In computers, parity refers
to a technique of checking whether data has been lost or
written over when it's moved from one place in storage to
another or when transmitted between computers.
Here's how it works:
An additional binary digit, the parity bit, is added to
a group of bits that are moved together. This bit is used
only for the purpose of identifying whether the bits being
moved arrived successfully. Before the bits are sent, they
are counted and if the total number of data bits is even,
the parity bit is set to one so that the total number of
bits transmitted will form an odd number. If the total number
of data bits is already an odd number, the parity bit remains
or is set to 0. At the receiving end, each group of incoming
bits is checked to see if the group totals to an odd number.
If the total is even, a transmission error has occurred
and either the transmission is retried or the system halts
and an error message is sent to the user.
The description above describes how parity checking works
within a computer. Specifically, the Peripheral Component
Interconnect bus and the I/O bus controller use the odd
parity method of error checking. Parity bit checking is
not an infallible error-checking method since it's possible
that two bits could be in error in a transmission, offsetting
each other. For transmissions within a personal computer,
this possibility is considered extremely remote. In some
large computer systems where data integrity is seen as extremely
important, three bits are allocated for parity checking.
Parity checking is also used in communication between modems.
Here, parity checking can be selected to be even (a successful
transmission will form an even number) or odd. Users may
also select no parity , meaning that the modems will not
transmit or check a parity bit. When no parity is selected
(or defaulted), it's assumed that there are other forms
of checking that will detect any errors in transmission.
No parity also usually means that the parity bit can be
used for data, speeding up transmission. In modem-to-modem
communication, the type of parity is coordinated by the
sending and receiving modems before the transmission takes