What is the Process Control Block? What are its fields?

The task controlling block, switch frame or task struct are the names of one and the same thing that we commonly called as the PCB or the process control block. 

- This data structure belongs to the kernel of the operating system and consists of the information that is required for managing a specific process. 

- The process control block is responsible for manifesting the processes in the operating system. 

- The operating system needs to be regularly informed about resources’ and processes’ statuses since managing the resources of the computer system for the processes is a part of its purpose. 

- The common approach to this issue is the creation and updating of the status table for every process and resource and objects which are relevant such as the files, I/O devices and so on:

1.  Memory tables are one such example as they consist of information regarding how the main memory and the virtual or the secondary memory has been allocated to each of the processes. It may also contain the authorization attributes given to each process for accessing the shared memory areas.

2.   I/O tables are another such example of the tables. The entries in these tables state about the availability of the device required for the process or of what has been assigned to the process. the status of the I/O operations taking place is also mentioned here along with address of the memory buffers they are using.

3.   Then we have the file tables that contain the information regarding the status of the files and their locations in memory.

4. Lastly, we have the process tables for storing the data that the operating systems require for the management of the processes. The main memory contains at least a part of the process control block even though its configuration and location keeps on varying with the operating system and the techniques it uses for memory management.

- Physical manifestation of a process consists of program data areas both dynamic and static, instructions, task management info etc. and this is what that actually forms the process control block. 

- PCB has got a central role to play in process management. 

- Operating system utilities access and modify it such as memory utilities, performance monitoring utilities, resource access utilities and scheduling utilities etc. 

- The current state of the operating system is defined by the set of process control blocks. 

- It is in the terms of PCBs that the data structuring is carried out. 

- In today’s sophisticated operating systems that are capable of multi-tasking, many different types of data items are stored in process control block. 

- These are the data items that are necessary for efficient and proper process management. 

- Even though the details of the PCBs depend up on the system, the common parts can still be identified and classified in to the following three classes:

1.  Process identification data: This includes the unique identifier of the process that is usually a number. In multi-tasking systems it may consists of user group identifier, parent process identifier, user identifier and so on. These IDs are very much important since they let the OS cross check with the tables.

2.   Process state data: This information defines the process status when it is not being executed. This makes it easy for the operating system to resume the process from appropriate point later. Therefore, this data consists of CPU process status word, CPU general purpose registers, stack pointer, frame pointers and so on.

3.   Process control data: This includes process scheduling state, priority value and amount of time elapsed since its suspension. 

Overview Of Process

A process is an activity which takes place over time and which has a precise aim regarding the result to be achieved. The concept of a process is hierarchical which means that a process may consist of a partially ordered set of sub-processes. A process is rather abstract. It describes the essentials of the purpose, structure, rationale, roles and timing, leaving plenty of implementation freedom. The power of a process is its abstraction, which enables its application in a wide range of applications, by tailoring its implementation to the specific application. A process can be tailored and elaborated in one or more procedures, which describe cookbook-like what need to be done when and by whom.
The process has been given many definitions for instance :
* A program in Execution.
* An asynchronous activity.
* The 'animated sprit' of a procedure in execution.
* The entity to which processors are assigned.
* The 'dispatchable' unit.
In Process model, all software on the computer is organized into a number of sequential processes. A process includes PC, registers, and variables. Conceptually, each process has its own virtual CPU. In reality, the CPU switches back and forth among processes.
A process goes through a series of discrete process states :
* New State: The process being created.
* Running State: A process is said to be running if it has the CPU, that is, process actually using the CPU at that particular instant.
* Blocked (or waiting) State: A process is said to be blocked if it is waiting for some event to happen such that as an I/O completion before it can proceed. Note that a process is unable to run until some external event happens.
* Ready State: A process is said to be ready if it use a CPU if one were available. A ready state process is runable but temporarily stopped running to let another process run.
* Terminated state: The process has finished execution.

A process in an operating system is represented by a data structure known as a process control block (PCB) or process descriptor. The PCB contains important information about the specific process including :
* The current state of the process i.e., whether it is ready, running, waiting, or whatever.
* Unique identification of the process in order to track "which is which" information. * A pointer to parent process.
* Similarly, a pointer to child process (if it exists).
* The priority of process (a part of CPU scheduling information).
* Pointers to locate memory of processes.
* A register save area.
* The processor it is running on.