Module Review: OS Foundations

Key Takeaways

  1. OS Duality: The OS has two jobs: Abstraction (API for apps) and Arbitration (Manager for hardware).
  2. Kernel Architecture:
    • Monolithic: Everything in Kernel Space (Fast, Risky).
    • Microkernel: Minimal Kernel, Drivers in User Space (Stable, Slow).
  3. Protection Rings:
    • Ring 0 (Kernel): Absolute power.
    • Ring 3 (User): Restricted access.
    • CPL Register: The hardware enforcement mechanism.
  4. System Calls: The only way for User Space to talk to Kernel Space. Expensive due to Context Switches.

  5. Boot Chain: Firmware (UEFI) → Bootloader (GRUB) → Kernel (vmlinuz) → Init (Systemd).

Module Review: OS Foundations

[!NOTE] This module explores the core principles of Module Review: OS Foundations, deriving solutions from first principles and hardware constraints to build world-class, production-ready expertise.

1. Cheat Sheet

Concept Description Command/Tool
System Call Request kernel service (e.g., read, write) strace -p <pid>
Context Switch CPU saving state to switch processes vmstat 1
Kernel Ring Ring 0 (Privileged Mode) dmesg (Kernel Logs)
User Ring Ring 3 (Restricted Mode) ps aux
PID 1 The first process (init or systemd) systemctl status
Bootloader Loads the kernel into RAM update-grub

2. Flashcards

Test your understanding. Click to flip.

🛡️

What happens if Ring 3 code executes 'CLI'?

General Protection Fault (GPF)

The CPU checks CPL. If CPL ≠ 0, it triggers an exception. The OS catches this and kills the process.

🐌

Why are Microkernels slower than Monolithic?

IPC Overhead

Components (like FS and Network) run as separate processes. Communication requires message passing and context switches.

📞

How does an App ask the Disk for data?

System Call

The app executes a SYSCALL instruction (trap). The CPU switches to Ring 0 and jumps to the kernel's handler.

🚀

What is the first process started by the Kernel?

Init (PID 1)

Usually /sbin/init, which is often a symlink to Systemd. It starts all other user-space services.