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Volume 4: Base IA-32 Instruction Reference
BT—Bit Test
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
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 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 A (4
(BitOffset DIV 32))
Or, it may access 2 bytes starting from the memory address for a 16-bit operand, using
this relationship:
Effective A (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.
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
ib
BT
r/m32,imm8
Store selected bit in CF flag
Содержание ITANIUM ARCHITECTURE
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Страница 7: ...402 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Страница 199: ...4 192 Volume 4 Base IA 32 Instruction Reference FWAIT Wait See entry for WAIT ...
Страница 269: ...4 262 Volume 4 Base IA 32 Instruction Reference LES Load Full Pointer See entry for LDS LES LFS LGS LSS ...
Страница 270: ...Volume 4 Base IA 32 Instruction Reference 4 263 LFS Load Full Pointer See entry for LDS LES LFS LGS LSS ...
Страница 273: ...4 266 Volume 4 Base IA 32 Instruction Reference LGS Load Full Pointer See entry for LDS LES LFS LGS LSS ...
Страница 288: ...Volume 4 Base IA 32 Instruction Reference 4 281 LSS Load Full Pointer See entry for LDS LES LFS LGS LSS ...
Страница 352: ...Volume 4 Base IA 32 Instruction Reference 4 345 ROL ROR Rotate See entry for RCL RCR ROL ROR ...
Страница 368: ...Volume 4 Base IA 32 Instruction Reference 4 361 SHL SHR Shift Instructions See entry for SAL SAR SHL SHR ...
Страница 373: ...4 366 Volume 4 Base IA 32 Instruction Reference SIDT Store Interrupt Descriptor Table Register See entry for SGDT SIDT ...
Страница 589: ...4 582 Volume 4 IA 32 SSE Instruction Reference ...
Страница 590: ...Index Intel Itanium Architecture Software Developer s Manual Rev 2 3 Index ...
Страница 591: ...Index Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Страница 603: ...INDEX Index 12 Index for Volumes 1 2 3 and 4 ...
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