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What is an Operating System?
A program that acts as an intermediary between a user of a
computer and the computer hardware
Operating system goals:
Execute user programs and make solving user problems
easier
Make the computer system convenient to use
Use the computer hardware in an efficient manner

Computer System Structure
Computer system can be divided into four components:
Hardware – provides basic computing resources
 CPU, memory, I/O devices
Operating system
 Controls and coordinates use of hardware among various
applications and users
Application programs – define the ways in which the system
resources are used to solve the computing problems of the
users
 Word processors, compilers, web browsers, database
systems, video games
Users
 People, machines, other computers

Four Components of a Computer System

Operating System Definition
OS is a resource allocator
Manages all resources
Decides between conflicting requests for efficient and
fair resource use
OS is a control program
Controls execution of programs to prevent errors and
improper use of the computer

Computer System Organization
Computer-system operation
One or more CPUs, device controllers connect through common
bus providing access to shared memory
Concurrent execution of CPUs and devices competing for
memory cycles

Review
Computer System Components
What is an OS?
Why do we need to have OS?
Agenda
Computer System Operations
Interrupts
Storage Hierarchy
Computer Architecture

Computer-System Operation
I/O devices and the CPU can execute concurrently
Each device controller is in charge of a particular device type
Each device controller has a local buffer
CPU moves data from/to main memory to/from local buffers
I/O is from the device to local buffer of controller
Device controller informs CPU that it has finished its
operation by causing an interrupt

Common Functions of Interrupts
Interrupt transfers control to the interrupt service routine
generally, through the interrupt vector, which contains the
addresses of all the service routines
Interrupt architecture must save the address of the
interrupted instruction
A trap or exception is a software-generated interrupt
caused either by an error or a user request
An operating system is interrupt driven

Storage Structure
Main memory – only large storage media that the CPU can access
directly
Random access
Typically volatile
Secondary storage – extension of main memory that provides large
nonvolatile storage capacity
Hard disks – rigid metal or glass platters covered with magnetic
recording material
Disk surface is logically divided into tracks, which are subdivided into
sectors
The disk controller determines the logical interaction between the device
and the computer
Solid-state disks – faster than hard disks, nonvolatile
Various technologies
Becoming more popular

Storage Hierarchy
Storage systems organized in hierarchy
Speed
Cost
Volatility
Caching – copying information into faster storage system;
main memory can be viewed as a cache for secondary
storage
Device Driver for each device controller to manage I/O
Provides uniform interface between controller and
kernel

Computer-System Architecture
Most systems use a single general-purpose processor
Most systems have special-purpose processors as well
Multiprocessors systems growing in use and importance
Also known as parallel systems, tightly-coupled systems
Advantages include:
1. Increased throughput
2. Economy of scale
3. Increased reliability – graceful degradation or fault tolerance
Two types:
1. Asymmetric Multiprocessing – each processor is assigned a
specie task.
2. Symmetric Multiprocessing – each processor performs all tasks

Symmetric Multiprocessing Architecture

A Dual-Core Design
Multi-chip and multicore
Systems containing all chips

Review
Computer System Operations
Interrupts
Storage Hierarchy
Computer Architecture
Agenda
Protection
Computing Environments

Protection and Security
Protection – any mechanism for controlling access of processes or
users to resources defined by the OS
Security – defense of the system against internal and external attacks
Huge range, including denial-of-service, worms, viruses, identity
theft, theft of service
Systems generally first distinguish among users, to determine who
can do what
User identities (user IDs, security IDs) include name and
associated number, one per user
User ID then associated with all files, processes of that user to
determine access control
Group identifier (group ID) allows set of users to be defined and
controls managed, then also associated with each process, file

Computing Environments – Distributed
Distributed computing
Collection of separate, possibly heterogeneous, systems
networked together
 Network is a communications path, TCP/IP most common
– Local Area Network (LAN)
– Wide Area Network (WAN)
Network Operating System provides features between
systems across network
 Communication scheme allows systems to exchange
messages
 Illusion of a single system

Computing Environments – Client-Server
Client-Server Computing
Many systems now servers, responding to requests generated
by clients

Computing Environments - Virtualization

Computing Environments – Cloud Computing
Delivers computing, storage, even apps as a service across a network
Logical extension of virtualization because it uses virtualization as the base
for it functionality.
Many types
Public cloud – available via Internet to anyone willing to pay
Private cloud – run by a company for the company’s own use
Hybrid cloud – includes both public and private cloud components
Software as a Service (SaaS) – one or more applications available via
the Internet (i.e., word processor)
Platform as a Service (PaaS) – software stack ready for application use
via the Internet (i.e., a database server)
Infrastructure as a Service (IaaS) – servers or storage available over
Internet (i.e., storage available for backup use)

Computing Environments – Real-Time Embedded Systems
Real-time embedded systems most prevalent form of computers
Vary considerable, special purpose, limited purpose OS,
real-time OS
Use expanding
Real-time OS has well-defined fixed time constraints
Processing must be done within constraint
Correct operation only if constraints met

Open-Source Operating Systems
Operating systems made available in source-code format rather
than just binary closed-source
Counter to the copy protection and Digital Rights
Management (DRM) movement
Started by Free Software Foundation (FSF), which has
“copyleft” GNU Public License (GPL)
Examples include GNU/Linux and BSD UNIX (including core of
Mac OS X), and many more
Can use VMM like VMware Player (Free on Windows), Virtualbox
(open source and free on many platforms -
http://www.virtualbox.com)
Use to run guest operating systems for exploration

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Intro to Operating Systems - Introductory Lectures - Software Engineering

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