Selects the bit in a bit string (specified with the first operand, called the bit base) at the bitposition designated by the bit offset operand (second operand), stores the value of the bit in the CF flag, and clears the selected bit in the bit string to 0. 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 or 32 (depending on the register size) of the bit offset operand, allowing any bit position to be selected in a 16- or 32-bit register, respectively. If the bit base operand specifies a memory location, it represents the address of the byte in memory that contains the bit base (bit 0 of the specified byte) of the bit string. The offset operand then selects a bit position within the range -2^31 to 2^31 - 1 for a register offset and 0 to 31 for an immediate offset.
Some assemblers support immediate bit offsets larger than 31 by using the immediate bit offset field in combination 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.