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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.
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The FLAGS register is the status register that contains the current state of a x86 CPU. The size and meanings of the flag bits are architecture dependent. It usually reflects the result of arithmetic operations as well as information about restrictions placed on the CPU operation at the current time.
Parity Flag (PF) - this flag is set to 1 when there is even number of one bits in result, and to 0 when there is odd number of one bits. Even if result is a word only 8 low bits are analyzed! Auxiliary Flag (AF) - set to 1 when there is an unsigned overflow for low nibble (4 bits).
A flag is a component of a programming language's data structure. A computer interprets a flag value in relative terms or based on the data structure presented during processing, and uses the flag to mark a specific data structure. Thus, the flag value directly impacts the processing outcome.
Prerequisite – Registers of 8085 microprocessor The Flag register is a Special Purpose Register. Depending upon the value of the result after any arithmetic and logical operation, the flag bits become set (1) or reset (0). In 8085 microprocessor, the flag register consists of 8 bits and only 5 of them are useful.
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.
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