Overflow Flag

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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 operandSign of second operandSign of resultResulting value of Overflow FlagNotes
0000
0011Adding two positive numbers produced a negative result
0100
0110
1000
1010
1101Adding two negative numbers produced a positive result
1110
Subtraction
Sign of value
being subtracted
(subtrahend)
Sign of value
being subtracted from
(minuend)
Sign of resultResulting value of Overflow Flag
0000
0010
0101Subtracting a positive number from a negative number gave a positive result.
0110
1000
1011Subtracting a negative number from a positive number gave a negative result.
1100
1110
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