Comprehensive Summary: Operating Systems

Lecture 1: OS Introduction

• Goal: Act as an intermediary between user and hardware. Prioritize efficiency, convenience, and program execution.
• Components: Hardware, OS, Applications, Users.
• Roles: Resource Allocator (manages CPU, memory) and Control Program (prevents errors).

Lecture 2 & 3: Startup & Interrupts

[Image of computer memory hierarchy]

• Bootstrap: Small program in ROM/Firmware that starts the OS kernel.
• Interrupts: Signals that halt CPU to run an Interrupt Service Routine (ISR).
  • Hardware: From devices (Keyboard).
  • Software (Trap/Exception): Errors or system calls.
• DMA (Direct Memory Access): High-speed devices transfer data blocks to memory directly, sending only one interrupt per block.

Lecture 4 & 5: Architectures & Deadlocks

• Multiprocessing:
  • Symmetric (SMP): All CPUs are equal.
  • Asymmetric (AMP): Master-slave relationship.
• Dual-Mode: User mode (Mode bit 1) vs. Kernel mode (Mode bit 0). Privileged instructions only run in Kernel mode.
• Deadlock Handling:
  1. Prevention (impossible to happen).
  2. Avoidance (checks risk before granting).
  3. Detection (lets it happen then fixes).
  4. Ignorance (restarts if it occurs).

Lecture 6: Review of Basics

• Covers definitions of Process (active program), System Bus (Data, Address, Control), and volatile vs. non-volatile memory.

Lecture 7: Scheduling Basics

• Schedulers: Long-term (admit to memory), Short-term (CPU allocation), Medium-term (swapping).
• Processes: CPU-bound (long bursts) vs. I/O-bound (many short bursts).
• Criteria: Throughput, Turnaround Time (TAT), Waiting Time (WT), Response Time (RT).

Lecture 8: FCFS & SJF Algorithms

• FCFS: Simple but causes "Convoy Effect" (long tasks block short ones).
• SJF: Optimal for minimizing average WT.
  • Preemptive version is called SRTF (Shortest Remaining Time First).

Lecture 9: Round Robin (RR)

• Concept: Designed for time-sharing. Each process gets a "Time Quantum" (q).
• Mechanism: If not finished within 'q', it moves to the end of the ready queue.
• Quantum Size: If 'q' is too large, it becomes FCFS. If too small, too many "Context Switches" (overhead) occur.