What is "parity bit"

parity bit \par"i*ty bit`\, n. (Computers) The bit within a data structure which is assigned a value of 1 or 0 so as to make the parity[7] of the data structure odd or even. Data structures may or may not have parity bits, dpending on whether the system does or does not perform parity checking. The most commonly used parity bit is the eigth (higher-order) bit of a byte, which is used when data transmission uses only the 7 lower-order bits of each byte as significant data; some memory systems use a ninth bit as a parity bit for each eight bits (one byte) of significant data in memory.

n. (context computing English) A bit (binary digit) used to indicate whether the number of bits in a given set with value of one is even or odd. Parity bits are used as a simple error detection code.

n. (computer science) abit that is used in an error detection procedure in which a 0 or 1 is added to each group of bits so that it will have either an odd number of 1's or an even number of 1's; e.g., if the parity is odd then any group of bits that arrives with an even number of 1's must contain an error [syn: parity, check bit]

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7 bits of data

(count of 1-bits)

8 bits including parity

even

odd

0000000

0

00000000

1010001

3

10100011

1101001

4

11010010

1111111

7

11111111

A parity bit, or check bit, is a bit added to a string of binary code that indicates whether the number of 1-bits in the string is even or odd. Parity bits are used as the simplest form of error detecting code.

There are two variants of parity bits: even parity bit and odd parity bit.

In the case of even parity, for a given set of bits, the occurrences of bits whose value is 1 is counted. If that count is odd, the parity bit value is set to 1, making the total count of occurrences of 1's in the whole set (including the parity bit) an even number. If the count of 1's in a given set of bits is already even, the parity bit's value is 0.

In the case of odd parity, the coding is reversed. For a given set of bits, if the count of bits with a value of 1 is even, the parity bit value is set to 1 making the total count of 1's in the whole set (including the parity bit) an odd number. If the count of bits with a value of 1 is odd, the count is already odd so the parity bit's value is 0.

Even parity is a special case of a cyclic redundancy check (CRC), where the 1-bit CRC is generated by the polynomial x+1.

If the parity bit is present but not used, it may be referred to as mark parity (when the parity bit is always 1) or space parity (the bit is always 0).

Parity bits are generally applied to the smallest units of a communication protocol, typically 8-bit octets (bytes), although they can also be applied separately to an entire message string of bits.