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 | |
