Process Control Block in Operating System

April 18, 2025

What is a Process Control Block in Operating System or PCB?

Process Control Block in Operating System is a data structure which has all the information that is needed by a scheduler to schedule a particular process, this data structure rests in the operating system Kernel.

When we use a computer or a mobile phone, we often open multiple apps or perform many tasks at once – like listening to music while browsing the internet or downloading a file. But have you ever wondered how your system keeps track of all these running tasks without getting confused?

The answer lies in something called a Process Control Block, or PCB.

Process Control block in operating system(OS)

Structure of Process Control Block

A Process Control Block (PCB) is like an identity card or profile for each task (also known as a process) running in the operating system. It stores all the important details about a process, such as:

  • What the process is doing
  • Where it is in the memory
  • What resources it’s using
  • Its current state (like running, waiting, or completed)
Process Control block in os

For Example:

Think of the PCB as a small file that helps the operating system manage and switch between different processes smoothly, without losing track of anything. Without PCBs, the system wouldn’t know what each task was doing – leading to confusion and errors.

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Priority Scheduling

CPU Scheduling

FCFS Scheduling

Preemptive Scheduling

Process ID or PID

This basically is an unique integer ID for a particular process being carried out. The PCB keeps the track of this and other relevant information related to any process.

The following are kept track of by the PCB –

  1. Process ID or PID – Unique Integer Id for each process in any stage of execution.
  2. Process Stage – The state any process currently is in, like Ready, wait, exit etc
  3. Process Privileges – The special access to different resources to the memory or devices the process has.
  4. Pointer – Pointer location to the parent process.
  5. Program Counter – It will always have the address of the next instruction in line of the processes
  6. CPU Registers – Before the execution of the program the CPU registered where the process needs to be stored at.
  7. Scheduling Information – There are different scheduling algorithms for a process based on which they will be selected in priority. This section contains all the information about the scheduling.
  8. Memory Management Information – The operating system will use a lot of memory and it needs to know information like – page table, memory limits, Segment table to execute different programs MIM has all the information about this.
  9. Accounting Information – As the name suggest it will contain all the information about the time process took, Execution ID, Limits etc.
  10. I/O Status – The list of all the information of I/O the process can use.

The PCB architecture is completely different for different OSes. The above is the most generalised architecture.

Why is PCB Important in Operating System?

Without PCB, the OS would forget the progress of a process once it switches to another. The PCB stores the current state, helping the OS resume exactly from where it left off.

This is especially important when multiple processes are running – it keeps everything organized and efficient.

PCB and Context Switching

When the CPU switches from one process to another (called context switching), the current process’s PCB is updated with its last state. Then, the OS loads the next process’s PCB to start/resume that task.

This mechanism is what allows multitasking in modern operating systems.

States of a Process and PCB’s Role

A process can be in one of the following states:

  • New – Just created
  • Ready – Waiting for CPU
  • Running – Currently being executed
  • Waiting – Waiting for input/output
  • Terminated – Process has ended

The PCB always reflects the current state of the process and updates it in real time.

✍️ Conclusion

The Process Control Block (PCB) is one of the most important parts of process management in an operating system. Without it, your system would not be able to multitask, manage resources, or remember where each task left off. Understanding PCB gives you a clear picture of how your OS runs behind the scenes – efficiently and silently.

FAQs

In multitasking, the CPU switches between multiple processes. The PCB helps the OS remember the progress of each process – like saving the game before switching off the console.
Without PCB, the system would forget where the process left off, causing confusion and errors.

A PCB contains:

  • Process ID (PID) – Unique number for each process
  • Process State – Whether it’s running, waiting, or ready
  • Program Counter – Where the process last stopped
  • Registers & CPU Info – Temporary data used during execution
  • Memory Info – Data about memory space used
  • I/O Info – Files/devices in use by the process

These components help the OS control and track the process effectively.

When the CPU switches from one process to another (context switch), the OS saves the current process’s PCB and loads the next one’s PCB.
This allows each process to continue from where it left off, without losing any data or progress.

No, users cannot directly see or change the PCB.
It is managed by the kernel (the core of the OS) to ensure process safety and system stability. Allowing access might risk system crashes or data loss.