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DRAM Size and Quality
For most people, it is not terribly important to pay attention to the number and type of DRAM chips that are used to make the SIMM or DIMM they plug into their PC. After all, the whole point of using these modules was so that it wouldn't be necessary to worry about all the little chips. However, all SIMMs are not created equal. Some older PCs will only work properly with SIMMs that have the right configuration and types of chips. There are also important quality issues associated with the DRAMs that the SIMM is made from.
Much as SIMMs are specified using a "depth x width" notation, the individual DRAM chips are as well. There are DRAM chips of various sizes available on the market, which have different depths and widths as well, and a SIMM can be made up (in general) of any combination of DRAM chips that adds up to the proper depth and width. For example, a 1x9 30-pin 1 MB parity SIMM is typically made up of either 9 1Mx1 DRAMs, or 2 1Mx4 DRAMs plus 1 1Mx1. In both cases, the total memory is the same: 1Mx9.
DRAMs are labeled using part numbers, usually a long string of letters and numbers. The exact part number is manufacturer-specific; however, many manufacturers' part numbers are similar in their last few digits, which can help you to identify the exact chip type and size if you need it. However, first, you have to be able to identify the manufacturer! This isn't always that easy, for two reasons.
First, they use a short code instead of the company name in many cases. Second, manufacturers often resell lesser-quality parts under a different name, a little-known fact. Manufacturers will often sell their top-quality parts (sometimes called "A grade") to their bigger customers and label them with their primary name. They will also have lower-quality parts (sometimes called "C grade"), perhaps not with as much margin on their marked speed or maybe not tested as well, that they will sell under a different label (for less money of course). The exact difference between high and low quality parts depends on the manufacturer. The high-quality parts may have passed more tests, or may have more speed margin compared to their rating. Either way, know that there is a difference.
This table shows common DRAM manufacturers and what the codes are that they typically use on their chips. I only know of some of the codes for the second-quality chips; another thing to look for in general is the name of a country on a chip. If it just says "JAPAN" or "KOREA", then the manufacturer has decided to leave their name off of their product--why do you suppose they would want to do that?:
It is important to realize that not all DRAMs are created equal. For the mostpart, a SIMM made from DRAMs manufactured by one of the major manufacturers is likely to be a good quality part. DRAMs that are either seconds as identified above or "mystery parts" with no known markings on them may easily be of lower quality (however, the latter could also be perfectly good parts made by a good company whose markings you don't recognize). Some companies have unfortunately even been known to sell partially damaged DRAMs on the market, which are bought by "price conscious" SIMM manufacturers and made into poor-quality memory modules.
The following table shows the different sizes of DRAM chips commonly used in memory modules, and the different bit widths that each size is typically available in. For each one, the configuration of the chip is shown (depth x width) and then in parentheses, the most commonly found last four digits of the part number found on the chip:
Note: The four digit codes above are the typical ones you'll find on FPM memory modules. EDO memory chips will have different numbers, and usually the pattern with 16 Mbit EDO chips is that they have a non-zero last digit where the FPM digit is zero. For example, a 4Mx4 FPM chip may have a 7400 code, while an EDO chip may have a code like 7404 or 7405. See this web site for a pretty comprehensive list of chip part numbers that you can use to find out specifically what's in your modules.