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;
    CF ← 0;
DEST ← temp;

Flags Affected

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


Other Exceptions

See Section 2.5.1, “Exception Conditions for VEX-Encoded GPR Instructions”, Table 2-29; additionally

#UD If VEX.W = 1.