The primary function of a parity bit in a programming a programmable controller or computer is to?

A parity bit is a primitive checksum-like tool, used to verify the integrity of a group of data bits. For example, some memories support a parity bit to cover an 8-, 16- or 32-bit bus, and on serial data lines, parity bits (typically covering 8 data bits) are sometimes used to help detect transmission errors.

A parity bit is, as the name suggests, a single bit, and is only effective with a number of simple errors, while a great number of other possible errors will slip through a parity-based integrity check undetected.

Stronger integrity checks use a cyclic redundancy check (CRC) or more sophisticated methods, such as SHA-2 and others. Priority bits still have their place where those more sophisticated methods are unsuitable or unappealing due to their cost or processing requirements; priority bits are very cheap to implement and validate in hardware, and can at least provide some level of integrity where the alternative might have been no integrity checks at all.