It is important to realize that only one process can be running on any processor at any instant.
要记住每一时刻一个处理器只能有一个进程运行。
Process Control Block 进程控制块
Each process is represented in the operating system by a process control block (PCB)—also called a task control block.
操作系统使用进程控制块表示进程，进程控制块也叫任务控制块。
PCB内容：
1 Process state.
2 Program counter.
3 CPU registers.
4 CPU-scheduling information.
5 Memory-management information.
6 Accounting information.
7 I/O status information.
In brief, the PCB simply serves as the repository for any information that may vary from process to process.
简而言之，PCB存储各进程所有相关的信息，而这些信息在各个进程中不尽相同。
Reference :
Operating System Concepts