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| <title>FCHS—Change Sign </title></head> |
| <body> |
| <h1>FCHS—Change Sign</h1> |
| <table> |
| <tr> |
| <th>Opcode</th> |
| <th>Instruction</th> |
| <th>64-Bit Mode</th> |
| <th>Compat/Leg Mode</th> |
| <th>Description</th></tr> |
| <tr> |
| <td>D9 E0</td> |
| <td>FCHS</td> |
| <td>Valid</td> |
| <td>Valid</td> |
| <td>Complements sign of ST(0).</td></tr></table> |
| <h2>Description</h2> |
| <p>Complements the sign bit of ST(0). This operation changes a positive value into a negative value of equal magni-tude or vice versa. The following table shows the results obtained when changing the sign of various classes of numbers.</p> |
| <h3>Table 3-30. FCHS Results</h3> |
| <table> |
| <tr> |
| <th>ST(0) SRC</th> |
| <th>ST(0) DEST</th></tr> |
| <tr> |
| <td>− ∞</td> |
| <td>+ ∞</td></tr> |
| <tr> |
| <td>− F</td> |
| <td>+ F</td></tr> |
| <tr> |
| <td>− 0</td> |
| <td>+ 0</td></tr> |
| <tr> |
| <td>+ 0</td> |
| <td>− 0</td></tr> |
| <tr> |
| <td>+ F</td> |
| <td>− F</td></tr> |
| <tr> |
| <td>+ ∞</td> |
| <td>− ∞</td></tr> |
| <tr> |
| <td>NaN</td> |
| <td>NaN</td></tr></table> |
| <p><strong>NOTES:</strong></p> |
| <p>*</p> |
| <p>F means finite floating-point value.</p> |
| <p>This instruction’s operation is the same in non-64-bit modes and 64-bit mode.</p> |
| <h2>Operation</h2> |
| <pre>SignBit(ST(0)) ← NOT (SignBit(ST(0)));</pre> |
| <h2>FPU Flags Affected</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>C1</td> |
| <td>Set to 0.</td></tr> |
| <tr> |
| <td>C0, C2, C3</td> |
| <td>Undefined.</td></tr></table> |
| <h2>Floating-Point Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#IS</td> |
| <td>Stack underflow occurred.</td></tr></table> |
| <h2>Protected Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#NM</td> |
| <td>CR0.EM[bit 2] or CR0.TS[bit 3] = 1.</td></tr> |
| <tr> |
| <td>#UD</td> |
| <td>If the LOCK prefix is used.</td></tr></table> |
| <h2>Real-Address Mode Exceptions</h2> |
| <p>Same exceptions as in protected mode.</p> |
| <h2>Virtual-8086 Mode Exceptions</h2> |
| <p>Same exceptions as in protected mode.</p> |
| <h2>Compatibility Mode Exceptions</h2> |
| <p>Same exceptions as in protected mode.</p> |
| <h2>64-Bit Mode Exceptions</h2> |
| <p>Same exceptions as in protected mode.</p></body></html> |