Difference between revisions of "Overflow Flag"
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| − | The Overflow Flag indicates that the previous operation resulted in a signed number that | + | 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. 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: | ||
| + | <center> | ||
| + | <table border=1> | ||
| + | <tr><th COLSPAN=4>Addition</TH></tr> | ||
| + | <tr><th>Sign of first operand</th><th>Sign of second operand</th><th>Sign of result</th><th>Resulting value of Overflow Flag</th></tr> | ||
| + | <tr><td>0</td><td>0</td><td>0</td><td>0</td></tr> | ||
| + | <tr><td>0</td><td>0</td><td>1</td><td>1</td></tr> | ||
| + | <tr><td>0</td><td>1</td><td>0</td><td>0</td></tr> | ||
| + | <tr><td>0</td><td>1</td><td>1</td><td>0</td></tr> | ||
| + | <tr><td>1</td><td>0</td><td>0</td><td>0</td></tr> | ||
| + | <tr><td>1</td><td>0</td><td>1</td><td>0</td></tr> | ||
| + | <tr><td>1</td><td>1</td><td>0</td><td>1</td></tr> | ||
| + | <tr><td>1</td><td>1</td><td>1</td><td>0</td></tr> | ||
| + | <tr><th COLSPAN=4>Subtraction</TH></tr> | ||
| + | <tr><th>Sign of value being subtracted (subtrahend)</th><th>Sign of value being subtracted from (minuend)</th><th>Sign of result</th><th>Resulting value of Overflow Flag</th></tr> | ||
| + | <tr><td>0</td><td>0</td><td>0</td><td>0</td></tr> | ||
| + | <tr><td>0</td><td>0</td><td>1</td><td>0</td></tr> | ||
| + | <tr><td>0</td><td>1</td><td>0</td><td>1</td></tr> | ||
| + | <tr><td>0</td><td>1</td><td>1</td><td>0</td></tr> | ||
| + | <tr><td>1</td><td>0</td><td>0</td><td>0</td></tr> | ||
| + | <tr><td>1</td><td>0</td><td>1</td><td>1</td></tr> | ||
| + | <tr><td>1</td><td>1</td><td>0</td><td>0</td></tr> | ||
| + | <tr><td>1</td><td>1</td><td>1</td><td>0</td></tr> | ||
| + | </table></center> | ||
Revision as of 00:36, 12 January 2005
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. 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 |
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 |
| 0 | 1 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 1 |
| 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 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 0 |
| 1 | 0 | 1 | 1 |
| 1 | 1 | 0 | 0 |
| 1 | 1 | 1 | 0 |
