1. Introduction to PCB
The Process Control Block (PCB) is a data structure maintained by the Operating System (OS) for each process. It contains important information required to manage and control a process.
💡 Think of PCB as an identity card for a process! It helps the OS track the process state, resources, and execution details.
2. Role of PCB in Process Management
The PCB is created when a process is created and deleted when a process terminates.
It helps the OS perform:
✅ Process scheduling – Decides which process runs next.
✅ Resource allocation – Manages CPU, memory, and I/O devices.
✅ Process state tracking – Keeps track of execution progress.
✅ Inter-process communication (IPC) – Helps processes exchange data.
3. Structure of a PCB (Components)
The PCB contains various fields to store essential information about a process.
📌 Key Components of a PCB:
| Field | Description |
|---|---|
| Process ID (PID) | A unique identifier assigned to each process. |
| Process State | Current status: New, Ready, Running, Waiting, Terminated. |
| Program Counter (PC) | Stores the address of the next instruction to be executed. |
| CPU Registers | Stores registers like accumulator, stack pointer, etc. |
| Memory Management Info | Base and limit registers, page tables. |
| I/O Status Info | List of allocated I/O devices and files. |
| CPU Scheduling Info | Priority, scheduling algorithms, process queues. |
| Accounting Info | CPU time used, process start time, execution details. |
| Parent Process ID | Identifies the parent process (if created using fork()). |
✔ Example: When a program like “Google Chrome” runs, the OS creates a PCB to track its execution.
4. Diagram of a PCB
Here’s a visual representation of a PCB:
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| Process ID (PID) | 101 |
| Process State | Running |
| Program Counter (PC) | 0x005A4F |
| CPU Registers | R1, R2, R3, ... |
| Memory Info | Base: 0x2000, Limit: 0x4000 |
| I/O Devices | Printer, Disk, Monitor |
| CPU Scheduling Info | Priority: High |
| Accounting Info | CPU Time: 5 ms |
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Each process has its own PCB stored in the Process Table.
5. How PCB Helps in Context Switching?
🔄 Context Switching occurs when the CPU switches from one process to another.
- The current process’s state and register values are stored in the PCB.
- The next process’s PCB is loaded, restoring its state and registers.
✔ Example:
If you’re running MS Word and a browser, the OS switches between their PCBs to manage execution.
6. Conclusion
✔ The Process Control Block (PCB) is a vital data structure used for process management.
✔ It stores process state, memory details, and CPU scheduling information.
✔ It helps in context switching, scheduling, and resource allocation.