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Wednesday, May 15, 2013

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. 

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