BTS—Bit Test and SetInstruction Operand EncodingDescriptionSelects the bit in a bit string (specified with the first operand, called the bit base) at the bit-position designated by the bit offset operand (second operand), stores the value of the bit in the CF flag, and sets the selected bit in the bit string to 1. The bit base operand can be a register or a memory location; the bit offset operand can be a register or an immediate value: •If the bit base operand specifies a register, the instruction takes the modulo 16, 32, or 64 of the bit offset operand (modulo size depends on the mode and register size; 64-bit operands are available only in 64-bit mode). This allows any bit position to be selected.•If the bit base operand specifies a memory location, the operand represents the address of the byte in memory that contains the bit base (bit 0 of the specified byte) of the bit string. The range of the bit position that can be referenced by the offset operand depends on the operand size.See also: Bit(BitBase, BitOffset) on page 3-11. Some assemblers support immediate bit offsets larger than 31 by using the immediate bit offset field in combina-tion with the displacement field of the memory operand. See “BT—Bit Test” in this chapter for more information on this addressing mechanism.This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically.In 64-bit mode, the instruction’s default operation size is 32 bits. Using a REX prefix in the form of REX.R permits access to additional registers (R8-R15). Using a REX prefix in the form of REX.W promotes operation to 64 bits. See the summary chart at the beginning of this section for encoding data and limits.OperationCF := Bit(BitBase, BitOffset);Bit(BitBase, BitOffset) := 1;Flags AffectedThe CF flag contains the value of the selected bit before it is set. The ZF flag is unaffected. The OF, SF, AF, and PF flags are undefined.OpcodeInstructionOp/ En64-bit ModeCompat/Leg ModeDescription0F AB /rBTS r/m16, r16MRValidValidStore selected bit in CF flag and set.0F AB /rBTS r/m32, r32MRValidValidStore selected bit in CF flag and set.REX.W + 0F AB /rBTS r/m64, r64MRValidN.E.Store selected bit in CF flag and set.0F BA /5 ibBTS r/m16, imm8MIValidValidStore selected bit in CF flag and set.0F BA /5 ibBTS r/m32, imm8MIValidValidStore selected bit in CF flag and set.REX.W + 0F BA /5 ibBTS r/m64, imm8MIValidN.E.Store selected bit in CF flag and set.Op/EnOperand 1Operand 2Operand 3Operand 4MRModRM:r/m (r, w)ModRM:reg (r)NANAMIModRM:r/m (r, w)imm8NANA
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