misc. an instruction word
指示字（符）是提示某个数据项位置的一种标识符。
Pointer is an identifier that indicates the location of an item of data .
指示器的指示字还包含一个值，当相应行不在存贮器中时，它引起硬设备或软设备中断。
The pointer to a row can contain some value which causes a hardware or software interrupt in case the row is not in memory.
listen调用指示协议，服务器进程准备接受套接字上任何新传入的连接。
The listen call indicates to the protocol that the server process is ready to accept any new incoming connections on the socket.
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