BT (Bit Test)

Opcodes

Opcode Instruction Op/En 64-bit Mode Compat/Leg Mode Description
0F A3 /r BT r/m16, r16 MR Valid Valid Store selected bit in CF flag.
0F A3 /r BT r/m32, r32 MR Valid Valid Store selected bit in CF flag.
REX.W + 0F A3 /r BT r/m64, r64 MR Valid N.E. Store selected bit in CF flag.
0F BA /4 ib BT r/m16, imm8 MI Valid Valid Store selected bit in CF flag.
0F BA /4 ib BT r/m32, imm8 MI Valid Valid Store selected bit in CF flag.
REX.W + 0F BA /4 ib BT r/m64, imm8 MI Valid N.E. Store selected bit in CF flag.

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
MR ModRM:r/m (r) ModRM:reg (r) NA NA
MI ModRM:r/m (r) imm8 NA NA

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:

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 combina-tion with the displacement field of the memory operand. In this case, the low-order 3 or 5 bits (3 for 16-bit oper-ands, 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))

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 partic-ular, it should avoid references to memory-mapped I/O registers. Instead, software should use the MOV instruc-tions 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 oper-ands. See the summary chart at the beginning of this section for encoding data and limits.

Operation

CF ← Bit(BitBase, BitOffset);

Flags Affected

The CF flag contains the value of the selected bit. The ZF flag is unaffected. The OF, SF, AF, and PF flags are undefined.

Protected Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#PF(fault-code) If a page fault occurs.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS If a memory operand effective address is outside the SS segment limit.
#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS(0) If a memory operand effective address is outside the SS segment limit.
#PF(fault-code) If a page fault occurs.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made.
#UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#SS(0) If a memory address referencing the SS segment is in a non-canonical form.
#GP(0) If the memory address is in a non-canonical form.
#PF(fault-code) If a page fault occurs.
#AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#UD If the LOCK prefix is used.