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(nasm.info)Section B.4.314

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B.4.314. `SYSENTER': Fast System Call
-------------------------------------

     SYSENTER                      ; 0F 34                [P6]

   `SYSENTER' executes a fast call to a level 0 system procedure or
routine. Before using this instruction, various MSRs need to be set up:

   * `SYSENTER_CS_MSR' contains the 32-bit segment selector for the
     privilege level 0 code segment. (This value is also used to
     compute the segment selector of the privilege level 0 stack
     segment.)

   * `SYSENTER_EIP_MSR' contains the 32-bit offset into the privilege
     level 0 code segment to the first instruction of the selected
     operating procedure or routine.

   * `SYSENTER_ESP_MSR' contains the 32-bit stack pointer for the
     privilege level 0 stack.

   `SYSENTER' performs the following sequence of operations:

   * Loads the segment selector from the `SYSENTER_CS_MSR' into the
     `CS' register.

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

   * Adds 8 to the value in `SYSENTER_CS_MSR' and loads it into the
     `SS' register.

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

   * Switches to privilege level 0.

   * Clears the `VM' flag in the `EFLAGS' register, if the flag is set.

   * Begins executing the selected system procedure.

   In particular, note that this instruction des not save the values of
`CS' or `(E)IP'. If you need to return to the calling code, you need to
write your code to cater for this.

   For more information, see the Intel Architecture Software Developer's
Manual, Volume 2.