| <!DOCTYPE html> |
| |
| <html> |
| <head> |
| <meta charset="UTF-8"> |
| <link href="style.css" type="text/css" rel="stylesheet"> |
| <title>FDECSTP—Decrement Stack-Top Pointer </title></head> |
| <body> |
| <h1>FDECSTP—Decrement Stack-Top Pointer</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 F6</td> |
| <td>FDECSTP</td> |
| <td>Valid</td> |
| <td>Valid</td> |
| <td>Decrement TOP field in FPU status word.</td></tr></table> |
| <h2>Description</h2> |
| <p>Subtracts one from the TOP field of the FPU status word (decrements the top-of-stack pointer). If the TOP field contains a 0, it is set to 7. The effect of this instruction is to rotate the stack by one position. The contents of the FPU data registers and tag register are not affected.</p> |
| <p>This instruction’s operation is the same in non-64-bit modes and 64-bit mode.</p> |
| <h2>Operation</h2> |
| <pre>IF TOP = 0 |
| THEN TOP ← 7; |
| ELSE TOP ← TOP – 1; |
| FI;</pre> |
| <h2>FPU Flags Affected</h2> |
| <p>The C1 flag is set to 0. The C0, C2, and C3 flags are undefined.</p> |
| <h2>Floating-Point Exceptions</h2> |
| <p>None.</p> |
| <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>#MF</td> |
| <td>If there is a pending x87 FPU exception.</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> |