Difference between revisions of "Overflow Flag"
(→Overflow's complicated!) |
|||
Line 1: | Line 1: | ||
− | 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 |