关于IRET指令

当使用IRET指令返回到相同保护级别的任务时，IRET会从堆栈弹出代码段选择子及指令指针分别到CS与IP寄存器，并弹出标志寄存器内容到EFLAGS寄存器。

当使用IRET指令返回到一个不同的保护级别时，IRET不仅会从堆栈弹出以上内容，还会弹出堆栈段选择子及堆栈指针分别到SS与SP寄存器。

IA-32指令手册关于这一点描述的很清楚，原文如下：

the IRET instruction pops the return instruction pointer, return code segment selector, and EFLAGS image from the stack to the EIP, CS, and EFLAGS registers, respectively, and then resumes execution of the interrupted program or procedure. If the return is to another privilege level, the IRET instruction also pops the stack pointer and SS from the stack, before resuming program execution.