Organization of processes: Control blocks, program segments, and data segments

A process is the unit of resource allocation and independent operation of the operating system. It is generally made up of the following three parts.

Process Control block

When the process is created, the operating system creates a new PCB structure, which is then resident memory, which can be accessed at any one time and deleted at the end of the process. The PCB is part of the process entity and is the only sign that the process exists.

When a process is created, the system establishes a PCB for the process, and when the process executes, the system uses its PCB to understand the current state information of the process so that it can be controlled and managed; When the process ends, the system reclaims its PCB and the process dies. The operating system manages and controls processes through the PCB table.

What the PCB typically contains

Process description Information	Process Control and management information	Resource allocation Checklist	processor-related information
Process Identifier (PID)	process Current state	code snippet pointer	Universal Register Value
User Identifier (UID)	process priority	data segment pointer	address register value
	code Run entry address	stack segment pointer	control register Value
	Program's external memory address	file descriptor	Flag Register Value
	Enter memory time	keyboard	status word
	processor Elapsed time	mouse
	semaphore use

The above table is an example of a PCB, mainly including process description information, Process Control and management information, resource allocation list and processor-related information. The main explanations of the sections are as follows:

1) Process Description information
Process identifier: Flags each process, each of which has one and unique identification number.
User identifier: The user that the process belongs to, and the user identifier is primarily a shared and protected service.

2) Process Control and management information
Process Current state: Describes the status of the process information, as the basis of the processor allocation schedule.
Process priority: Describes the priority of a process preemption processor, and a high-priority process is given priority to get the processor.

3) A resource allocation checklist that describes the status of the memory address space or virtual address space, the list of open files, and the input/output device information used.

4) Processor related information, mainly refers to the processor in the register value, when the process is switched, the processor status information must be stored in the corresponding PCB, so that the process re-execution, can again from the breakpoint to continue execution.

In a system, there are usually many processes, some are in a ready state, some are in a blocking state, and the reasons for blocking are different. In order to facilitate the scheduling and management of the process, the PCB of each process needs to be organized in an appropriate way. At present, there are two kinds of organization methods, such as Link way and index way. Link the same state of the PCB link into a queue, different states corresponding to different queues, can also be in the blocking process of the PCB, depending on the cause of the blocking of the different, into a number of blocking queues. Indexing is a process in which the same state is organized in an index table, the table entry of the index table points to the corresponding PCB, and the different states correspond to different index tables, such as the Ready Index table and the blocking index table.

Program Segment

The program segment is the program code snippet that can be dispatched by the process scheduler to the CPU execution. Note that programs can be shared by multiple processes, which means that multiple processes can run the same program.

Data segment

The data segment of a process can be the raw data processed by the process's corresponding program, or it can be the intermediate or final result when the program executes.

Organization of processes: Control blocks, program segments, and data segments