.. include:: global.rst

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Chapter 3: Processes
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Process Concept and State Diagram
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A **process** is a program in execution. It consists of the program code, current activity,
and the associated resources (such as memory, files, and I/O devices).

Key aspects:
- A process has its own program counter, stack, and data section.
- It represents the basic unit of work in a system.


**Process State Diagram**
A process moves between different states during execution:

- **New**: Process is being created.
- **Ready**: Process is waiting to be assigned to the CPU.
- **Running**: Instructions are being executed on the CPU.
- **Waiting/Blocked**: Process is waiting for some event (e.g., I/O completion).
- **Terminated**: Process has finished execution.

Process Control Block (PCB)
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The **Process Control Block (PCB)** is a data structure maintained by the operating system
to represent a process. It stores essential information needed to manage the process.

Contents of PCB typically include:
- **Process ID (PID)**
- **Process state**
- **Program counter**
- **CPU registers**
- **CPU scheduling information**
- **Memory management information**
- **I/O status information**
- **Accounting information**


Process Scheduling Queues
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The operating system maintains different **queues** to manage processes:

- **Job Queue**: Set of all processes in the system.
- **Ready Queue**: Processes in main memory, ready to execute, waiting for CPU time.
- **Device Queue**: Processes waiting for I/O devices.

Schedulers:
- **Long-term scheduler**: Selects which jobs are admitted into the system for processing.
- **Medium-term scheduler**: Temporarily suspends or resumes processes.
- **Short-term scheduler**: Selects which process will be executed next on the CPU.


Context Switching
=====================
**Context switching** occurs when the CPU switches from one process to another.

Steps involved:
1. Save the state of the current process (into its PCB).
2. Load the state of the next process (from its PCB).
3. Resume execution of the new process.

Key points:
- Context switching adds **overhead** because the system does not do useful work while switching.
- Efficient context switching is critical for system performance.