Microsoft KB Archive/62398

Works: Meaning of Parity Error

PSS ID Number: Q62398 Article last modified on 06-12-1996

2.00

MS-DOS

= SUMMARY =

The syntax of a parity error message varies depending on the machine. The message can appear as “Memory Parity Error,” “Fatal Parity Error,” “Parity Check 2,” and in a variety of other forms.

This is a DOS-level error message indicating that a defective or damaged RAM memory chip has been addressed.

To test the RAM, run a recursive system diagnostics program on the computer, which will accurately check all the RAM memory addresses.

= MORE INFORMATION =

When information is sent from the CPU to RAM, the number of bits must be an even number. The parity bit is set by the parity chip, so that an even number of bits are always being sent.

Data Bits Parity Bit ——— ———-

1 1 0 1 1 0 1 0 1 (parity bit set)

1 1 0 1 1 0 0 0 0 (no parity bit set)

When the information is read back from RAM to the CPU, the number of bits is checked by the parity chip. If the information was written to a bad chip, the bit’s value will change, causing the parity to change, and the parity chip to take notice.

The following is an example:

G = Good chip B = Defective chip

Memory Chips G G G B G G G G Parity Bit ———— - - - - - - - - ———-

CPU->RAM 1 1 1 1 1 0 0 0 1

RAM->CPU 1 1 1 0 1 0 0 0 1 /

Data Bit Changes

In the above example, the fourth data bit was changed in RAM (due to a faulty chip) and now the sum of the bits is odd. When this information is read back into the CPU, the parity chip notices this and sends a fatal trap error to the CPU, which generates the parity error message and locks the computer up, requiring a cold boot.

KBCategory: kbother KBSubcategory: dworkskb Additional reference words:

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================================================================ Copyright Microsoft Corporation 1996.