3-45
INSTRUCTION SET REFERENCE
BT—Bit Test
Description
This instruction 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 operand (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 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 (refer to Figure
3-1). 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 (refer to Figure
3-2). 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. In this case, the low-
order three or five bits (three for 16-bit operands, five 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 four 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 two 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.
Operation
CF
<
Bit(BitBase, BitOffset)
Opcode
Instruction
Description
0F A3
BT r/m16,r16
Store selected bit in CF flag
0F A3
BT r/m32,r32
Store selected bit in CF flag
0F BA /4 ib BT r/m16,imm8
Store selected bit in CF flag
0F BA /4 ibBT r/m32,imm8
Store selected bit in CF flag
