---
title: "Process control block"
subject: "Operating System"
chapter: 2
key: "semester_6/operating_system:pcb-explain"
source: https://www.bctengineer.com/q/semester_6/operating_system/pcb-explain
site: BCT Engineering
---

# Process control block

Operating System · Chapter 2: Process Management

- Asked in: 2069 Bhadra, 2 marks; year not given
- Video: [Process Control Block](https://www.youtube.com/watch?v=4s2MKuVYKV8) — Neso Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/operating_system/pcb-explain
- Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-2

## Question

What is Process Control Block? Explain in detail.

## Short answer

A Process Control Block (PCB) is a kernel data structure storing all process information: PID, state, program counter, CPU registers, memory info, open files, scheduling priority, and accounting data. It is saved during context switches, enabling multitasking.

## Answer

A **Process Control Block (PCB)**, also called a task control block, is a data structure kept by the operating system for every process. It holds all the information needed to manage the process and to resume it correctly after it has been stopped. The PCB is created when the process is created and removed when it terminates; it is the "identity card" of the process. All PCBs are kept in the process table.

#### Contents of a PCB

```text
+---------------------------+
| Process ID (PID), parent  |
| Process state             |
| Program counter           |
| CPU registers, stack ptr  |
| Scheduling info: priority |
| Memory info: base/limit,  |
|   page tables             |
| I/O status: open files,   |
|   devices allocated       |
| Accounting: CPU time used |
+---------------------------+
```

- **Process identification**: PID, parent PID, user ID.
- **Process state**: new, ready, running, waiting, terminated.
- **Program counter**: address of the next instruction.
- **CPU registers**: accumulators, index registers, stack pointers, status flags; saved on interrupt.
- **CPU-scheduling information**: priority, pointers to scheduling queues.
- **Memory-management information**: base/limit registers, segment or page tables.
- **I/O status information**: list of open files and allocated I/O devices.
- **Accounting information**: CPU time used, time limits, job number.

#### Use

During a **context switch** the OS saves the state of the running process in its PCB and loads the state from the PCB of the next process. Hence PCB makes multitasking possible.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is a Process Control Block?
   A. A hardware register that stores the CPU clock
   B. A data structure kept by the OS for each process holding the information needed to manage and resume it
   C. A queue of processes waiting for I/O only
   D. A file on disk containing the program's source code
   Answer: B. The PCB is the per-process data structure (the process's identity card) that lets the OS manage it and resume it after it is stopped. (Key point: Purpose of PCB)

2. Which of the following is NOT stored in a PCB?
   A. Program counter
   B. Page tables / base-limit registers
   C. Open files list
   D. The complete text of other processes' memory pages
   Answer: D. A PCB holds PID, state, program counter, registers, scheduling, memory-management, I/O and accounting information, not other processes' data. (Key point: PCB contents)

3. What happens to the PCB during a context switch?
   A. The PCB of the running process is deleted
   B. The state of the running process is saved in its PCB and the next process's state is loaded from its PCB
   C. Both PCBs are merged into one
   D. The PCB is copied to the disk and the process restarted from the beginning
   Answer: B. Saving to and loading from PCBs is how the OS resumes processes correctly, making multitasking possible. (Key point: PCB in context switch)

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- [Case study: critical factors in process control](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-critical-factors) — Instrumentation II, chapter 10, asked 2×
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- [PID control operation with block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apid-operation-block-diagram) — Embedded System, chapter 7, asked 4×