sysenter

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.