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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"
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:
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.