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Mixing Parity/ECC and Non-Parity Memory

Under most circumstances, you will want to avoid mixing parity and non-parity memory in the same machine. Certainly, no vendor should ever sell you a machine that comes equipped with mixed memory in this manner. However, many folks who upgrade machines, particularly older ones, find themselves in a situation where they may not be able to find expansion memory that is identical to that which is already in the machine. This can include problems finding parity memory for an older parity machine. Also, in some cases parity memory of a particular style may be available but only at a prohibitive price.

Here are some general guidelines for mixing parity/ECC and regular non-parity memory in the same machine:

• If possible, avoid mixing memory at all. Memory is cheap enough now that it may be cheaper to replace all of the memory in a PC than to upgrade it with a more expensive type.
• Do not mix parity and non-parity SIMMs in the same bank of memory on any machine. This can lead to all sorts of problems due to the modules being different electrically.
• If your machine currently uses parity memory and has no way to disable parity checking in the BIOS, you must use either proper parity memory in the machine, or if you must, false parity memory to fool the system into thinking it has all parity memory. If you put regular non-parity memory into a parity system, the machine will halt shortly after it starts to boot.
• If your machine either does not allow parity checking, or it can be disabled in the BIOS, you can use either parity memory or non-parity memory, and probably can mix them as well. Parity memory placed into a non-parity system will normally function as non-parity; the extra parity bits are simply ignored.
• There are probably some machines out there that will act strange if you mix parity and non-parity memory in them. This is more likely to be the case for older machines than newer ones.
• Watch out for certain big-name PCs that require proprietary modules.
• It is not possible to mix parity and non-parity memory in a parity machine and have the parity memory still function as parity memory; you must turn off parity checking. However, if you mix true parity memory with false parity memory and leave parity checking enabled, then the true parity memory will still be protected.

As with all hardware upgrades, you should back up your hard drive before adding memory to your PC, especially if mixing different types.

Next: Logical Memory Layout

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