What is the purpose of the Parity Flag on a CPU?

Question

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.

Answer 1

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.