sysexit

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.