Hardware Reference
In-Depth Information
• ECC
Nonparity systems have no fault tolerance. The only reason they are used is because they
have the lowest inherent cost. No additional memory is necessary, as is the case with par-
ity or ECC techniques. Because a parity-type data byte has 9 bits versus 8 for nonparity,
memory cost is approximately 12.5% higher. Also, the nonparity memory controller is
simplified because it does not need the logic gates to calculate parity or ECC check bits.
Portable systems that place a premium on minimizing power might benefit from the re-
ductioninmemorypowerresultingfromfewerDRAMchips.Finally,thememorysystem
data bus is narrower, which reduces the number of data buffers. The statistical probab-
ility of memory failures in a modern office desktop computer is now estimated at about
one error every few months. Errors will be more or less frequent depending on how much
memory you have.
Thiserrorratemightbetolerableforlow-endsystemsthatarenotusedformission-critical
applications. Inthiscase,theextreme market sensitivity topriceprobablycan'tjustifythe
extra cost of parity or ECC memory, and such errors then must be tolerated.
Parity Checking
One standard IBM set for the industry is that the memory chips in a bank of nine each
handle1bitofdata:8bitspercharacter plus1extrabitcalledthe parity bit .Theparitybit
enables memory-control circuitry to keep tabs on the other 8 bits—a built-in cross-check
for the integrity of each byte in the system.
Originally, all PC systems used parity-checked memory to ensure accuracy. Starting in
1994, most vendors began shipping systems without parity checking or any other means
ofdetectingorcorrectingerrorsonthefly.Thesesystemsusedcheapernonparitymemory
modules, which saved about 10%-15% on memory costs for a system.
Parity memory results in increased initial system cost, primarily because of the additional
memory bits involved. Parity can't correct system errors, but because parity can detect er-
rors, it can make the user aware of memory errors when they happen.
Since then, Intel and other chipset manufacturers have put support for ECC memory in
many chipsets (especially so in their higher-end models). The low-end chipsets, however,
typically lack support for either parity or ECC. If more reliability is important to you,
make sure the systems you purchase have this ECC support.
How Parity Checking Works
IBM originally established the odd parity standard for error checking. The following ex-
planation might help you understand what is meant by odd parity. As the 8 individual bits
in a byte are stored in memory,a parity generator/checker,which is either part ofthe CPU
 
 
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