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(nasm.info)Section B.4.315

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B.4.315. `SYSEXIT': Fast Return From System Call
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     SYSEXIT                       ; 0F 35                [P6,PRIV]

   `SYSEXIT' executes a fast return to privilege level 3 user code. This
instruction is a companion instruction to the `SYSENTER' instruction,
and can only be executed by privilege level 0 code. Various registers
need to be set up before calling this instruction:

   * `SYSENTER_CS_MSR' contains the 32-bit segment selector for the
     privilege level 0 code segment in which the processor is currently
     executing. (This value is used to compute the segment selectors
     for the privilege level 3 code and stack segments.)

   * `EDX' contains the 32-bit offset into the privilege level 3 code
     segment to the first instruction to be executed in the user code.

   * `ECX' contains the 32-bit stack pointer for the privilege level 3
     stack.

   `SYSEXIT' performs the following sequence of operations:

   * Adds 16 to the value in `SYSENTER_CS_MSR' and loads the sum into
     the `CS' selector register.

   * Loads the instruction pointer from the `EDX' register into the
     `EIP' register.

   * Adds 24 to the value in `SYSENTER_CS_MSR' and loads the sum into
     the `SS' selector register.

   * Loads the stack pointer from the `ECX' register into the `ESP'
     register.

   * Switches to privilege level 3.

   * Begins executing the user code at the `EIP' address.

   For more information on the use of the `SYSENTER' and `SYSEXIT'
instructions, see the Intel Architecture Software Developer's Manual,
Volume 2.