BLSMSK (Get Mask Up to Lowest Set Bit)
|Opcode/Instruction||Op/En||64/32 -bit Mode||CPUID Feature Flag||Description|
|VEX.NDD.LZ.0F38.W0 F3 /2 BLSMSK r32, r/m32||VM||V/V||BMI1||Set all lower bits in r32 to “1” starting from bit 0 to lowest set bit in r/m32.|
|VEX.NDD.LZ.0F38.W1 F3 /2 BLSMSK r64, r/m64||VM||V/N.E.||BMI1||Set all lower bits in r64 to “1” starting from bit 0 to lowest set bit in r/m64.|
Instruction Operand Encoding
|Op/En||Operand 1||Operand 2||Operand 3||Operand 4|
|VM||VEX.vvvv (w)||ModRM:r/m (r)||NA||NA|
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.
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.
temp ← (SRC-1) XOR (SRC) ; SF ← temp[OperandSize -1]; ZF ← 0; IF SRC = 0 CF ← 1; ELSE CF ← 0; FI DEST ← temp;
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.
Intel C/C++ Compiler Intrinsic Equivalent
unsigned __int32 _blsmsk_u32(unsigned __int32 src);
unsigned __int64 _blsmsk_u64(unsigned __int64 src);
SIMD Floating-Point Exceptions
See Section 2.5.1, “Exception Conditions for VEX-Encoded GPR Instructions”, Table 2-29; additionally
|#UD||If VEX.W = 1.|