# Overflow Flag

The Overflow Flag indicates that the previous operation resulted in a signed number whose sign was inconsistent with the signs of the operands that produced it—signed integer overflow. For instance, the Overflow Flag gets set when adding two positive numbers produces a negative result, or adding two negative numbers produces a positive result.

The truth table below indicates when the CPU sets or clears the Overflow Flag:

Addition | ||||
---|---|---|---|---|

Sign of first operand | Sign of second operand | Sign of result | Resulting value of Overflow Flag | Notes |

0 | 0 | 0 | 0 | |

0 | 0 | 1 | 1 | Adding two positive numbers produced a negative result |

0 | 1 | 0 | 0 | |

0 | 1 | 1 | 0 | |

1 | 0 | 0 | 0 | |

1 | 0 | 1 | 0 | |

1 | 1 | 0 | 1 | Adding two negative numbers produced a positive result |

1 | 1 | 1 | 0 | |

Subtraction | ||||

Sign of value being subtracted (subtrahend) | Sign of value being subtracted from (minuend) | Sign of result | Resulting value of Overflow Flag | |

0 | 0 | 0 | 0 | |

0 | 0 | 1 | 0 | |

0 | 1 | 0 | 1 | Subtracting a positive number from a negative number gave a positive result. |

0 | 1 | 1 | 0 | |

1 | 0 | 0 | 0 | |

1 | 0 | 1 | 1 | Subtracting a negative number from a positive number gave a negative result. |

1 | 1 | 0 | 0 | |

1 | 1 | 1 | 0 |