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[ The PC Guide | Systems and Components Reference Guide | System Memory | Memory Packaging ]

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:

  • DRAM Quality: Inspect the manufacturer of the DRAMs used on each module you buy, or if buying by phone ask who makes the chips. If the person you are talking to doesn't even know what you are talking about, then you probably should consider buying somewhere else. If you can't determine who made the DRAMs, or if they are not marked as first quality DRAMs made by one of the big manufacturers listed in the table in the section on DRAM size and quality, keep shopping.
  • Number of Chips on the Module: Avoid SIMMs that have more than 12 chips on them if at all possible.
  • Module Quality: Do not assume that because you buy a SIMM that has chips saying "Toshiba" all over them, that the module itself was made by Toshiba. While DRAMs are made by big companies in large plants, the SIMMs themselves are assembled by many different kinds of companies. Some of them are rather unscrupulous and will cut corners in any way possible to save on cost. You are always best off to buy from a large reputable company, which is far more likely to be selective about where they get memory chips from, and conscientious about how they assemble them onto SIMMs.
  • Warranty: A good company will give you a one-year warranty on your memory. Very good companies offer a lifetime warranty (even though these are rarely ever used if the memory doesn't fail in the first 90 days). If someone offers you the deal of the century on modules with no warranty, well, think about it.

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.

Next: Memory Errors, Detection and Correction


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