Introduction to Operating Systems

1.1 Role of the Operating System

An operating system (OS) is system software that manages hardware resources and provides services To application programs. It serves as an intermediary between users and the underlying hardware.

Core responsibilities:

• Process management: Create, schedule, and terminate processes.
• Memory management: Allocate and protect memory; implement virtual memory.
• File system management: Organise, store, retrieve, and protect data.
• I/O management: Control and mediate access to I/O devices.
• Protection and security: Enforce access controls and resource isolation.

1.2 Kernel Architectures

Monolithic Kernel. All OS services (file system, device drivers, network stack, scheduler) run In a single address space in kernel mode. Examples: Linux, FreeBSD.

• Advantage: Low overhead from function-call latency.
• Disadvantage: A bug in any component can crash the entire system.

Microkernel. Only essential services (IPC, scheduling, basic memory management) run in kernel Mode. Other services run in user mode as separate processes. Examples: MINIX 3, seL4, QNX.

• Advantage: Fault isolation; easier to verify formally.
• Disadvantage: Higher overhead from message passing between user-mode servers.

Hybrid Kernel. A pragmatic compromise combining monolithic and microkernel ideas. Some Non-essential services run in kernel mode for performance, but the architecture is more modular Than a pure monolith. Examples: Windows NT, macOS XNU.

1.3 System Calls

System calls provide the interface between user-mode applications and kernel-mode OS services. They Are invoked via software interrupts (e.g., syscall on x86-64, svc on ARM).

Categories:

System call overhead. A transition from user mode to kernel mode involves saving user Registers, switching to the kernel stack, validating arguments, executing kernel code, and Returning. Typical overhead: $100\mathrm{--1000}$ ns on modern hardware.