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| <title>RDFSBASE/RDGSBASE—Read FS/GS Segment Base </title></head> |
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| <h1>RDFSBASE/RDGSBASE—Read FS/GS Segment Base</h1> |
| <table> |
| <tr> |
| <th>Opcode/Instruction</th> |
| <th>Op/En</th> |
| <th>64/32-bit Mode</th> |
| <th>CPUID Fea-ture Flag</th> |
| <th>Description</th></tr> |
| <tr> |
| <td>F3 0F AE /0 RDFSBASE r32</td> |
| <td>M</td> |
| <td>V/I</td> |
| <td>FSGSBASE</td> |
| <td>Load the 32-bit destination register with the FS base address.</td></tr> |
| <tr> |
| <td>REX.W + F3 0F AE /0 RDFSBASE r64</td> |
| <td>M</td> |
| <td>V/I</td> |
| <td>FSGSBASE</td> |
| <td>Load the 64-bit destination register with the FS base address.</td></tr> |
| <tr> |
| <td>F3 0F AE /1 RDGSBASE <em>r32</em></td> |
| <td>M</td> |
| <td>V/I</td> |
| <td>FSGSBASE</td> |
| <td>Load the 32-bit destination register with the GS base address.</td></tr> |
| <tr> |
| <td>REX.W + F3 0F AE /1 RDGSBASE <em>r64</em></td> |
| <td>M</td> |
| <td>V/I</td> |
| <td>FSGSBASE</td> |
| <td>Load the 64-bit destination register with the GS base address.</td></tr></table> |
| <h3>Instruction Operand Encoding</h3> |
| <table> |
| <tr> |
| <td>Op/En</td> |
| <td>Operand 1</td> |
| <td>Operand 2</td> |
| <td>Operand 3</td> |
| <td>Operand 4</td></tr> |
| <tr> |
| <td>M</td> |
| <td>ModRM:r/m (w)</td> |
| <td>NA</td> |
| <td>NA</td> |
| <td>NA</td></tr></table> |
| <h2>Description</h2> |
| <p>Loads the general-purpose register indicated by the modR/M:r/m field with the FS or GS segment base address.</p> |
| <p>The destination operand may be either a 32-bit or a 64-bit general-purpose register. The REX.W prefix indicates the operand size is 64 bits. If no REX.W prefix is used, the operand size is 32 bits; the upper 32 bits of the source base address (for FS or GS) are ignored and upper 32 bits of the destination register are cleared.</p> |
| <p>This instruction is supported only in 64-bit mode.</p> |
| <h2>Operation</h2> |
| <pre>DEST ← FS/GS segment base address;</pre> |
| <h2>Flags Affected</h2> |
| <p>None</p> |
| <h2>C/C++ Compiler Intrinsic Equivalent</h2> |
| <p>RDFSBASE:</p> |
| <p>unsigned int _readfsbase_u32(void );</p> |
| <p>RDFSBASE:</p> |
| <p>unsigned __int64 _readfsbase_u64(void );</p> |
| <p>RDGSBASE:</p> |
| <p>unsigned int _readgsbase_u32(void );</p> |
| <p>RDGSBASE:</p> |
| <p>unsigned __int64 _readgsbase_u64(void );</p> |
| <h2>Protected Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>The RDFSBASE and RDGSBASE instructions are not recognized in protected mode.</td></tr></table> |
| <h2>Real-Address Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>The RDFSBASE and RDGSBASE instructions are not recognized in real-address mode.</td></tr></table> |
| <h2>Virtual-8086 Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>The RDFSBASE and RDGSBASE instructions are not recognized in virtual-8086 mode.</td></tr></table> |
| <h2>Compatibility Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>The RDFSBASE and RDGSBASE instructions are not recognized in compatibility mode.</td></tr></table> |
| <h2>64-Bit Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td> |
| <p>If the LOCK prefix is used.</p> |
| <p>If CR4.FSGSBASE[bit 16] = 0.</p> |
| <p>If CPUID.07H.0H:EBX.FSGSBASE[bit 0] = 0.</p></td></tr></table></body></html> |