POPCNT (Return the Count of Number of Bits Set to 1)

Opcodes

Opcode Instruction Op/En 64-Bit Mode Compat/Leg Mode Description
F3 0F B8 /r POPCNT r16, r/m16 RM Valid Valid POPCNT on r/m16
F3 0F B8 /r POPCNT r32, r/m32 RM Valid Valid POPCNT on r/m32
F3 REX.W 0F B8 /r POPCNT r64, r/m64 RM Valid N.E. POPCNT on r/m64

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
RM ModRM:reg (w) ModRM:r/m (r) NA NA

Description

This instruction calculates of number of bits set to 1 in the second operand (source) and returns the count in the first operand (a destination register).

Operation

Count = 0;
For (i=0; i 
 OperandSize; i++)
{
         IF (SRC[ i] = 1) // i’th bit
         THEN Count++; FI;
}
DEST ← Count;

Flags Affected

OF, SF, ZF, AF, CF, PF are all cleared. ZF is set if SRC = 0, otherwise ZF is cleared.

Intel C/C++ Compiler Intrinsic Equivalent

POPCNT:

int _mm_popcnt_u32(unsigned int a);

POPCNT:

int64_t _mm_popcnt_u64(unsigned __int64 a);

Protected Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS or GS segments.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#PF (fault-code) For a page fault.
#AC(0) If an unaligned memory reference is made while the current privilege level is 3 and alignment checking is enabled.
#UD If CPUID.01H:ECX.POPCNT [Bit 23] = 0. If LOCK prefix is used.

Real-Address Mode Exceptions

#GP(0) If any part of the operand lies outside of the effective address space from 0 to 0FFFFH.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#UD If CPUID.01H:ECX.POPCNT [Bit 23] = 0. If LOCK prefix is used.

Virtual 8086 Mode Exceptions

#GP(0) If any part of the operand lies outside of the effective address space from 0 to 0FFFFH.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#PF (fault-code) For a page fault.
#AC(0) If an unaligned memory reference is made while alignment checking is enabled.
#UD If CPUID.01H:ECX.POPCNT [Bit 23] = 0. If LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in Protected Mode.

64-Bit Mode Exceptions

#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 CPUID.01H:ECX.POPCNT [Bit 23] = 0. If LOCK prefix is used.