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Memory Module Quality Factors
Assuming that "memory is memory" is one of the biggest mistakes made by people who buy RAM upgrades for their PCs. I know, because I used to make it myself. It's easy to walk into a computer show, or scan on the net for the lowest prices, and just buy whatever is cheapest. Even the vendors often have little understanding of the product they are selling, and some of them say as little as possible about the memory intentionally, because they know its quality is so low. So all they say is "16 MB EDO $xx". I've seen buyers make the decision to spend over 300 dollars on SIMMs at company A based entirely on the fact that they had a SIMM for $150 and company B's were $152. They just handed over the money, no questions asked (well, maybe "were these tested?", which always is answered in the affirmative by every vendor.) Would these people purchase a car, or even a stereo system in this manner? Unlikely.
The reason this happens is that the competition amongst vendors is fierce, and buyers have little knowledge of what they are purchasing. The perception is that all memory is the same, and so many people just buy whatever is cheapest. This is exacerbated by the fact that most systems today do not use parity memory, so users often don't realize that the random system problems they are having are due to poor quality memory. Many vendors are quick to pass off errors that are being caused by their memory as a "software glitch" or "motherboard defect", and in fact some of them are counting on you doing this when they sell you their junk (not all or even most, but some). The result of all this is that a lot of junk is being sold on the open market, especially by small companies to home builders and upgraders, and especially at computer shows or swapmeets.
Here are some quality factors to bear in mind specifically when evaluating memory modules:
Warning: Beware of parity
modules that are actually what is called "logic parity";
they are not real parity memory and provide no error detection capabilities at all.
One final tip: beware of modules that have too many chips on them. I have seen (and even bought, before I realized what they were) 4x36 SIMMs with 12 DRAM chips on them. Now 12 chips is a normal configuration for a parity SIMM, but 4 of the chips should be smaller than the other 8. These chips were all the same size, and turned out to be 4Mx4 DRAMs, yielding a total of 4Mx48 bits worth of memory. Since the SIMM only needs 36 bits of width, why would they waste money putting the extra chips on the SIMM? The reason is simple: because only part of the DRAMs actually works. Some companies will sell damaged 4Mx4 DRAMs, where 3 of the 4 quadrants are still functional, as 4Mx3 chips. Then 12 of these can be used to make a 4x36 SIMM. You can see small resistors on these SIMMs which are used to control which portions of these damaged 4Mx4 are actually used. I saw these for sale at a "great price" at a computer show a long time ago and I got suckered. TANSTAAFL.