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[ The PC Guide | Systems and Components Reference Guide | System Memory | Memory Errors, Detection and Correction ]

The Market's Change from Parity to Non-Parity Memory

At one time, all computers used parity memory. In fact, until well into the fourth generation of PC systems it was impossible to even turn off parity checking on a PC. This changed rapidly, and in a few short years parity went from the default and the standard to the minority in new systems. Most Pentium class systems not only do not use parity memory, most cannot support parity checking (or ECC) at all. What happened?

The answer is simple: parity checking was cut out of the modern PC to save a few bucks--in essence, it is a corner-cutting measure. There are valid technical reasons to not do parity checking, as I discuss here, but the original motivation was cost savings. By not having to include an extra bit of parity storage for every eight bits of data, non-parity memory is approximately 11% cheaper than parity memory. Note that there is no savings in not having a parity generator/checker; it is part of the chipset and is an extremely cheap and simple logic circuit. And the enormous savings as a result of taking away error detecting capability? The typical system at the time that this was done had about 8 MB of memory costing around $200. So, about 20 bucks on a system costing over $2,000 in most cases. Data corruption risk for a savings of 1%. Today, the difference in the cost of the chips to make a 32 MB machine parity instead of non-parity is even less: $10 at most.

Now the PC industry is extremely competitive, and there have been many ways that vendors have tried to reduce system cost. In many cases the less reputable dealers would cut costs and the better ones wouldn't; however the move to non-parity memory seemed to be nearly universal. There were a few reasons for this:

  • When Intel introduced the Pentium it also introduced its new chipsets for Pentium motherboards. These early chipsets were the standard from the early going, and they did not support parity checking. (Intel's 430HX Pentium chipset supports parity and ECC, but was not introduced until two years after Intel's first Pentium chipsets).
  • Removing parity checking was easy to do because few PC buyers have enough computer knowledge to understand the difference or what the implications are of not having parity memory; it just isn't something most people would pay any attention to.
  • Keeping parity memory had no positive impact on performance, so it wasn't something to use as a club against the competition ("we still use performance-enhancing parity memory!").
  • The negative effects of not having parity memory (risk of unreliable operation) are virtually impossible for the average person to see--they tend to blame some other component.

Overall, the market became oriented so that a vendor deciding to include parity memory would incur an additional cost and get no sales benefit for it. The end result of all of this was a self-fulfilling prophesy of sorts: as parity memory grew out of favor, demand for it (and hence production) dropped dramatically. This caused the price to go up relative to non-parity memory, which in turn led to its becoming even less popular. Most of the reasons for parity memory costing more today are supply and demand issues; with DRAM prices so cheap today the material difference in cost between parity and non-parity 8 MB SIMMs is less than five dollars.

I am pleased to report that parity/ECC is making a comeback, particularly in systems running the Pentium Pro and Pentium II. In part this is because of the superior capabilities of ECC over parity, and in part due to the fact that Pentium Pros are often used in servers, where data integrity is paramount. In fact, not only is ECC getting renewed interest in memory, Intel is adding it to its secondary cache bus on faster Pentium II processor modules, due to the need for data integrity with these higher-speed processors.

Next: Parity vs. Non-Parity: Pros and Cons


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