Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual page 1342

BT—Bit Test
Opcode
0F A3
0F A3
0F BA /4 ib
0F BA /4 ib
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 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. 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
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))
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.
Operation
CF  Bit(BitBase, BitOffset)
Flags Affected
The CF flag contains the value of the selected bit. The OF, SF, ZF, AF, and PF flags are
undefined.
4:40
Instruction
BT r/m16,r16
BT r/m32,r32
BT r/m16,imm8
BT r/m32,imm8
31 31
to 2  1 for a register offset and 0 to 31 for an immediate offset.
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
Volume 4: Base IA-32 Instruction Reference