sysret

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).