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Unformatted text preview: E, A-M BT--Bit Test
Opcode 0F A3 0F A3 REX.W + 0F A3 0F BA /4 ib 0F BA /4 ib REX.W + 0F BA /4 ib Instruction BT r/m16, r16 BT r/m32, r32 BT r/m64, r64 BT r/m16, imm8 BT r/m32, imm8 BT r/m64, imm8 64-Bit Mode Valid Valid Valid Valid Valid Valid Compat/ Leg Mode Valid Valid N.E. Valid Valid N.E. Description Store selected bit in CF flag. Store selected bit in CF flag. Store selected bit in CF flag. Store selected bit in CF flag. Store selected bit in CF flag. Store selected bit in CF flag. Description
Selects the bit in a bit string (specified with the first operand, called the bit base) at the bit-position designated by the bit offset (specified by the second operand) and stores the value of the bit in the CF flag. 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). 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 3-10. 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. In this case, the low-order 3 or 5 bits (3 for 16-bit operands, 5 for 32-bit operands) of the immediate bit offset are stored in the immediate bit offset field, and the high-order bits are shifted and combined with the byte displacement in the addressing mode by the assembler. The processor will ignore the high order bits if they are not zero. When accessing a bit in memory, the processor may access 4 bytes starting from the memory address for a 32-bit operand size, using by the following relationship: Effective Address + (4 (BitOffset DIV 32)) 3-74 Vol. 2 INSTRUCTION SET REFERENCE, A-M Or, it may access 2 bytes starting from the memory address for a 16-bit operand, using this relationship: Effective Address + (2 (BitOffset DIV 16)) It may do so even when only a single byte needs to be accessed to reach the given bit. When using this bit addressing mechanism, software should avoid referencing areas of memory close to address space holes. In particular, it should avoid references to memory-mapped I/O registers. Instead, software should use the MOV instructions to load from or store to these addresses, and use the register form of these instructions to manipulate the data. 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 bit operands...
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This note was uploaded on 10/01/2013 for the course CPE 103 taught by Professor Watlins during the Winter '11 term at Mississippi State.
- Winter '11