What is the purpose of the Parity Flag on a CPU?
Some CPUs (notably x86 CPUs) feature a parity flag on their status register. This flag indicates whether the number of bits of the result of an operation is odd or even.
What actual practical purpose does the parity flag serve in a programming context?
Side note: I'm presuming it's intended to be used in conjunction with a parity bit in order to perform basic error checking, but such a task seems to uncommon to warrant an entire CPU flag.
Answer
Back in the "old days" when performance was always a concern, it made more sense. It was used in communication to verify integrity (do error checking) and a substantial portion of communication was serial, which makes more use of parity than parallel communications. In any case, it was trivial for the CPU to compute it using just 8 XOR gates, but otherwise was rather hard to compute without CPU support. Without hardware support it took an actual loop (possibly unrolled) or a lookup table, both of which were very time consuming, so the benefits outweighed the costs. Now though, it is more like a vestige.