Fundamental Concepts of Operating Systems

Introduction

An operating system is system software that manages computer hardware, software resources, and common services for programs. It acts as the interface between the user, application software, and physical hardware such as the CPU, main memory, and storage devices.

Without an operating system, a computer cannot load programs, organize files, manage devices, or process user input efficiently. For Prelims, the most important areas are kernel structure, process management, memory handling, file systems, and system calls.

Core Architecture of an Operating System

The operating system is built in layers, with the kernel at the center and the shell at the user-facing edge. These layers work together to control hardware and provide a usable environment for applications.

• Kernel: The central core program that loads first during startup and remains in primary memory.
• Hardware control: Manages CPU time, memory allocation, and responses to hardware interrupts.
• System services: Provides essential services such as process handling, device management, and memory control.
• Shell: The outer software layer that interprets user commands and passes them to the kernel.
• System calls: The formal interface through which applications request services from the kernel.

Kernel, Shell and System Calls

The kernel is the most privileged part of the operating system. It manages direct communication with hardware and decides how system resources are distributed among processes.

• Monolithic kernel: Runs operating system services such as file systems, virtual memory, device drivers, and system calls in one privileged memory space. Examples include Linux and traditional Unix.
• Microkernel: Keeps only minimal services in kernel space, such as low-level memory management and basic inter-process communication, while running drivers and file systems in user space. Examples include QNX and MINIX.
• Hybrid kernel: Combines microkernel modularity with monolithic performance. Examples include the Windows NT kernel and Apple macOS XNU kernel.
• Command-Line Interface (CLI) shell: Uses text-based commands. Bash in Linux and PowerShell in Windows are standard examples.
• Graphical User Interface (GUI) shell: Provides interaction through windows, icons, menus, and pointers.
• System calls: Common examples include fork to create child processes, exec to run new programs, read and write for input-output operations, and exit to terminate a process.

Process and CPU Scheduling

A process is a program in active execution. It includes program code, current activity, processor registers, and an allocated memory stack. The CPU scheduler selects which ready process gets processor time.

• First-Come, First-Served: Processes are executed in order of arrival without preemption.
• Shortest Job Next: The process with the least CPU burst time is executed first.
• Round Robin: Each process gets a fixed time slice or quantum in cyclic order, supporting multitasking.
• Priority Scheduling: CPU time is assigned according to priority levels and may be preemptive or non-preemptive.
• Scheduling goal: Improve CPU utilization, responsiveness, and fairness among processes.

Memory Management and Virtual Memory

The memory manager allocates and deallocates memory blocks for running processes. It ensures that active programs can use RAM efficiently while maintaining isolation and stability.

• Paging: Divides physical memory into fixed-size frames and logical memory into equal-sized pages.
• Segmentation: Divides logical memory into variable-sized segments that match logical modules such as functions, arrays, or data stacks.
• Virtual memory: Uses hard disk or solid-state drive space to simulate extra RAM.
• MMU: The Memory Management Unit uses page tables to translate virtual addresses into physical RAM addresses.
• Page fault: Occurs when a program accesses a page mapped in virtual space but not currently loaded into physical RAM.

File, Storage and Device Management

The operating system also manages files, storage allocation, and input-output devices. These functions are essential for data organization and hardware communication.

• File system: Organizes digital files into directory hierarchies and manages naming, storage allocation, and read-write permissions.
• Contiguous allocation: Stores a file in continuous blocks of disk space.
• Linked list allocation: Stores file blocks in linked form.
• Indexed allocation: Uses pointers through an index block.
• Common file systems: FAT32, NTFS, and ext4 are standard file systems used across storage media.
• Device drivers: Specialized programs that let the kernel communicate with hardware peripherals.
• Interrupt handlers: Process hardware signals that temporarily pause the CPU for urgent requests.
• DMA: Direct Memory Access controllers transfer data directly between peripheral devices and main memory without continuous CPU intervention.

Dual-Mode Operation and Deadlock

Modern microprocessors use dual-mode operation to protect the system from unauthorized access. The two modes are User Mode and Kernel Mode.

• User Mode: Runs standard applications with restricted instruction sets and isolated memory areas.
• Kernel Mode: Also called Supervisor Mode or Privileged Mode; gives direct access to CPU instructions and physical memory addresses.
• Mode transition: Occurs through hardware interrupts, software exceptions, or explicit system calls.
• Purpose: Prevents user applications from making unauthorized system modifications.
• Deadlock: A condition in which a set of concurrent processes is permanently blocked because each process holds a resource while waiting for another resource.

Exam point: The kernel is loaded first and remains in primary memory, while system calls are the formal link between user programs and the kernel.

Key Prelims Takeaways

• Operating system: System software that manages hardware, software resources, and common program services.
• Kernel vs shell: The kernel controls hardware; the shell provides the user interface.
• System calls: fork, exec, read, write, and exit are standard examples.
• CPU scheduling: FCFS, SJN, Round Robin, and Priority Scheduling are core algorithms.
• Memory concepts: Paging uses fixed-size frames and pages; segmentation uses variable-sized segments.
• Virtual memory: Extends effective memory using storage space and is supported by the MMU.
• Protection: Dual-mode operation separates User Mode from Kernel Mode for system security.