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(nasm.info)Section B.4.316

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B.4.316. `SYSRET': Return From Operating System
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     SYSRET                        ; 0F 07                [P6,AMD,PRIV]

   `SYSRET' is the return instruction used in conjunction with the
`SYSCALL' instruction to provide fast entry/exit to an operating system.

   * The `ECX' register, which points to the next sequential instruction
     after the corresponding `SYSCALL' instruction, is copied into the
     `EIP' register.

   * Bits [63-48] of the `STAR' register specify the selector that is
     copied into the `CS' register.

   * Bits [63-48]+1000b of the `STAR' register specify the selector
     that is copied into the `SS' register.

   * Bits [1-0] of the `SS' register are set to 11b (RPL of 3)
     regardless of the value of bits [49-48] of the `STAR' register.

   The `CS' and `SS' registers should not be modified by the operating
system between the execution of the `SYSCALL' instruction and its
corresponding `SYSRET' instruction.

   For more information, see the `SYSCALL and SYSRET Instruction
Specification' (AMD document number 21086.pdf).