1. Process life cycle:

+ Ready state: The process has acquired all other resources except the cpu, waiting for the CPU to schedule in the ready queue

+ Execution status: the CPU has been obtained and all required resources are running

+ Blocking state (waiting state): A process abandons the processor because it waits for the required resources, or the process does not own the processor and other resources are not satisfied.

2. Necessity of PCB

PCB is a part of process entity and the most important record data structure in OS. It records all the information needed by OS to describe the process and control the operation of the process. The function of PCB is to make a program (including data) that can not run independently in a multi-programming environment become a basic unit that can run independently and a process that can run concurrently with other processes. In other words, OS controls and manages concurrent processes according to PCB. In the whole life of a process, the system always controls the process through PCB. That is to say, the system perceives the existence of the process according to the PCB of the process. So, PCB is the symbol of the existence of the process.