Volume 3: General-Purpose and System Instructions - Stanford ...

AMD64 Technology 24594 Rev. 3.10 February 2005BSWAPByte SwapReverses the byte order of the specified register. This action converts the contents ofthe register from little endian to big endian or vice versa. In a doubleword, bits 7–0 areexchanged with bits 31–24, and bits 15–8 are exchanged with bits 23–16. In aquadword, bits 7–0 are exchanged with bits 63–56, bits 15–8 with bits 55–48, bits 23–16with bits 47–40, and bits 31–24 with bits 39–32. A subsequent use of the BSWAPinstruction with the same operand restores the original value of the operand.The result of applying the BSWAP instruction to a 16-bit register is undefined. To swapthe bytes of a 16-bit register, use the XCHG instruction and specify the respective bytehalves of the 16-bit register as the two operands. For example, to swap the bytes of AX,use XCHG AL, AH.Mnemonic Opcode DescriptionBSWAP reg32 0F C8 +rd Reverse the byte order of reg32.BSWAP reg64 0F C8 +rq Reverse the byte order of reg64.Related InstructionsXCHGrFLAGS AffectedNoneExceptionsNone78 BSWAP