RET, and its exact synonym RETN, pop IP or EIP from the stack and transfer control to the new address. Optionally, if a numeric second operand is provided, they increment the stack pointer by a further imm16 bytes after popping the return address.
ret：也可以叫做近返回，即段内返回。处理器从堆栈中弹出IP或者EIP，然后根据当前的CS：IP跳转到新的执行地址。如果之前压栈的还有其余的参数，则这些参数也会被弹出。
RETF executes a far return: after popping IP/EIP, it then pops CS, and then increments the stack pointer by the optional argument if present.
retf：也叫远返回，从一个段返回到另一个段。先弹出堆栈中的IP/EIP，然后弹出CS，有之前压栈的参数也会弹出。（近跳转与远跳转的区别就在于CS是否压栈。）
IRET returns from an interrupt (hardware or software) by means of popping IP (or EIP), CS and the flags off the stack and then continuing execution from the new CS:IP.
iret：用于从中断返回，会弹出IP/EIP，然后CS，以及一些标志。然后从CS：IP执行。
IRETW pops IP, CS and the flags as 2 bytes each, taking 6 bytes off the stack in total.
iretw：先后弹出IP，CS和标志位，每次都以2个字节为一个单位弹出，总共6个字节。
IRETD pops EIP as 4 bytes, pops a further 4 bytes of which the top two are discarded and the bottom two go into CS, and pops the flags as 4 bytes as well, taking 12 bytes off the stack.
iretd：以4字节为单位弹出EIP，然后再弹4个字节，这四个字节的前两个字节被抛弃，低两个字节留下写入CS，然后弹出4字节的标志。（主要因为CS只有16位）
IRET is a shorthand for either IRETW or IRETD, depending on the default BITS setting at the time.
iret是iretw或者iretd的缩写，取决于一个段被申明为16位段还是32位
从异常或中断处理例程返回必须使用IRET（或IRETD）指令。IRET 指令与RET 指
令的唯一不同在于前者将恢复保存的EFLAGS 寄存器。只有当CPL 为0 时，才恢复EFLAGS
寄存器的IOPL 域。只有CPL 小于或等于IOPL 时才改变IF 标志。
转自：http://linux.cn/home-space-uid-11720-do-blog-id-5748.html