RAM modules for Apple notebooks: the test

Materials
Most of laptops use DDR SO-DIMM RAM modules (Double Date rate synchronous SDRAM Small Outline Dual-In-line Memory Modules) as memory modules.
SwissBit has provided us with 3 high-density/high-speed modules featuring the " Chip-On-Board" technology :

1GB DDR333 SODIMM PC2700
512MB DDR333 SODIMM PC2700
1GB DDR266 SODIMM PC2100

To illustrate the difference linked to the high-density technology, here is a photos displaying a standard 1GB DDR333 PC2700 module and a SwissBit module of the same capacity. You can appreciate the difference of the chips.

1GB DDR333 SODIMM PC2700 (standard)

With each module we also received information related to their specifications (example). We will describe , in a little bit, what they mean , especially those defining the capacities and performance level of RAM modules. If you wish to go into more details, please...
The four most important parameters to know when comparing RAM modules, or looking for one, are often mentioned by resellers, and are even advertised by manufacturers selling "aggressive settings" RAM modules, which are supposed to boost performance. The RAM modules we have been testing are labeled: 2.5-3-3-10 . We will now explain the meaning of those numbers.

• The first value defines the CAS latency: for sure the most important parameter, since its value indicates the RAM module response time and more precisely the number of cycles necessary to transfer the information upon memory controller request from the buffer to the processor. The smaller the CAS latency value, the better it is and as a consequence the faster the RAM module will be (for this parameter). One will consider CAS latency 2.0 as an aggressive setting , while slower settings will correspond to value ranging from 3.0 to 4.0.
• The second value is the tRCD (ACTIVE to READ or WRITE delay or RAS to CAS): this parameter is also important since it defines the time necessary for the RAM module to be available for further requests. As with the CAS latency, the smaller the value is, the better. The optimal value for the tRCD will range between 2 and 3 cycles; this is of course really important for notebooks using a rather "slow" bus frequency (133MHz for the iBook and 166MHz for the PowerBook).
• The third value is the tRP (PRECHARGE command period): it indicates the latency to deactivate the open row in a particular bank or the open row in all banks. The bank(s) will be available for a subsequent row access   at some specified time (tRP) after the PRECHARGE command is issued. Once a bank has been precharged, it is in the idle state and must be activated prior to any READ or WRITE commands being issued to that bank. The precharging command line is an electrical necessity.
• The last but not least important value is the tRAS (ACTIVE to PRECHARGE command): it simply defines the time necessary between 2 RAM access requests , the optimal value being around 6-8 cycles.

So why not always looking for the most aggressive settings RAM module? Very short, or too short latencies are endangering the overall stability. Latencies are a kind of security gap between commands, like taking a breath in between before singing the next line. The performance gain from reducing the latencies is usually marginal except when there is a big gap between 2 RAM modules specifications. Short latencies are usually associated to a higher risk of potential instabilities. One need to test these parameters carefully before using them in a system.


This RAM modules information are available directly from the System information panel:


The number 25330 indicates 2.5-3-3-10. Users with CAS3.0 RAM module should have 30330 displayed.