16.1 Purposes of an Operating System (OS)

An Operating System (OS) is a fundamental piece of software that manages computer hardware and software resources. It provides a platform for applications to run and acts as an intermediary between the user and the hardware. This section focuses on the purpose of an OS, with a particular emphasis on process management.

Core Purposes of an Operating System

Operating systems serve several crucial purposes. These can be broadly categorized as:

• Resource Management: The OS controls and allocates system resources such as the CPU, memory, storage devices, and input/output devices among competing processes.
• User Interface: The OS provides a way for users to interact with the computer, either through a command-line interface (CLI) or a graphical user interface (GUI).
• Process Management: The OS is responsible for creating, scheduling, and terminating processes. This is a core function and will be discussed in detail below.
• Storage Management: The OS manages the storage of files and data on various storage devices.
• Security and Protection: The OS enforces security policies and protects the system from unauthorized access.
• Error Detection and Handling: The OS detects and handles errors that occur during system operation.

Process Management: A Key Function

Process management is a critical aspect of operating system functionality. A process is a program in execution. It represents an instance of a program that is being actively carried out by the CPU.

Process Lifecycle

A process typically goes through several stages during its lifecycle:

1. New: The process is being created.
2. Ready: The process is waiting to be assigned to the CPU.
3. Running: The process is currently being executed by the CPU.
4. Blocked/Waiting: The process is waiting for an event to occur (e.g., I/O completion, signal).
5. Terminated: The process has finished execution.

Process States

The different states a process can be in are crucial for the OS to manage them effectively. These states are often represented in a process control block (PCB), which is a data structure maintained by the OS for each process.

Process Control Block (PCB)

A PCB contains all the necessary information about a process, including:

Field	Description
Process ID (PID)	A unique identifier for the process.
Process State	The current state of the process (e.g., Ready, Running, Blocked).
Program Counter (PC)	Indicates the address of the next instruction to be executed.
CPU Registers	The values of the CPU registers at the time the process was last executed.
Memory Management Information	Details about the memory allocated to the process.
Accounting Information	Information used for billing and resource tracking.
Process Control Block (PCB) Priority	The priority of the process relative to other processes.

Process Scheduling

Process scheduling is the process of selecting which process to run next. The OS uses various scheduling algorithms to make these decisions. Common scheduling algorithms include:

• First-Come, First-Served (FCFS): Processes are executed in the order they arrive.
• Shortest Job First (SJF): The process with the shortest estimated execution time is executed next.
• Priority Scheduling: Processes are executed based on their priority.
• Round Robin: Each process is given a fixed time slice to execute.

The choice of scheduling algorithm significantly impacts system performance, affecting metrics like turnaround time, waiting time, and throughput.

Context Switching

Context switching is the process of saving the state of the current process and loading the state of the next process to be executed. This allows the OS to switch between multiple processes, giving the illusion of parallel execution. Context switching involves saving the contents of the CPU registers, the program counter, and other relevant information into the PCB.

Efficient context switching is crucial for maximizing CPU utilization and responsiveness of the operating system.