LZCNT — Count the Number of Leading Zero Bits

LZCNT counts the number of leading most significant zero bits in a source operand (second operand) and returns the result in the destination (first operand). LZCNT is an extension of the BSR instruction. The key difference between the LZCNT and BSR instructions is that when the source operand is zero, LZCNT outputs the operand size to the destination operand, whereas BSR leaves the destination operand unmodified.

On processors that do not support LZCNT, the instruction byte encoding is executed as BSR.

temp ← OperandSize - 1
DEST ← 0
WHILE (temp >= 0) AND (Bit(SRC, temp) = 0)
DO
    temp ← temp - 1
    DEST ← DEST+ 1
OD
IF DEST = OperandSize
    CF ← 1
ELSE
    CF ← 0
FI
IF DEST = 0
    ZF ← 1
ELSE
    ZF ← 0
FI

ZF flag is set to 1 in case of zero output (most significant bit of the source is set), and to 0 otherwise, CF flag is set to 1 if input was zero and cleared otherwise. OF, SF, PF, and AF flags are undefined.

LZCNT unsigned __int32 _lzcnt_u32(unsigned __int32 src);
LZCNT unsigned __int64 _lzcnt_u64(unsigned __int64 src);

#GP(0) For an illegal memory operand effective address in the CS, DS, ES, FS or GS segments. If the DS, ES, FS, or GS register is used to access memory and it contains a null segment selector.

#SS(0) For an illegal address in the SS segment.

#PF (fault-code) For a page fault.

#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.

#UD If LOCK prefix is used.

#GP(0) If any part of the operand lies outside of the effective address space from 0 to 0FFFFH.

#SS(0) For an illegal address in the SS segment.

#UD If LOCK prefix is used.

#GP(0) If any part of the operand lies outside of the effective address space from 0 to 0FFFFH.

#SS(0) For an illegal address in the SS segment.

#PF (fault-code) For a page fault.

#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.

#UD If LOCK prefix is used.

Same exceptions as in Protected Mode.

#GP(0) If the memory address is in a non-canonical form.

#SS(0) If a memory address referencing the SS segment is in a non-canonical form.

#PF (fault-code) For a page fault.

#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.

#UD If LOCK prefix is used.

Source: Intel® 64 and IA-32 Architectures Software Developer's Manual, Combined Volumes (Order Number 325462-091US, March 2026)
Generated: 7-6-2026