OS Fundamentals

An Operating System (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Main Functions of OS:
• Process Management - Creating, scheduling, terminating processes
• Memory Management - Allocating and deallocating memory
• File System Management - Managing files and directories
• I/O Management - Managing input/output devices
• Security & Protection - Protecting data and resources OS as an Interface: Goals of OS: 1. Convenience - Make computer easy to use 2. Efficiency - Use resources efficiently 3. Ability to Evolve - Allow updates without affecting users

1. Batch Operating System
• Jobs are grouped and processed in batches
• No interaction with user during execution
• Example: Payroll systems, bank statements 2. Multiprogramming OS
• Multiple programs in memory simultaneously
• CPU switches between programs when one waits for I/O
• Increases CPU utilization 3. Multitasking/Time-Sharing OS
• CPU time is shared among multiple users/processes
• Each gets a small time quantum (time slice)
• Creates illusion of parallel execution
• Example: Windows, Linux, macOS 4. Real-Time OS (RTOS)
• Guarantees response within strict time constraints
• Hard RTOS: Missing deadline = system failure (missile systems)
• Soft RTOS: Missing deadline = degraded performance (video streaming) 5. Distributed OS
• Multiple computers work together as single system
• Resources shared across network
• Example: Google's infrastructure 6. Embedded OS
• Designed for specific hardware/purpose
• Limited resources, optimized for task
• Example: Smart TV, washing machine, car systems

System Call is the interface between a process and the operating system. When a program needs OS services, it makes a system call. How System Calls Work: Categories of System Calls: | Category | Examples | Purpose | |----------|----------|---------| | Process Control | fork(), exec(), exit(), wait() | Create, manage, terminate processes | | File Management | open(), read(), write(), close() | File operations | | Device Management | ioctl(), read(), write() | Device I/O | | Information | getpid(), alarm(), sleep() | System info | | Communication | pipe(), shmget(), mmap() | IPC mechanisms | User Mode vs Kernel Mode:
• User Mode: Limited access, can't access hardware directly
• Kernel Mode: Full access, can execute privileged instructions
• System call causes mode switch (trap) from user to kernel

Kernel is the core of the operating system that has complete control over everything in the system. Functions of Kernel:
• Process scheduling and management
• Memory management
• Device driver management
• System call handling
• Interrupt handling Types of Kernels: 1. Monolithic Kernel `` ┌─────────────────────────────┐ │ User Space │ ├─────────────────────────────┤ │ ┌───────────────────────┐ │ │ │ Monolithic Kernel │ │ │ │ (All services here) │ │ │ │ - File System │ │ │ │ - Device Drivers │ │ │ │ - Networking │ │ │ │ - Process Mgmt │ │ │ └───────────────────────┘ │ ├─────────────────────────────┤ │ Hardware │ └─────────────────────────────┘ `
• All OS services run in kernel space
• Fast (no context switching)
• Large size, one bug can crash system
• Example: Linux, Unix 2. Microkernel ` ┌─────────────────────────────────────┐ │ User Space │ │ [File System] [Drivers] [Network] │ ├─────────────────────────────────────┤ │ ┌─────────────────────┐ │ │ │ Microkernel │ │ │ │ (IPC, Scheduling) │ │ │ └─────────────────────┘ │ ├─────────────────────────────────────┤ │ Hardware │ └─────────────────────────────────────┘ ``
• Minimal kernel - only essential services
• Other services in user space
• More stable, slower (more context switches)
• Example: Minix, QNX 3. Hybrid Kernel
• Combines monolithic and microkernel
• Performance of monolithic + stability of micro
• Example: Windows NT, macOS

Interrupt is a signal that tells the CPU to stop current execution and handle an event. Why Interrupts? Without interrupts, CPU would have to constantly check (poll) if devices need attention. Interrupts allow devices to notify CPU only when needed. Types of Interrupts: 1. Hardware Interrupts
• Generated by hardware devices
• Examples: Keyboard press, mouse click, disk read complete
• Asynchronous (can occur anytime) 2. Software Interrupts (Traps)
• Generated by programs
• System calls are implemented using software interrupts
• Synchronous (predictable) 3. Exceptions
• Generated due to errors
• Examples: Division by zero, page fault, invalid instruction Interrupt Handling Process: `` 1. Interrupt occurs │ ▼ 2. CPU stops current task │ ▼ 3. Save current state (PC, registers) │ ▼ 4. Look up ISR in Interrupt Vector Table │ ▼ 5. Execute Interrupt Service Routine (ISR) │ ▼ 6. Restore state │ ▼ 7. Resume previous task `` Interrupt Vector Table (IVT):
• Table containing addresses of ISRs
• Each interrupt type has unique number
• CPU uses this number to find correct handler

Operating System Structures: 1. Simple Structure (MS-DOS)
• No clear separation between layers
• Applications can access hardware directly
• Not secure, one crash affects everything 2. Layered Structure `` Layer 5: User Interface Layer 4: I/O Management Layer 3: Device Drivers Layer 2: Memory Management Layer 1: CPU Scheduling Layer 0: Hardware `
• Each layer uses services of lower layer
• Easy to debug and maintain
• Slower due to multiple layers 3. Modular Structure (Modern Linux)
• Core kernel + loadable modules
• Modules can be loaded/unloaded at runtime
• Flexible and efficient
• Example: Device drivers as modules Booting Process: ` 1. Power On │ ▼ 2. BIOS/UEFI runs POST (Power-On Self Test) │ ▼ 3. Load bootloader from boot device │ ▼ 4. Bootloader loads kernel into memory │ ▼ 5. Kernel initializes hardware │ ▼ 6. Start init process (PID 1) │ ▼ 7. Load system services │ ▼ 8. Display login prompt `` Key Terms:
• BIOS: Basic Input/Output System (legacy)
• UEFI: Unified Extensible Firmware Interface (modern)
• Bootloader: GRUB (Linux), Windows Boot Manager