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Vol. 2A 3-73
INSTRUCTION SET REFERENCE, A-M
BSWAP—Byte Swap
BSWAP—Byte Swap
Description
Reverses the byte order of a 32-bit or 64-bit (destination) register. This instruction is
provided for converting little-endian values to big-endian format and vice versa. To
swap bytes in a word value (16-bit register), use the XCHG instruction. When the
BSWAP instruction references a 16-bit register, the result is undefined.
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 bits. See the summary chart at
the beginning of this section for encoding data and limits.
IA-32 Architecture Legacy Compatibility
The BSWAP instruction is not supported on IA-32 processors earlier than the
Intel486™ processor family. For compatibility with this instruction, software
should include functionally equivalent code for execution on Intel processors earlier
than the Intel486 processor family.
Operation
TEMP ← DEST
IF 64-bit mode AND OperandSize = 64
THEN
DEST[7:0] ← TEMP[63:56];
DEST[15:8] ← TEMP[55:48];
DEST[23:16] ← TEMP[47:40];
DEST[31:24] ← TEMP[39:32];
DEST[39:32] ← TEMP[31:24];
DEST[47:40] ← TEMP[23:16];
DEST[55:48] ← TEMP[15:8];
DEST[63:56] ← TEMP[7:0];
ELSE
DEST[7:0] ← TEMP[31:24];
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
0F C8+rd BSWAP r32 Valid* Valid Reverses the byte order of a 32-
bit register.
REX.W + 0F
C8+rd
BSWAP r64 Valid N.E. Reverses the byte order of a 64-
bit register.
NOTES:
* See IA-32 Architecture Compatibility section below.