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| <title>BLSMSK — Get Mask Up to Lowest Set Bit </title></head> |
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| <h1>BLSMSK — Get Mask Up to Lowest Set Bit</h1> |
| <table> |
| <tr> |
| <th>Opcode/Instruction</th> |
| <th>Op/En</th> |
| <th>64/32 -bit Mode</th> |
| <th>CPUID Feature Flag</th> |
| <th>Description</th></tr> |
| <tr> |
| <td> |
| <p>VEX.NDD.LZ.0F38.W0 F3 /2</p> |
| <p>BLSMSK r32, r/m32</p></td> |
| <td>VM</td> |
| <td>V/V</td> |
| <td>BMI1</td> |
| <td>Set all lower bits in r32 to “1” starting from bit 0 to lowest set bit in r/m32.</td></tr> |
| <tr> |
| <td> |
| <p>VEX.NDD.LZ.0F38.W1 F3 /2</p> |
| <p>BLSMSK r64, r/m64</p></td> |
| <td>VM</td> |
| <td>V/N.E.</td> |
| <td>BMI1</td> |
| <td>Set all lower bits in r64 to “1” starting from bit 0 to lowest set bit in r/m64.</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>VM</td> |
| <td>VEX.vvvv (w)</td> |
| <td>ModRM:r/m (r)</td> |
| <td>NA</td> |
| <td>NA</td></tr></table> |
| <h2>Description</h2> |
| <p>Sets all the lower bits of the destination operand to “1” up to and including lowest set bit (=1) in the source operand. If source operand is zero, BLSMSK sets all bits of the destination operand to 1 and also sets CF to 1.</p> |
| <p>This instruction is not supported in real mode and virtual-8086 mode. The operand size is always 32 bits if not in 64-bit mode. In 64-bit mode operand size 64 requires VEX.W1. VEX.W1 is ignored in non-64-bit modes. An attempt to execute this instruction with VEX.L not equal to 0 will cause #UD.</p> |
| <h2>Operation</h2> |
| <pre>temp ← (SRC-1) XOR (SRC) ; |
| SF ← temp[OperandSize -1]; |
| ZF ← 0; |
| IF SRC = 0 |
| CF ← 1; |
| ELSE |
| CF ← 0; |
| FI |
| DEST ← temp;</pre> |
| <h2>Flags Affected</h2> |
| <p>SF is updated based on the result. CF is set if the source if zero. ZF and OF flags are cleared. AF and PF flag are undefined.</p> |
| <h2>Intel C/C++ Compiler Intrinsic Equivalent</h2> |
| <p>BLSMSK:</p> |
| <p>unsigned __int32 _blsmsk_u32(unsigned __int32 src);</p> |
| <p>BLSMSK:</p> |
| <p>unsigned __int64 _blsmsk_u64(unsigned __int64 src);</p> |
| <h2>SIMD Floating-Point Exceptions</h2> |
| <p>None</p> |
| <h2>Other Exceptions</h2> |
| <p>See Section 2.5.1, “Exception Conditions for VEX-Encoded GPR Instructions”, Table 2-29; additionally</p> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>If VEX.W = 1.</td></tr></table></body></html> |