Notes/Operating Systems/Introductory lecture.md

Operating System

• Manages hardware and software
• Resource allocator
  • Manages and routes resources and requests
• Control program
  • Prevents errors and improper use

User v. System

User

graph TD;

	A["user"] <--> B["application programs"]
	B <--> C["operating system"]
	C <--> D["computer hardware"]

System

An operating system is a control program

Major OS Components

Name	Description
Kernel	Core part of the OS; manages CPU, memory, I/O
Process manager	Handles multitasking, scheduling, and execution
Memory Manager	Allocates/deallocates RAM, manages virtual memory.
File System	Organizes, stores, and secures files.
Device Manager	Interfaces with hardware peripherals
User Interface (CLI/GUI)	Allows users to interact with the OS

Management of resources

CPU

• A program can do nothing unless its instructions are executed by a CPU
• An executing program becomes a process
• The process needs resources
• All processes can potentially be executed concurrently by multiplexing on one CPU core or in parallel across multiple

Memory

• For a program to be executed, it must be loaded into memory.
• To improve the CPU utilization, several programs are kept in memory.

OS should:

• Know which parts of memory are currently being used and which process is using htem
• Allocate and deallocate memory accordingly
• Decide which processes and data moves in and out of memory

Cache

• Cache operates at different levels (L1, L2, L3) and is managed primarily by hardware (CPU cache controllers), not the operating system.
• HOWEVER, the OS can influence this by
  • Managing process scheduling which affects cache locality¹
  • Optimizing memory access patterns
  • Supporting cache-aware algorithms²

File System

• The operating system abstracts from the physical properties of its storage devices to define a logical storage unit, the file

Mass Storage

The operating system is responsible for:

• Mounting and unmounting
• Free-space management
• Storage allocation
• Disk scheduling
• Partitioning
• Protection

I/O

The I/O subsystem consists of several components:

• A memory-management component that includes buffering, caching, and spooling
• A general device-driver interface
• Drivers for specific hardware devices

User and Operating System Interface

Command Interpreters

Special programs that run when a user first logs on. They read and interpret user commands and execute programs.

On Linux, the command interpreter is called a shell. The most common shell is the Bourne Again Shell (bash)

System calls

• System calls are the interface between the user and the operating system
• They are the only way a user program can request a service from the operating system
• They are typically written in a high-level language (C) and are accessed via a library

E.g.

#include <stdio.h>
#include <unistd.h>

int main() {
	write(1, "Hello, World!\n", 14); // 1 is the file descriptor for stdout, 14 is the number of bytes to write
	return 0;
}

APIs in the OS

• APIs are a set of functions that allow the programmer to interact with the OS
• They are typically written in a high-level language (C) and are accessed via a library
• They are used to interact with the OS and its services

i.e. Under the hood, widely used libraries like stdlib.h and stdio.h are just APIs that interact with the OS

Performance Optimization

A computer system executes a program by:

• Loading the program into memory
• FDE cycle (Fetch, Decode, Execute)
• Repeat until the program is done
• The OS must manage resources to ensure that the program runs efficiently

Instruction Types

• CPU-bound: spends most of its time executing instructions
• I/O-bound: spends most of its time waiting for I/O operations to complete

Keyword - waiting, hence I/O operations are very slow.

How do we sped it up?

• Replace slow I/O devices
• Perform I/O operations independently of the CPU
• Multiprogramming/multitasking

Spooling

• Simultaneous Peripheral Operations On-Line
• A technique that uses a buffer to hold data for devices that are not currently in use

graph TD;

	A["User"] --> B["Spooling"]
	B --> C["Printer"]
	B --> D["Disk"]

Multiprogramming

• The OS keeps several jobs in memory simultaneously
• Needed for efficiency
• Single users cannot keep the CPU and I/O devices busy at all times
• Scheduling is key

graph TD;

	A["User"] --> B["Multiprogramming"]
	B --> C["Job 1"]
	B --> D["Job 2"]

Multitasking/Timesharing

• The OS switches between jobs so quickly that it appears as if they are running simultaneously
• If several jobs are ready to run at the same time, the OS must decide which one to run

graph TD;

	A["User"] --> B["Multitasking"]
	B --> C["Job 1"]
	B --> D["Job 2"]

---

1. is the tendency of a processor to access the same set of memory locations repetitively over a short period of time ↩︎

2. https://stackoverflow.com/questions/473137/a-simple-example-of-a-cache-aware-algorithm ↩︎