🪴 TJ's Notes 1.0

Operating Systems Lecture 6

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Continued from Operating System Lecture 5

Operating Systems Lecture 6

Process Control Block (PCB)

The Process Control Block (PCB) is a critical data structure maintained by the operating system for each process. It contains all the information about a process, allowing the OS to manage and control it effectively. The PCB serves as a repository of key process attributes and facilitates context switching, process scheduling, and execution.

Structure of a PCB

A typical PCB consists of the following fields:

  1. Process Identification Information:

    • Process ID (PID): Unique identifier for the process.
    • Parent Process ID (PPID): ID of the process that spawned it.
    • User ID and Group ID: Identifies the user and group associated with the process.

  2. Process State:

    • Represents the current state of the process:
      • New: Process is being created.
      • Ready: Process is ready to execute.
      • Running: Process is currently executing on the CPU.
      • Waiting/Blocked: Process is waiting for an I/O operation or event.
      • Terminated: Process has completed execution.

  3. Program Counter (PC):

    • Stores the address of the next instruction to be executed for the process.

  4. CPU Registers:

    • Contains the values of the CPU registers when the process is not executing.
    • Includes general-purpose registers, index registers, stack pointers, and flags.

  5. Memory Management Information:

    • Includes details about the process’s memory allocation:
      • Base and Limit Registers: Define the address space allocated to the process.
      • Page Tables/Segment Tables: Used in virtual memory systems.
      • Memory Boundaries: Information on the start and end addresses of the process’s memory.

  6. Accounting Information:

    • Tracks resource usage:
      • CPU Usage: Time the process has spent on the CPU.
      • I/O Usage: I/O operations performed by the process.
      • Execution Time: Total runtime of the process.
    • Used for billing and performance monitoring.

  7. I/O Status Information:

    • List of I/O devices allocated to the process.
    • Status of I/O requests (e.g., pending, completed).

  8. Process Priority:

    • Indicates the priority level of the process for scheduling purposes.

  9. Scheduling Information:

    • Information used by the scheduler:
      • Priority: Determines the process’s execution order.
      • Queue Pointers: Links to ready or waiting queues.
      • Scheduling Policy: Time-sharing, round-robin, etc.

  10. File Management Information:

    • List of files opened by the process.
    • File descriptors and permissions.

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Role of PCB in Process Management

  1. Process Tracking:

    • The OS uses the PCB to track the status and details of every active process.

  2. Context Switching:

    • During a context switch, the OS saves the current process’s state in its PCB and loads the next process’s state from its PCB.

  3. Scheduling:

    • The scheduler uses PCB information (e.g., priority, state) to determine the next process to execute.

  4. Resource Allocation:

    • The PCB ensures proper allocation and deallocation of resources like memory, I/O devices, and CPU time.

PCB and Multitasking

  • In multitasking systems, the PCB allows the OS to maintain information for multiple processes simultaneously.
  • Each process has its own PCB, which is stored in a process table managed by the OS.

Advantages of PCB

  1. Efficient Process Management:
    • Simplifies tracking and managing processes.
  2. Facilitates Multitasking:
    • Enables the OS to switch between processes seamlessly.
  3. Resource Allocation:
    • Ensures accurate and fair resource allocation for processes.

Challenges with PCB

  1. Memory Overhead:
    • Maintaining a PCB for each process consumes additional memory, especially in systems with a high number of processes.
  2. Complexity in Context Switching:
    • Frequent context switching increases overhead due to constant saving and restoring of PCB data.

Process Scheduling

There are queues that processes line up in to get their instructions executed.

  • Job Queues: All Processes in the system.
  • Ready Queue: Processes waiting to be executed in memory.
  • Device Queue: Processes waiting for an I/O Signal
  • Migration of process throughout the different queues is possible

References

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References

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  • date: 2025.01.23
  • time: 11:05

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