Memory Parity and ECC

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The integrity of memory and its contents are crucial to the successful operation of a computing system. There are two logical diagnostic memory tools that serve as a system of checks and balances for the contents of DRAM. These two forms of memory checking are called 'parity' and 'error correction code,' or Error Checking and Correction (ECC).

Parity

As you may recall, there are 8 bits in 1 byte. Parity checking adds another bit, called a parity bit (ninth bit), to each byte of information that is stored in memory to verify its integrity. In other words, an even or odd parity bit is added to every byte of information (8 bits). The parity bit for each byte of information is made to force all bits or units to have either an odd or even number of bits. Later, when the byte of information is needed, the computer checks to verify the even or odd state of the byte; if it does not match its original assignment of even or odd parity, a memory parity error occurs, and the system may halt. Some parity errors may show up on the computer screen if the parity check fails. A parity error 1 indicates that the parity error or check has failed on the motherboard. A parity error 2 indicates that the error has most likely occurred on a memory expansion board. An important fact to remember is that parity is a tool used only to detect errors in memory-parity does not fix memory problems.

Calculating the number of parity bits associated with memory is fairly simple. If one parity bit is assigned to every byte (8 bits) of data, we can calculate that 16 bits have 2 parity bits, 32 bits have 4 parity bits, and 64 bits have 8 parity checking bits.

Less expensive memory modules are available that provide fake parity. Fake parity does not provide a valid test of data stored in memory; it simply fools the system into believing that any results from memory are acceptable.

Another way of manufacturing a less expensive memory module or chip is to disable parity altogether.

Inexpensive or used memory is often to blame for parity errors and General Protection Faults (GPFs). Generally, GPFs occur when more than one application or program attempts to access or write to an area of memory already assigned to another application.

Error Correction Code or Error Checking and Correction

ECC works in conjunction with the memory controller to not only detect errors found in data as it passes out of memory, but also to fix single-bit errors with its built-in logic. ECC adds a special bit to data, called an error correction code bit, which is decoded by the memory controller for accuracy.

Some SDRAM chips support ECC. DIMMs (which normally have eight chips on a circuit board) that have a ninth chip show the presence of ECC. You can look in the BIOS configuration to see if ECC is enabled. ECC is worth the extra money it costs, because of its reliability.



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A+ Complete Study Guide, Third Edition (220-301 and 220-302)
The A+ Exams Guide: (Exam 220-301), (Exam 220-302) (TestTakers Guides)
ISBN: 1584503076
EAN: 2147483647
Year: 2003
Pages: 237

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