Parity flag

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In computer processors the parity flag indicates if the number of set bits is odd or even in the binary representation of the result of the last operation. It is normally a single bit in a processor status register.

For example, assume a machine where a set parity flag indicates even parity. If the result of the last operation were 26 (11010 in binary), the parity flag would be 0 since the number of set bits is odd. Similarly, if the result were 102 (1100110 in binary) then the parity flag would be 1.

[edit] Example (x86 Processor Family)

The parity flag is changed in the x86 processor family by the following instructions (from an 80386 Intel manual):

  • All arithmetic instructions;
  • Compare instruction (equivalent to a subtract instruction without storing the result);
  • Logical instructions - XOR, AND, OR;
  • the TEST instruction (equivalent to the AND instruction without storing the result).
  • the POPF instruction
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