from it. Systems don't just "detect" the latency capabilities of a module when you install it.

There's a discernible difference in memory throughput between CAS2, 2.5 and CAS3 timings. Wether you will notice any benefit from it or not depends on what you're doing. Some applications might only see a performance increase in the order of 1-2% while stuff that involves alot of constant reading/writing to/from memory (like encoding) tends to fare much better (5-6%).

There's a nice article on ExtremeTech that looks at all this stuff. Read their conclusions here (http://www.extremetech.com/article2/0,1697,1637776,00.asp)

I've drawn my conclusions from personal experiences and experimentation with my PC.

kim jong il
23rd October 2005, 06:21 PM
Nice complimentry post xsive :) Sadly there is nothing in OS X that allows for control of RAM timing that I know of. It MAY be controllable from open firmware; I have no idea?


23rd October 2005, 09:38 PM
Unfortunately no - there isn't a way to alter CAS settings under any software or open firmware.

I stuffed up my original post - I was referring to RAM with a general setting of CAS 2.5-3-3 as required for these iBooks - sort of had CAS 3 in my mind as I typed it :)


23rd October 2005, 10:24 PM
Just to clarify -

xsive - you said that systems don't detect the timings of RAM... well as you said you're talking about your PC experience, and x86 BIOS and open firmware are very different beasts AFAIK - my understanding was the OSX does indeed detect the speed of your RAM and run it at either 2.5 or 3 CAS latency depending on the RAM. Can anyone speak from actual experience here?

I'm choosing between brand name CAS 3 (Corsair Value or whatever it's called) and brand name CAS 2.5, so reliability is not a concern.... I'm just wondering if we are talking like 1% speed difference, or 5%. Does anyone have any figures on this?