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

Description

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.

Operation

temp ← (SRC-1) XOR (SRC) ;
SF ← temp[OperandSize -1];
ZF ← 0;
IF SRC = 0
  CF ← 1;
ELSE
  CF ← 0;
FI
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

BLSMSK:	unsigned __int32 _blsmsk_u32(unsigned __int32 src);
BLSMSK:	unsigned __int64 _blsmsk_u64(unsigned __int64 src);

SIMD Floating-Point Exceptions

None

Other Exceptions

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

#UD	If VEX.W = 1.