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False Parity Memory (a.k.a. "Logic Parity")

Parity memory was replaced by non-parity memory as a cost-saving measure. However, the penny pinchers responsible for this had a problem--the large number of installed systems that had no way to turn off parity checking. So they came up with a "solution"--false parity memory. It is also called "logic parity" or "parity generator memory"--fancier names for the same crapola.

Regular parity checking works by storing a parity bit when a byte of data is written to memory, and then using it for error detection when the byte is read from memory. False parity memory replaces all the extra parity bits on a memory module with a special circuit. This circuit generates the correct parity bit each time any memory byte is read. So it is generating the bit at read time, instead of write time. The result is that a parity error will never occur--it can't, because the parity bit is always calculated as the data is read so that this won't happen.

What is the point of bothering with this? Simple: the little circuit on the SIMM costs less than the bits of parity memory it replaces. This made more sense many years ago when this stuff became prevalent, because memory was more expensive than it is today. False parity memory is in many ways worse than non-parity memory, because it was often sold to unsuspecting buyers as real parity memory. In simple terms, this is fraudulent, and was done to let the memory vendor pocket a few extra bucks at the expense of his customer. At least with non-parity memory you know what you are getting.

Warning: What false parity memory is doing is defeating the parity checking scheme. It is--quite literally--making sure to always "tell the system what it wants to hear". If you want to do error detection and correction on your system, avoid this garbage at all costs. Fortunately, the word is now out about false parity memory, and with most systems now having a BIOS setting to control parity checking, false parity memory is becoming rather hard to find. Still, you should always make sure you buy real parity memory if you are running with parity checking enabled. Some vendors call this "true parity" memory.

There is only one legitimate application of false parity memory: if you are absolutely sure that you want to run without parity protection, and you have no way to turn parity checking off, you can use false parity memory to fool your system into working "properly". It's still not recommended but at least if you know your parity memory is fake, you won't be tricked into thinking you have protection that isn't there.

The best way to avoid the chance of buying false parity memory is to specify "true parity" memory and buy from a good vendor. No reputable vendor is going to try to hoodwink a buyer by selling logic parity. If you think you may have false parity memory in your system, there's a couple of things you can do to check:

• Enable ECC: Most of these things are designed to fake out parity circuits, but they can't fake out the ECC circuits on newer chipsets. Using this BIOS setting, enable ECC. If the system works with parity enabled but gives errors when you turn on ECC, the chances are high that you have fake parity memory.
• Inspect the Module: The logic circuit that does the faking out on these modules looks different from the other chips. It is usually smaller and closer to the memory module's circuit board, and will be marked very differently than the other chips on the module. While it is normal for there to be different sizes of DRAM chips on a parity module, false parity modules will have a logic chip which is completely different in size and shape than the real DRAM chips.
• Count the Chips: For 30-pin SIMMs, an odd number of chips means that the SIMM is parity, and usually true parity. For 72-pin SIMMs, true parity SIMMs usually have 12, 18, 24 or 36 chips. If you are sold a 72-pin SIMM (not 30) with 9 chips or 17 chips, and one of them has totally different looking markings and a different physical appearance, this may be a logic parity module. 72-pin ECC modules can have 9 chips, but they should all be DRAM chips; usually with false parity the ninth chip is much smaller and may even be attached to the module differently.

If you suspect you have been sold false parity, call up the vendor and ask. They often will surprise you by admitting it up front, as some of them think selling this junk is legitimate (I even had one guy try to tell me "well, you didn't specify true parity memory!"... uh, sure, so why did you charge me 20% extra for it?) Have it replaced or get your money back.

Next: Parity in Advanced Memory Technologies

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