Course Introduction

Operating Systems Primer

CS-446/646

C. Papachristos

Robotic Workers (RoboWork) Lab

University of Nevada, Reno

Your Instructor

Christos Papachristos

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Associate Professor

Department of Computer Science & Engineering

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• Areas of Activity:

Robotic Workers Lab (https://www.roboticworkerslab.com/) - (RoboWork)

Research / Coding / SW Engineering for Field Robotics

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• Education

MS in Electrical & Computer Engineering

PhD in Autonomous (Aerial) Robotics

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• Find me at:

WPEB - 309 (Office) or WPEB – 316/302 (the RoboWork Lab)

cpapachristos@unr.edu , papachric@gmail.com

CS446/646 C. Papachristos

Course Rules

Grading Policy:

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You cannot earn a C in the course without having earned a C on both:

• The weighted average of the Midterm and Final exams.
• The weighted average of the Homework Assignments (& Grad Presentations if applicable).

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Plus/Minus grading will be assigned as indicated in the Syllabus.

Grade re-scaling may be assigned based on an outstanding or inferior Final exam.

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Note: Also, you cannot earn a passing grade (D-) in the course without a passing grade on both categories.

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Class participation will be factored in. As per the University Administrative Manual (UAM) 3,020, students are expected to attend classes in which they are enrolled.

CS446/646 C. Papachristos

Course Rules

Tasks & Responsibilities:

Weekly / Bi-Weekly Homework Assignments

Turn in via WebCampus! LATE submission policy:

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Academic Dishonesty:

Cheating, plagiarism or otherwise obtaining grades under false pretenses constitute academic dishonesty according to the code of this university. Academic dishonesty will not be tolerated and penalties can include filing a final grade of "F"; reducing the student's final course grade one or two full grade points; awarding a failing mark on the coursework in question; or requiring the student to retake or resubmit the coursework. For more details, see the University of Nevada, Reno General Catalog.

(Also, refer to Academic Standards in course syllabus and online)

CS446/646 C. Papachristos

• 1 day late: 15% penalty
• 2 days late: 30% penalty
• 3 days late: 60% penalty
• >3 days : Zero credit

Course Rules

Tasks & Responsibilities:

Weekly / Bi-Weekly Homework Assignments

Turn in via WebCampus! LATE submission policy:

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Academic Dishonesty:

Note:

There exist widely accessible and reliable tools to cross-compare you code:

vs your classmates’ code !

vs students’ code from previous semesters !

Semantic analysis and comparison means IT WILL CATCH similarly structured code, � even if you rename your variables / change indentation / etc. !

CS446/646 C. Papachristos

• 1 day late: 15% penalty
• 2 days late: 30% penalty
• 3 days late: 60% penalty
• >3 days : Zero credit

Course Rules

Academic Standards Policy for Writing Code – CSE Department:

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• Sharing ideas with other students is fine, but you should write your own code. Never copy or read other students’ code, including code from previous years. Cosmetic changes, such as rewriting comments, changing variable names, and so forth to disguise the fact that your work is copied from someone else, are easy to detect and not allowed.
• It is your responsibility to keep your code private. Sharing your code in public is prohibited, and may result in zero credit for the assignment.
• If you find some external code (such as an open-sourced project) that could be reused as part of your assignment, you should first contact the instructor to see whether it is fine to reuse it. If the instructor permits it, she/he may announce it to the entire class so that all students could use it. And if you decide to reuse the external code, you should clearly cite it in comments and keep the original copyright in your code, if applicable.
• You should be prepared to explain any code you submit, including code copied/modified from external sources.

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• Every student turning in a submission is assumed to be signing the following statement:�“This code is my own work. It was written without consulting a tutor or code written by other students.”

CS446/646 C. Papachristos

Course Rules

Generative AI use is NOT allowed for any purpose :

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• For the purposes of this course, any and all uses of generative artificial intelligence (AI)/large language model tools (such as ChatGPT, DALL-E, Gemini, Microsoft Copilot, etc.) will be considered a violation of the UNR Academic Integrity Policy (UAM 6,502), specifically the prohibition against cheating or submitting work that is not your own.
• This applies to all assessments in the course, including case studies, written assignments, discussions, quizzes, exams, and problem sets.
• The following actions are prohibited:
• Submitting any part or all of an assignment statement or test questions as part of a prompt to a large language model AI tool.
• Incorporating any part of an AI-written response into a submitted assignment or assignment component.
• Using AI to summarize or contextualize reading assignments or source materials.
• Submitting your own work for this class to a large language model AI tool for iteration or improvement.

CS446/646 C. Papachristos

Course Objectives

Theoretical Understanding of:

• Core Operating Systems (OS) Principles

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• Virtualization : Processes, Scheduling, Virtual Memory, Paging, Dynamic Memory
• Concurrency : Threads, Synchronization, Semaphores & Monitors
• Persistence : I/O, Disks, File Systems
• Networking : Protocols, Network File System
• Security : Authentication, Access Control
• Virtual Machines : Virtual Machine Monitors, HW-Assisted Virtualization

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Gain a holistic understanding of the big picture:

• How do hardware/architecture, programming language, compiler, algorithms, OS work together.

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Coding in / Bash Scripting – level application of relevant concepts

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Learn some transferrable skills

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CS446/646 C. Papachristos

Course Resources

Operating Systems – Three Easy Pieces

Operating System Concepts

Modern Operating Systems

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Also, Webcampus - Slides developed following content of� i) Stanford CS 140 by David Mazières,� ii) Johns Hopkin’s CS 318 by Ryan Huang,� iii) Columbia Engineering W4118 by Junfeng Yang

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Course Help

Course & Projects:

Teaching Assistants:

Md Omer Danish

mdanish@unr.edu

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Disability Services:

Any student with a disability needing academic adjustments or accommodations is requested to speak with the Disability Resource Center as soon as possible to arrange for appropriate accommodations.

CS446/646 C. Papachristos

Challenges

Get comfortable with:

The Linux environment. All code must compile & run on (X)Ubuntu (min version: 18.04)

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Start with your Homework Assignments early on !

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(X)Ubuntu systems available at the ECC (SEM 231) and WPEB Labs.

To access Ubuntu on one of those machines, you have to hard restart the computer by pushing the

button on the tower, and then select CS135 from the available partition options.

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Don’t wait to seek help. Benefit from all sources.

Your TA(s).

Your Instructor.

WebCampus material (Lectures, Samples, etc.) & Discussions.

CS446/646 C. Papachristos

Operating System Basics

What is an Operating System (OS)

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Layer between Applications and Hardware

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• “Middleware”

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I.e. all the foundation on which you can develop your Programs/Apps.

Operating System

App(lication)

Hard

ware

CS446/646 C. Papachristos

Internet

Game

User

Operating System Basics

OS and Hardware

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Management of Hardware Resources

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Computation Volatile Storage Persistent Storage Communication I/O

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Provides Abstractions to hide details of Hardware from Applications:

• Processes, Threads
• Virtual Memory
• File Systems
• …

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Operating System Basics

OS and Hardware

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Mediates Access from different applications

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• Who has Access at which point and for how long

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Why?

Provides benefits to Applications by offering:

• Simplicity (no need to tweak Device Registers at User Level)
• Device Independence (all Network Interface Cards (NICs) “look” the same)
• Portability (across Win95/-Win10 versions)

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Operating System Basics

OS and Applications

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Virtual Machine interface

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• Each Program thinks it owns the entire computer / system Resources

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Provides Protection

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• Prevents Processes (/Users) from clobbering one another

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Provides Sharing

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• Concurrent execution of multiple Programs (“Time Slicing”)
• Communication among multiple Programs (“Pipes”, Cut & Paste functionality)
• Shared implementations of common Resources & Facilities (e.g. Filesystem)

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Operating System Basics

Different OSs

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Numerous popular OS options exist

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• Free / Commercial
• Open-source / Closed-source
• Embedded OSs
• Real-Time OSs
• Different capabilities (e.g. UNIX named pipes)
• Different Security architectures

CS446/646 C. Papachristos

Operating System Basics

Requirements for a functional Operating System

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A primitive OS idea:

• e.g. an implementation as a library of standard services

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Issues:

• Support for single-Program execution at a time
• Waiting for Program execution makes poor utilization of Resources�(CPU and other Hardware, even the User’s engagement time)
• No accounting for malicious Programs / Users

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CS446/646 C. Papachristos

Applications

Hardware

Operating System

Operating System Overview

Multitasking

The ability for more than once Processes to be executing (running) at the same time

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Note: Concurrently does NOT (necessarily) mean in parallel

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• When one Process “Blocks” (waits for disk/network/user input), run another Process

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Mechanism: Context-Switching

• When a Process resumes, it can continue from its last execution point (State)

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Requirements:

• Scheduling

Avoid Processes going into infinite loop and never relinquishing CPU

• Limiting: Fair sharing, take CPU away from lengthy Process
• Virtual Memory

Avoid Processes scribbling over each other’s data

• Isolation: Protect one Processes’ used Memory from another

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CS446/646 C. Papachristos

Operating System Overview

Multitasking

The ability for more than once Processes to be executing (running) at the same time

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Note: Concurrently does NOT (necessarily) mean in parallel

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• When one Process “Blocks” (waits for disk/network/user input), run another Process

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Mechanism: Context-Switching

• In a traditional CPU each Process utilizes the CPU Registers to store data and hold the current State. In a Multitasking OS, the Operating System “Context Switches” between Processes (/Threads) to allow the execution of multiple Processes simultaneously. For every Switch, it saves the State of the current Process, followed by loading the next Process’ State, which will run on the CPU.
• Context Switches are usually computationally intensive; Context Switching from one Process to another requires time for doing the administration (saving and loading Registers and Memory Maps, updating various tables and lists, etc.)
• What is actually involved in a Context Switch depends on the Architectures, the OS-specific design, and the number of Resources shared (e.g. Threads of the same Process share many Resources, as opposed to unrelated non-cooperating Processes).

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CS446/646 C. Papachristos

Operating System Overview

OS Structure

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• Most software runs as a User-Level Process

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• OS Kernel runs in Privileged mode

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Kernel

User

Network

Card

Console

Disk

Virtual Memory,

IPC, Scheduler,

Filesystem

Device Driver

Device Driver

Device Driver

Device File

Device File

Device File

Process 1

Process 2

Process …

Process N

Operating System Overview

User Mode (Unprivileged mode, Restricted mode, or Slave mode)

(User-Mode “bit”: 1)

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• No direct access to system Resources; in order to access the Resources, a “System Call” must be made
• If an Interrupt occurs, a single Process can fail
• Each Process gets a separate Virtual Address Spaces by the OS
• Only Memory allocated for User Mode can be referenced
• Not all CPU Instructions can be directly executed on their own, or reference any Memory Block

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Kernel Mode (Privileged mode, System mode, or Master mode)

(User-Mode “bit”: 0)

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• Direct & unrestricted access to system Resources
• If an Interrupt occurs, the entire system might go down (become unrecoverable)
• All Tasks (/Threads) share a single Virtual Address Space
• Memory allocated for both Kernel & User Mode can be referenced
• CPU Instructions can be directly executed, and any Memory Block is allowed to be referenced

CS446/646 C. Papachristos

Note: E.g., CS (Code Segment) Register’s Current Privilege Level is 2-bit field corresponding to Privileges 0 (highest)-3(lowest). Linux only uses levels 0 & 3, respectively called Kernel Mode & User Mode.

Operating System Overview

System Call ( “Syscall” )

Applications can invoke the OS Kernel through System Calls

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• Through a special CPU Instruction that serves to transfer control to the Kernel
• Subsequently, System Call Handlers are dispatched
• (and in most Unix-like systems are processed in Kernel Mode)

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Note: In most OSs, no Context Switch is necessary (but a Privilege Switch occurs)

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CS446/646 C. Papachristos

Kernel Space

User Space

User Application

System Call Interface

open( )

0xN

sys_open( )

Entry Point of open() System Call

return

#include <fcntl.h>

#include <unistd.h>

int main()

{

int fd = open("cs318.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);

if (fd < 0) {

write(2, "Failed to open cs318.txt\n", 25);

_exit(1);

}

write(fd, "Hello, OS!\n", 11);

close(fd);

return 0;

}

System Call Dispatch Table

i.e. OS sets/unsets User Mode “bit”

Operating System Overview

System Call ( “Syscall” )

The only way for an application to invoke OS services

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Goal: Do things application isn’t allowed to do in User Mode (Unprivileged)

• Like a library call, but “taps” into more Privileged Kernel code

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Kernel supplies well-defined System Call Interface

• User-Space Application sets up System Call arguments and “ Traps to Kernel ”
• Kernel performs operation and returns result

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Note: (from A Commentary on the 6^th Edition Unix OS - 9 Hardware Interrupts and Traps):�“trap” Instructions may be deliberately inserted in user mode programs to catch the attention of the Operating System with a request to perform a specified service. This mechanism is used as part of the facility known as “System Calls”. (http://warsus.github.io/lions-/lionc/sect0116.html)

CS446/646 C. Papachristos

Operating System Overview

System Call ( “Syscall” )

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C Standard Library calls are built on System Calls

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Kernel Space

User Space

System Call Interface

write( )

0xN

sys_write( )

Entry Point of of write() System Call

return

#include <stdio.h>

int main()

{

printf("Hello, OS!\n");

return 0;

}

printf( )

System Call Dispatch Table

Standard C library ( libc )

Operating System Overview

System Call ( “Syscall” )

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CS446/646 C. Papachristos

#include <unistd.h>

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int main() {

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write(1, "Hello\n", 6); // 1 is stdout

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return 0;

}

C Example:

Assembly Example:

section .data

msg db "Hello", 0xa ; 0xa is \n

len equ $ - msg

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section .text

global _start

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_start:

mov rax, 1 ; sys_write system call number

mov rdi, 1 ; file descriptor (stdout)

mov rsi, msg ; address of string

mov rdx, len ; length of string

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syscall ; invoke system call

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mov rax, 60 ; sys_exit system call number

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xor rdi, rdi ; exit code 0

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syscall ; invoke system call

The Linux Syscall Table (for 64-bit Architectures)

Operating System Overview

Trap / Software Interrupt (“Interrupts” (of all types) sometimes also referred to as “Exceptions”)

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Trap :

• Synchronous (from events internal to CPU) Interrupt generated by the CPU when a User Level program attempts to execute a Privileged Instruction or encounters an error (e.g. divide by zero, stack overflow in PDP-11 architectures, etc.)
• When a Trap occurs, the CPU transfers control to Kernel and executes a Trap Handler, which checks the type of Trap and takes appropriate action (call SIGFPE, terminate program, perform Privileged operation on behalf of it, etc.)

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System Call ( “Syscall” ):

• Made by a User Level program to the OS to perform a Privileged operation (read/write to File, allocate Memory, etc.)
• To make a Syscall, the program executes a special Instruction, e.g. syscall / int 0x2E (this triggers a Software Interrupt)
• Control is transferred to Kernel Mode – Trap to Kernel – and a System Call Handler is executed (based on the set-up arguments e.g. the %rax Register value), which takes appropriate action (read from File/Resource, allocate Memory, etc.)

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Both System Calls & Traps involve transferring control to the Kernel to perform Privileged operations, but:

• “Traps” describe a more general class; they are usually generated automatically by the CPU when a Program encounters an error or attempts to execute a Privileged Instruction
• “System Calls” are initiated by the Program itself to request Privileged operations

C. Papachristos

Time for Questions !

CS-446/646

CS446/646 C. Papachristos