Difference between revisions of "Overflow Flag"

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(Overflow's complicated!)
m (Protected "Overflow Flag" ([edit=autoconfirmed] (indefinite) [move=autoconfirmed] (indefinite)))
 
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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.  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.
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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:
 
The truth table below indicates when the CPU sets or clears the Overflow Flag:
 
<center>
 
<center>
 
<table border=1>
 
<table border=1>
<tr><th COLSPAN=4>Addition</TH></tr>
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<tr><th COLSPAN=5>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>
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<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><th>Notes</th></tr>
 
<tr><td>0</td><td>0</td><td>0</td><td>0</td></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>
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<tr><td>0</td><td>0</td><td>1</td><td>1</td><td>Adding two positive numbers produced a negative result</td></tr>
 
<tr><td>0</td><td>1</td><td>0</td><td>0</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>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>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>0</td><td>1</td><td>0</td></tr>
<tr><td>1</td><td>1</td><td>0</td><td>1</td></tr>
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<tr><td>1</td><td>1</td><td>0</td><td>1</td><td>Adding two negative numbers produced a positive result</td></tr>
 
<tr><td>1</td><td>1</td><td>1</td><td>0</td></tr>
 
<tr><td>1</td><td>1</td><td>1</td><td>0</td></tr>
<tr><th COLSPAN=4>Subtraction</TH></tr>
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<tr><th COLSPAN=5>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>
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<tr><th>Sign of value<br/>being subtracted<br/>(subtrahend)</th><th>Sign of value<br/>being subtracted from<br/>(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>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>0</td><td>1</td><td>0</td></tr>
<tr><td>0</td><td>1</td><td>0</td><td>1</td></tr>
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<tr><td>0</td><td>1</td><td>0</td><td>1</td><td>Subtracting a positive number from a negative number gave a positive result.</td></tr>
 
<tr><td>0</td><td>1</td><td>1</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>0</td><td>0</td></tr>
<tr><td>1</td><td>0</td><td>1</td><td>1</td></tr>
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<tr><td>1</td><td>0</td><td>1</td><td>1</td><td>Subtracting a negative number from a positive number gave a negative result.</td></tr>
 
<tr><td>1</td><td>1</td><td>0</td><td>0</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>
 
<tr><td>1</td><td>1</td><td>1</td><td>0</td></tr>
 
</table></center>
 
</table></center>

Latest revision as of 09:01, 4 December 2010

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