The purpose of process introduction is to improve the system resource utilization and increase the system throughput. There're 3 problems arising from concurrent execution of programs:1. Discontinuity. 2. Loss of closure. 3. Non existence. And the above problems have been well solved after the introduction of the concept of process.

 

The biggest differences between the process concept and the procedure concept are：

1. Processes are dynamic, while programs are static.

2. A process has a certain life cycle, and a program is a collection of instructions, and it has no meaning of "movement". A program without a process cannot be approved by the operating system as an independent unit.

3. A program can correspond to multiple processes, but a process can only correspond to one program. The relationship between process and procedure is like that between performance and scr ipt.

4. Processes and programs are different in composition. From the static point of view, the process is composed of program, data and process control block (PCB). A program is an ordered set of instructions.

Process is a running activity of a computer program on a data set. It is the basic unit of resource allocation and scheduling, and the basis of operating system structure.

 

It is necessary for the status of process to be divided into five states, because only by this why the operating system can run successfully and efficiently. The 5 status are:

Creation state: a blank PCB needs to be applied for when the process is created, and the information of control and management process is filled in to complete resource allocation. If the creation work cannot be completed, such as the resources cannot be satisfied, it cannot be scheduled to run. The state of the process at this time is called the creation state.

Ready state: the process is ready, allocated to the required resources, and can run as soon as it is allocated to the CPU.

Execution state: after the process is in the ready state and scheduled, the process enters the execution state.

Blocking state: the executing process is temporarily unable to run due to some events (I / O request, cache request failed), and the process is blocked. When the request is satisfied, it enters the ready state and waits for the system call.

Termination state: the process ends, or an error occurs, or is terminated by the system, and enters the termination state. Can no longer be executed.