What Are Check Digits?

The parity bit is also called a parity bit . It is a binary number that indicates whether one of the binary numbers of a given bit is odd or even. Parity is the simplest error detection code.

There are two types of parity bits : even parity and odd parity . ^[1]

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Because it is simple, parity bits are used in many computer hardware, and can be re-operated when in trouble, or where simple error detection can make a big difference. For example, the SCSI bus uses parity bits to detect transmission errors, and many microprocessors' instruction caches also include parity bit protection. Because the instruction cache data is a copy of the main memory data, it can discard the error data and retrieve it again when an error is found. ^[2]

Some redundant disk arrays (RAID) use parity blocks for redundancy. If a disk in the array fails, the data blocks in the working disk are used with the parity blocks to reconstruct the lost data.

Each column in the chart below represents a disk, assuming A1 = 00000111, A2 = 00000101, and A3 = 00000000. The Ap obtained by XOR of A1, A2, and A3 is equal to 00000010. If the second disk fails, A2 will not be accessible, but it can be rebuilt by XOR of A1, A3 and Ap:

A1 XOR A3 XOR Ap = 00000101

Redundant disk array

A1 A2 A3

Ap B1 B2

Bp C1 C2

C3 C4 Cp

Note: The data block is format A #, and the parity block is Ap.