OS

OS

an intermediary interface between user(s), software and

computer hardware.

1940s–1950s

No OS, programs loaded manually with punch cards

1960s

IBM OS/360 introduced batch processing

1970s–1980s

Multiprogramming & Time-Sharing introduced

1980s–1990s

Graphical User Interfaces (GUIs)

IBM OS 360

First family purpose computer & designed to be general purpose

Batch Operating System

Users submit jobs to an operator, who groups similar jobs and runs them together. Users do not interact directly with the computer.

Batch Operating System (—)

➢ Time Consuming

➢ Jobs are collected, grouped, and run sequentially (used in IBM OS/360)

➢ Priority/less priority categorization can’t be done

Multiprogramming Operating System

multiple programs run

in memory at the same time. The CPU switches between

programs, and reduces CPU response time.

Time-Sharing (Multitasking) Operating System

Multiple jobs are executed by the CPU by switching between them, but the

switches occur so frequently. Thus, the user can receive an immediate

response. Time quantum(TQ) introduced.

Example: Linux, MacOS, Windows

Multiprocessing Operating System (DOS)

In Multiprocessing OS, multiple CPU is used to execute multiple jobs.

RTOS

The Real-time Operating System (RTOS) is a type of operating system

that is designed to meet strict time constraints, with a guaranteed response

time for critical tasks.

RTOS are divided into 2 categories:

Soft RTOS:

It is mostly used when execution time(ET) is not so important

Example: Computer games, multimedia devices.

Hard RTOS:

When execution time is strict, and can’t be delayed.

Example: Air traffic, surgery, etc.

Distributed Operating System (DOS)

in this system multiple computers work together and share resources as if they were a single computer.

Network Operating System (NOS)

Network Operating System (NOS) is

a specialized operating system designed to

manage network resources, data, users,

groups, and applications across multiple

computers.

Example: Microsoft Windows Server

NOS and DOS difference

NOS connects separate computers, while DOS makes separate computers appear to be one unified computer system.

CLI

Command-line Interface/console

GUI

Graphical User Interface (Windows, MacOS)

Functions of Operating Systems

• Process, Memory, Device, File System Management

• Security and Access Control

• User Interface (CLI / GUI)

• Networking and Communication

32-bit and 64-bit Operating Systems

These types of processors tell us how much memory a processor can access from a CPU register.

For instance,

➢ A 32-bit system can access 2**32 different memory addresses. Ex: 4 GB of RAM or physical memory. Ideally, it can access more than 4 GB of RAM also.

➢ A 64-bit system can access 264 different memory addresses. Ex: 16 million TB (4 billion times more than 32-bit) of RAM. In short, any amount of memory greater than 4 GB can be easily handled by it

Kernel

is a core/heart of any Operating System. (It is the first program that’s loaded by

the bootloader when computer is turned on.)

Monolithic Kernel

OS services run in kernel space as a single large process.

Examples: Unix, Linux

Monolithic Kernel (+ and —)

Advantage: Execution time (ET) & CPU response time are very fast.

Disadvntage: Less reliable (a failure of one task can crash the entire system)

Micro kernel

Micro kernel consists of kernel level and user level tasks.

Note: Micro kernel is not fast as Monolithic kernel, but it is more reliable and

stable.

Hybrid Kernel

Hybrid kernel = Monolithic (speed) + Micro kernel (stable)

Hybrid kernel is both fast like monolithic and stable like Micro kernel.

Exo Kernel

The main idea behind Exo kernel is to let applications directly access to hardware resources Like CPU, I/O devices and memory management, etc.

(+) — fast

(—) — Harder to develop, security can be more difficult

Nano Kernel

➢ minimal kernel with only essential functions

➢ Used in systems requiring extremely small and fast kernel layers

ASCII stands for

“American Standard Code for Information Interchange”.

ASCII is a 7-bit character encoding standard that represents 128 characters using numbers from 0 to 127.

UTF-8

a variable-length character encoding that represents Unicode characters using 1–4 bytes. It is backward-compatible with ASCII.

System call

System call is a mechanism that allows a program to request a service from the operating system (OS). It translates commands into OS-level instructions.

Shell

Shell is a program that provides an interface between user and operating system.

CLI (Command Line) Shells

A CLI shell is a program that allows users to interact with an operating system by typing text commands instead of using a graphical interface.

Examples: Cmd.exe, Powershell, Bash, Sh, Zsh.

GUI (Graphical) Shells

GUI (Graphical) Shells are software interfaces that allow user interact with an operating system using buttons, icons, menus, etc.

Examples: Windows Explorer (File Explorer), macOS Finder, etc.

CPU scheduling

CPU scheduling is a process in which OS will decide which task / process is going to utilize CPU at a particular time.

Non-preemptive

process will leave the CPU with its own will, cannot be interrupted

Preemptive

Preemptive means that a process can be interrupted during execution

Arrival Time

point of time at which process enters Ready state or queue

Burst Time

duration of process inside the CPU

Turn Around Time

CT - AT
Amount of time spent by the process from its arrival till its completion time.

Waiting Time

TAT - BT

Process Lifecycle

• Ready → Run

  
  

• Run → Terminated

  
  

• Run → Waiting

  
  

• Waiting → Ready

response ratio

(WT+BT)/BT

diagram of a boot process

Power ON → BIOS/UEFI → POST → GRUB → Operating System

post in boot process

POST in the boot process stands for Power-On Self-Test.

It is the initial hardware check performed by the computer's firmware (BIOS/UEFI) immediately after you turn it on.

BIOS/UEFI

starts the computer and prepares the hardware before the operating system loads.

GRUB

is a bootloader that starts the operating system after BIOS/UEFI has finished its job.

Deadlock

Deadlock is a phenomenon in OS when 2 or more processes are

waiting each other forever (stuck/no progress) and none of them

can be completed

It happens when each process

is holding a resource that needs each other.

4 necessary conditions for Deadlock

1. Mutual Exclusion (only 1 process can use a resource at a time)

2. Hold and Wait (process is holding one resource and waiting for

another)

3. No Preemption (processes can’t be interrupted; must be

released voluntarily)

4. Circular Wait (a circular chain of 2+processes should exist)

Mutual Exclusion + Hold and Wait + No Preemption + Circular Wait = Deadlock

How to handle Deadlocks

1. Break Coffman Condition (avoid at least 1 of the 4 conditions)

2. Deadlock Avoidance (if a process needs 2 resources, it must

request both resources together, aka Banker’s algorithm. Example,

banks giving a loan to good creditors).

3. Eliminate “No Preemption” (OS can take away (preempt) the held

resources and give them to another process)

Example: If Process P1 holds the printer and now requests the scanner

(which is busy),

→ OS takes back the printer from P1 and gives it to someone else.

P1 will retry later.

Resource Allocation Graphs (RAG)

RAG