In order to facilitate the system to describe and manage the operation of the process, a data structure is specially defined for each process in the core of the OS - Process Control Block (PCB). As a part of the process entity, PCB records all the information required by the operating system to describe the current situation of the process and manage the running of the process. It is the most important record-type data structure in the operating system. The function of PCB is to make a program (including data) that cannot run independently in a multiprogramming environment become a basic unit that can run independently, a process that can be executed concurrently with other processes.

　　(1) PCB as a sign of the basic unit of independent operation. When a program (including data) is configured with a PCB, it means that it is already a legal basic unit that can run independently in a multi-program environment, and it also has the power to obtain OS services, such as opening files in the file system. , request to obtain I/O devices in the system, and communicate with other related processes. So when the system creates a new process, it builds a PCB for it. At the end of the process, its PCB is recycled and the process dies. The system senses the existence of the process through the PCB. In fact, the PCB has become the only sign that a process exists in the system.

　　(2) The PCB can realize intermittent operation. In the multiprogramming environment, the program runs in a stop-and-go intermittent operation mode. When a process is suspended due to blocking, it must retain information about the CPU context when it was running. After having the PCB, the system can save the CPU scene information in the PCB of the interrupted process for use when the process is scheduled and executed again to resume the CPU scene. From this, it can be made clear again that in the multiprogramming environment, as a static program in the traditional sense, because it does not have the means to protect or save its own running scene, it cannot guarantee the reproducibility of its running results, and thus loses its ability to run. significance.

　　(3) PCB provides the information required for process management. When the scheduler schedules a process to run, it can only find the corresponding program and data according to the starting address pointer of the program and data recorded in the process PCB in the memory or external memory; during the process of running, when the file needs to be accessed When files or I/O devices in the system are used, the information in the PCB is also needed. In addition, according to the resource list in the PCB, you can know all the resources required for the process, etc. It can be seen that in the whole life cycle of the process, the operating system always controls and manages the process according to the PCB.

　　(4) PCB provides the information needed for process scheduling. Only processes in the ready state can be scheduled for execution, and the PCB provides information about the state of the process. If the process is in the ready state, the system inserts it into the process ready queue and waits for the scheduler to schedule; in addition, other information about the process is often required when scheduling. For example, in the priority scheduling algorithm, it is necessary to know the process priority. In some relatively fair scheduling algorithms, it is also necessary to know the waiting time of the process and the events that have been executed.

　　(5) The PCB realizes synchronization and communication with other processes. The process synchronization mechanism is used to realize the coordinated operation of various processes. When the semaphore mechanism is used, it requires that a corresponding semaphore for synchronization be set in each process. There is also an area or a communication queue pointer, etc. for implementing process communication in the PCB.

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