POPCNT - Return the Count of Number of Bits Set to 1

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)

N/A

N/A

Description

This instruction calculates the 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.