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Operations on Processes
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Process Table and Process Control Block (PCB)

Last Updated : 27 Dec, 2024
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While creating a process, the operating system performs several operations. To identify the processes, it assigns a process identification number (PID) to each process. As the operating system supports multi-programming, it needs to keep track of all the processes. For this task, the process control block (PCB) is used to track the process’s execution status. Each block of memory contains information about the process state, program counter, stack pointer, status of opened files, scheduling algorithms, etc.

All this information is required and must be saved when the process is switched from one state to another. When the process makes a transition from one state to another, the operating system must update information in the process’s PCB. A Process Control Block (PCB) contains information about the process, i.e. registers, quantum, priority, etc. The Process Table is an array of PCBs, which logically contains a PCB for all of the current processes in the system.

Structure of the Process Control Block

A Process Control Block (PCB) is a data structure used by the operating system to manage information about a process. The process control keeps track of many important pieces of information needed to manage processes efficiently. The diagram helps explain some of these key data items.

process-control-block

Process Control Block

  • Pointer: It is a stack pointer that is required to be saved when the process is switched from one state to another to retain the current position of the process.
  • Process state: It stores the respective state of the process.
  • Process number: Every process is assigned a unique id known as process ID or PID which stores the process identifier.
  • Program counter: Program Counter stores the counter, which contains the address of the next instruction that is to be executed for the process.
  • Register: Registers in the PCB, it is a data structure. When a processes is running and it’s time slice expires, the current value of process specific registers would be stored in the PCB and the process would be swapped out. When the process is scheduled to be run, the register values is read from the PCB and written to the CPU registers. This is the main purpose of the registers in the PCB.
  • Memory limits: This field contains the information about memory management system used by the operating system. This may include page tables, segment tables, etc.
  • List of Open files: This information includes the list of files opened for a process.

Process Table and Process Control Block

Additional Points to Consider for Process Control Block (PCB)

  • Interrupt Handling: The PCB also contains information about the interrupts that a process may have generated and how they were handled by the operating system.
  • Context Switching: The process of switching from one process to another is called context switching. The PCB plays a crucial role in context switching by saving the state of the current process and restoring the state of the next process.
  • Real-Time Systems: Real-time operating systems may require additional information in the PCB, such as deadlines and priorities, to ensure that time-critical processes are executed in a timely manner.
  • Virtual Memory Management: The PCB may contain information about a process virtual memory management, such as page tables and page fault handling.
  • Fault Tolerance: Some operating systems may use multiple copies of the PCB to provide fault tolerance in case of hardware failures or software errors.

Location of The Process Control Block

The Process Control Block (PCB) is stored in a special part of memory that normal users can’t access. This is because it holds important information about the process. Some operating systems place the PCB at the start of the kernel stack for the process, as this is a safe and secure spot.

Advantages

Advantages of Process Table

  • Keeps Track of Processes: It helps the operating system know which processes are running, waiting, or completed.
  • Helps in Scheduling: The process table provides information needed to decide which process should run next.
  • Easy Process Management: It organizes all the details about processes in one place, making it simple for the OS to manage them.

Advantages of Process Control Block (PCB)

  • Stores Process Details: PCB keeps all the important information about a process, like its state, ID, and resources it uses.
  • Helps Resume Processes: When a process is paused, PCB saves its current state so it can continue later without losing data.
  • Ensures Smooth Execution: By storing all the necessary details, PCB helps the operating system run processes efficiently and without interruptions.

Disadvantages

Disadvantages of Process Table

  • Takes Up Memory : The process table needs space to store information about all processes, which can use a lot of memory in systems with many processes.
  • Slower Operations : When there are too many processes, searching or updating the table can take more time, slowing down the system.
  • Extra Work for the System : The operating system has to constantly update the process table, which adds extra work and can reduce overall system performance.

Disadvantages of Process Control Block (PCB)

  • Uses More Memory : Each process needs its own PCB, so having many processes can consume a lot of memory.
  • Slows Context Switching : During context switching , the system has to update the PCB of the old process and load the PCB of the new one, which takes time and affects performance.
  • Security Risks : If the PCB is not well-protected, someone could access or modify it, causing security problems for processes.

Conclusion

The Process Table and PCB are important for managing processes in an operating system. The Process Table keeps a list of all active processes, and the PCB holds details about each process. The PCB enables smooth process switching, effective multitasking, and efficient resource allocation. While they use some memory and processing power, they are essential for running multiple tasks smoothly.



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Operations on Processes

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Rajshree Srivastava
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