This guide discusses the principles underlying how to determine RAM compatibility.  Although the discussion focuses on 256MB SDRAM non-ECC 144-pin SoDIMM, the principles discussed are equally valid for DDR, DDR2, EDO or FPM memory; or for other types of SoDIMM, DIMM or SIMM; or for RAM modules of other sizes. The only type of RAM it does not apply to is RDRAM.

Before you begin you should ascertain your laptop can indeed accept 256MB SDRAM non-ECC 144-pin SoDIMM, and that the memory slot in your laptop has enough clearance space.  SoDIMMs are predominantly 1.25" (32mm) tall,  but can vary from 0.9" to 2.3" in height.

The first thing to remember is that it is rarely relevant whether the RAM is PC133, PC100 or PC66. All SDRAM SoDIMMs meets the PC66 standard. Those which can be safely used up to 100mhz FSB (front-side bus) are called PC100. PC100 memory which can be safely used up to 133mhz FSB (including 100mhz naturally) is called PC133. In other words, all other parameters being equal, PC133 RAM is always backward compatible with PC100 RAM, and PC100 RAM is always backward compatible with PC66 RAM. The only time PC133-PC100-PC66 becomes an issue is when lower-rated RAM is used in lieu of higher-rated RAM (e.g. using PC100 when PC133 is required), but never the other way around.

When it comes to compatibility, the most important parameter by far is the number of chips used to build the SoDIMM. Each laptop has a maximum limit on the largest RAM chip it can read. Since a SoDIMM can be built with 16, 8 or 4 chips, this limit can be found by dividing the maximum RAM per slot by 16. For example, if the specifications of your laptop say that it can take up to 256MB per slot, that implies it is limited to chips of 16MB each or less (256 divided by 16). Therefore you should not expect it to work with any 8-chip 256MB SoDIMM or 4-chip 128MB SoDIMM, since both are built with 32MB chips ##.

The next most important parameter is chip density. Every 256MB SoDIMM has the 32Mx64 configuration **. Within this are actually 4 sub-configurations (note: there is no 4-chip or 32-chip 256MB SDRAM SODIMM):

1. 16 chips of 16Mx8 each
2. 16 chips of 32Mx4 each
3. 8 chips of 16Mx16 each
4. 8 chips of 32Mx8 each

(1) and (3) are called low-density while (2) and (4) are called high-density. In addition to the maximum size per chip, your laptop may also be limited on the maximum chip density. For example, if your laptop is limited to 16M density and a 32M-density SoDIMM is installed, depending on how it was designed, your laptop may either reject it completely, or recognize only half of the memory because it would be treating each 32M-density chip as if it were only 16M-density (because it would be reading 32Mx64 RAM as if it is 16Mx64, or 128MB).

A third parameter which sometimes come into play is refresh rate. Some people who don't understand how chip density could cause a laptop to recognise only half of the RAM (see preceding paragraph) erroneously speculate that is caused by the RAM having too high a refresh rate.  The computer "determines" the size of the RAM during its boot-up process when it checks each of its hardware components. The refresh rate only comes into play after data has already been written to the RAM and needs to be refreshed; which happens later, typically when software is loaded or run. These two processes have nothing to do with each. The truth is that an incompatible refresh rate will cause your laptop to crash,  but cannot affect the size of RAM your laptop recognises.  In any case it is a very minor factor.