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Introduction to Operating Systems

An Operating System (OS) is software that manages computer hardware and provides services for computer programs. It serves as an intermediary between user applications and hardware.

Goals and Functions​

Primary Goals​

  1. Convenience: Make the computer system easy to use.
  2. Efficiency: Utilize computer hardware in an efficient manner.
  3. Throughput: Maximize the amount of work done per unit of time.

Core Functions​

  • Resource Management: CPU, memory, storage, and I/O devices.
  • Process Management: Creating, scheduling, and terminating processes.
  • Memory Management: Allocation and protection of memory spaces.
  • File System Management: Organizing data on persistent storage.
  • I/O Device Management: Handling hardware interrupts and driver operations.
  • Security and Protection: Ensuring system integrity and data privacy.

History of OS Evolution​

The development of operating systems can be traced through several key phases:

  1. Batch Systems (1950s): Similar jobs are grouped together and run as a batch. No user interaction during execution.
  2. Time-sharing Systems (1960s): Multiple users interact with the system simultaneously through terminals. CPU switches between jobs rapidly.
  3. Personal Computing (1970s-80s): Focused on user interface and responsiveness for individual users (e.g., DOS, early Windows/macOS).
  4. Microkernel & Distributed Systems (1990s): Minimizing kernel functionality and enabling computing across networked machines.
  5. Virtualization & Cloud (2000s-Present): Running multiple OS instances on the same hardware (Hypervisors) and containerization.

Operating System Structures​

Monolithic Structure​

The entire OS runs as a single, large program in kernel mode. All services share the same address space.

  • Pro: High performance due to low overhead.
  • Con: Hard to maintain; a failure in one part can crash the whole system.
  • Examples: Linux, traditional Unix, MS-DOS.

Microkernel Structure​

Removes all non-essential components from the kernel and implements them as system and user-level programs.

  • Pro: High reliability and extensibility.
  • Con: Performance overhead due to frequent IPC (Inter-Process Communication).
  • Examples: Mach, L4, QNX.

Modular & Hybrid Structure​

Modern OSs often use a hybrid approach. The core is monolithic for performance, but functionality is added via Loadable Kernel Modules (LKMs).

  • Examples: Windows NT, macOS (XNU kernel).

User vs. Kernel Mode​

Modern processors provide hardware support for protecting the OS from user programs via execution modes.

  • User Mode: Restricted access to hardware and memory. Applications run here.
  • Kernel Mode: Unrestricted access to all hardware, CPU instructions, and memory. The OS core runs here.

Mode Bit​

A hardware flag indicates the current mode. Certain "privileged" instructions can only execute in kernel mode.

System Calls​

System calls provide an interface between a running program and the OS. They are the only way for a user-mode program to request services from the kernel.

The System Call Mechanism (Trap)​

  1. User program places arguments in registers/stack.
  2. Execution of a special instruction (e.g., INT, SYSCALL).
  3. Hardware switches to kernel mode and jumps to a predefined trap handler.
  4. Kernel identifies the system call number and executes the corresponding routine.
  5. Kernel returns results and switches back to user mode.