Name of Faculty: Reena MuraliDept. of Computer science

RIT

OPERATING SYSTEMS S5R

Introduction and Overview

What is an operating system?

How have operating systems evolved?

Why study operating systems?

What is an Operating System?

• OS:Everything in system that isn’t an application or hardware

• OS:Software that converts hardware into a useful form for applications

• Not easy to define precisely…

Users

Hardware

Operating System

Applicationscompilersdatabasesword processors

CPUmemoryI/O devices

What is the role of the OS?

• Role #1: Resources provider

• What is a resource?

– Anything valuable (e.g., CPU, memory, disk)

What is the role of the OS?

• Role #2: Resource coordinator (I.e., manager)

• Advantages of resource coordinator

– Virtualize resources so multiple users or applications can share

– Protect applications from one another– Provide efficient and fair access to resources

What Functionality belongs in OS?

• No single right answer

• Desired functionality depends on outside factors– OS must adapt to both user expectations

and technology changes

OS Concepts

A Computer System:

Users

Hardware

Operating System

Applications compilersdatabasesword processors

CPUmemoryI/O devices

• OS can be viewed as

OPERATING SYSTEM OVERVIEW

The Layers Of A System

Program Interface

Humans

User Programs

O.S. Interface

O.S.

Hardware Interface/ Privileged Instructions

Disk/Tape/Memory

Conclusion

An Operating system is

• An interface between users and hardware

- an environment "architecture”

• Allows convenient and efficient usage of

resources (Gives each user a slice of the

resources)

• Provides information protection

• Acts as a control program.

Example OS

Common Operating Systems

• DOS

• Windows 95,98,2000, NT, XP, Vista …

• UNIX / LINUX (Distributions like Red Hat, Suse, Debian, Ubuntu…. ), MacOS

Evolution of OS

Evolution of Operating Systems• Early Systems (1950)• Simple Batch Systems (1960)• Multiprogrammed Batch Systems (1970)• Time-Sharing (1970)• Personal/Desktop Systems (1980)• Multiprocessor Systems (1980)• Networked/Distributed Systems (1980)• Real-Time (1970) and Handheld (1990)

Batch Processing

Batch: Group of jobs submitted together– Operator collects jobs; orders efficiently; runs one at a time

• Advantages– Keep machine busy while programmer thinks– Improves throughput and utilization

• Disadvantages– User must wait until batch is done for results– Machine idle when job is reading from cards and writing to printers

•

Multiprogrammed Systems

• Goal of OS– Improve performance by always running a job

– Keep multiple jobs resident in memory– When job waits for disk I/O, OS switches to another job

• OS Functionality– Job scheduling policies

– Memory management and protection

• Advantage: Improves throughput and utilization• Disadvantage: Machine not interactive

Interactive Multiprogramming

Time Sharing Systems (TSS)

• Batch multiprogramming does not support interaction with users.

• In time sharing systems multiple users simultaneously access the system through terminals .

• Processor’s time is shared among multiple users.

Why does Time-Sharing work?

• Because of slow human reaction time, a typical user needs 2 seconds of processing time per minute.

• Then many users should be able to share the same system without noticeable delay in the computer reaction time.

• The user should get a good response time.

Personal/Desktop Systems

• Personal computers – computer system dedicated to a single user.

• I/O devices – keyboards, mice, display screens, small printers.

• May run several different types of operating systems (Windows, MacOS, UNIX, Linux)

Other Systems

• Multi User Systems

• Multi Tasking Systems

Networked Systems• Requires networking infrastructure.

• Local area networks (LAN) or Wide area networks (WAN).

• May be either Centralized Sever or Client-Server or Peer-to-Peer (P2P) systems.

Local Area Network (LAN) structure

Wide Area Network (WAN) structure

Client/Server Environment

Client-Sever Systems

Peer-To-Peer (P2P) Systems

**Networked/Distributed Systems

• Distribute resources and the computation among several physical processors.

• Loosely coupled system:– each processor has its own local memory.– processors communicate with one another

through various communications lines.

• Advantages:– Resources Sharing – Computation speed up – load sharing – Reliability

Networked/Distributed System Structure

network

disk

disk

processors

disk

disk

processorsdisk

disk

processors

disk

disk

processors

…

node 1

node Nnode 3

node 2

• Network Operating System (NOS):– provides mainly file sharing.– Each computer runs independently from other

computers on the network.

• Distributed Operating System (DOS):– gives the impression there is a single operating

system controlling the network.– network is mostly transparent – it’s a powerful

virtual machine.

Networked/Distributed Operating Systems

Real-Time Systems (RTS)

• Note that not all Operating Systems are general-purpose systems.

• Real-Time (RT) systems are dedicated systems that need to adhere to deadlines , i.e., time constraints.

• Correctness of the computation depends not only on the logical result but also on the time at which the results are produced.

Hard Real-Time Systems• Hard real-time system:

– Must meet its deadline.

• Often used as a control device in a dedicated application: – Industrial control– Robotics

• Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM).

Soft Real-Time Systems• Soft real-time system:

– Deadline desirable but not mandatory.

– Limited utility in industrial control or robotics.

– Useful in modern applications (multimedia, virtual reality) requiring advanced operating-system features.

Handheld Systems

• Handheld systems are also dedicated.

– Personal Digital Assistants (PDAs).

– Cellular telephones.

• Issues:

– Limited memory

– Slow processors

– Small display screens

– Support for multimedia (images, video)

Migration of OS Concepts and Features

Operating-System Concepts

• functions of OS

• Operating System Services

• System Calls

• OS Structure

Functions of OS

• Process Management • Main Memory Management• File Management• I/O System Management• Secondary Management• Networking• Protection System• Command-Interpreter System

Process Management

• A process is a program in execution. A process needs certain resources, including CPU time, memory, files, and I/O devices, to accomplish its task.

• The operating system is responsible for the following activities in connection with process management.– Process creation and deletion.– process suspension and resumption.– Provision of mechanisms for:

• process synchronization• process communication

Main-Memory Management• Memory is a large array of words or bytes, each with its

own address. It is a repository of quickly accessible data shared by the CPU and I/O devices.

• Main memory is a volatile storage device. It loses its contents in the case of system failure.

• The operating system is responsible for the following activities in connections with memory management:– Keep track of which parts of memory are currently

being used and by whom.– Decide which processes to load when memory space

becomes available.– Allocate and deallocate memory space as needed.

File Management

• A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data.

• The operating system is responsible for the following activities in connections with file management:– File creation and deletion.– Directory creation and deletion.– Mapping files onto secondary storage.– File backup on stable (nonvolatile) storage media.

I/O System Management

• The I/O system consists of:– A buffer-caching system – A general device-driver interface– Drivers for specific hardware devices

Secondary-Storage Management

• Since main memory (primary storage) is volatile and too small to accommodate all data and programs permanently, the computer system must provide secondary storage to back up main memory.

• Most modern computer systems use disks as the principle on-line storage medium, for both programs and data.

• The operating system is responsible for the following activities in connection with disk management: – Free space management– Storage allocation– Disk scheduling

Networking (Distributed Systems)

• A distributed system is a collection processors that do not share memory or a clock. Each processor has its own local memory.

• The processors in the system are connected through a communication network.

• Communication takes place using a protocol.• A distributed system provides user access to various system

resources.• Access to a shared resource allows:

– Computation speed-up – Increased data availability– Enhanced reliability

Protection System

• Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resources.

• The protection mechanism must: – distinguish between authorized and

unauthorized usage.– specify the controls to be imposed.– provide a means of enforcement.

Command-Interpreter System

• Many commands are given to the operating system by control statements which deal with:– process creation and management– I/O handling– secondary-storage management– main-memory management– file-system access – protection – networking

Command-Interpreter System (Cont.)

• The program that reads and interprets control statements is called variously:

– command-line interpreter– shell (in UNIX)

Its function is to get and execute the next command statement.

Additional Operating System Functions

Additional functions exist not for helping the user, but rather for ensuring efficient system operations.

• Resource allocation – allocating resources to multiple users or multiple jobs running at the same time.

• Accounting – keep track of and record which users use how much and what kinds of computer resources for account billing or for accumulating usage statistics.

• Protection – ensuring that all access to system resources is controlled.

OS SERVICES

Operating System Services• Program execution – system capability to load a program

into memory and to run it.• I/O operations – since user programs cannot execute I/O

operations directly, the operating system must provide some means to perform I/O.

• File-system manipulation – program capability to read, write, create, and delete files.

• Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing.

• Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.

System Calls

System Calls• System calls provide the interface between a running

program and the operating system.– Generally available as assembly-language

instructions.– Languages defined to replace assembly language for

systems programming allow system calls to be made directly (e.g., C, C++)

• Three general methods are used to pass parameters between a running program and the operating system.– Pass parameters in registers.– Store the parameters in a table in memory, and the

table address is passed as a parameter in a register.– Push (store) the parameters onto the stack by the

program, and pop off the stack by operating system.

Passing of Parameters As A Table

Types of System Calls

• Process control

• File management

• Device management

• Information maintenance

• Communications

Components of OS

The OS Shell

• Defines interface between OS and users– Windows GUI– UNIX command line– UNIX users can choose among a variety of

shells• csh is the “C shell”• tcsh is an enhanced “C shell”

OS Shell interface

O / S

shell

Users

UsersUsers

The OS Kernel

• The internal part of the OS is often called the kernel

• Kernel Components– File Manager– Device Drivers– Memory Manager– Scheduler– Dispatcher

OS File Manager• Maintains information about the

files that are available on the system

• Where files are located in mass storage, their size and type and their protections, what part of mass storage is available

• Files usually allowed to be grouped in directories or folders. Allows hierarchical organization.

OS Device Drivers

• Software to communicate with peripheral devices or controllers

• Each driver is unique

• Translates general requests into specific steps for that device

OS Memory Manager

• Responsible for coordinating the use of the machine’s main memory

• Decides what area of memory is to be allocated for a program and its data

• Allocates and deallocates memory for different programs and always knows what areas are free

OS Scheduler

• Maintains a record of processes that are present, adds new processes, removes completed processes– memory area(s) assigned– priority– state of readiness to execute (ready/wait)

OS Dispatcher• Ensures that processes that are

ready to run are actually executed

• Time is divided into small (50 ms) segments called a time slice

• When the time slice is over, the dispatcher allows scheduler to update process state for each process, then selects the next process to run

OS structures

Design Approaches• Three common approaches:

– Kernel Approach• monolithic kernel• microkernel• exokernel

– Layered Approach– Virtual Machine Approach

Kernel approach

Kernel Based Approach• Kernel contains a collection of primitives which are used to build the OS• OS implements policy, Kernel implements mechanisms• The advantage is performance, the disadvantage are complexity and

maintainability• Why use this approach? If you have a relatively “small” kernel the gains in

performance and efficiency outweigh the disadvantages.

kernel (privileged)

hardwarehw-sw interface

kernel-user interface

User Applications

kernel (privileged)

hardwarehw-sw interface

kernel-user interface

User Apps

SysServices

References:

1. Brinch Hansen, P., "The Nucleus of a Multiprogramming System", Communications of the ACM, Apr. 1970, pp. 238-241.

2. D. Ritchie, and K. Thompson, “The UNIX Time-Sharing System”, Communications of the ACM, Vol. 17, No. 7, Jul. 1974, pp. 365-375.

3. Wulf, W., E. Cohen, W. Corwin, A. Jones, R. Levin, C. Pierson, and F. Pollack, "HYDRA: The Kernel of a Multiprocessor Operating System", Communications of the ACM, June 1974, pp. 337-345.

1. Monolithic Operating System

MS-DOS System Structure

• MS-DOS – written to provide the most functionality in the least space:– not divided into modules (monolithic).– Although MS-DOS has some structure, its

interfaces and levels of functionality are not well separated.

MS-DOS System Structure

UNIX System Structure

• UNIX – limited by hardware functionality, the original UNIX OS had limited structuring.

• The UNIX OS consists of two separable parts:

1. Systems Programs:

2. The Kernel:• Consists of everything below the system-call interface and

above the physical hardware• Provides the file system, CPU scheduling, memory

management, and other operating-system functions; a large number of functions for one level.

UNIX System Structure

Traditional UNIX Kernel [Bach86]

2. Microkernel System Structure (1)

• Move as much functionality as possible from the kernel into “user” space.

• Only a few essential functions in the kernel– primitive memory management (address space)– I/O and interrupt management– Inter-Process Communication (IPC)– basic scheduling

• Other OS services are provided by processes running in user mode (vertical servers) -– device drivers, file system, virtual memory…

2. Microkernel System Structure (2)

• Communication takes place between user modules using message passing.

• But a performance penalty caused by replacing service calls with message exchanges between process.

• More flexibility, extensibility, portability and reliability (details in next 4 slides).

Microkernel Operating System

Benefits of a Microkernel Organization (1)

• Extensibility/Reliability– modular design.– easy to add services.– small microkernel can be rigorously tested.

• Portability– changes needed to port the system to a new

processor is done in the microkernel - not in the other services.

Benefits of Microkernel Organization (2)

• Distributed system support–message are sent without knowing

what the target machine is.

• Object-oriented operating system–components are objects with clearly

defined interfaces that can be interconnected to form software.

Mach 3 Microkernel Structure

Windows NT Client-Server Structure

Windows NT 4.0 Architecture

The Neutrino Microkernel

3. Exokernel• Takes micro-kernel to the extreme• Services implemented as user space library linked against

application.• Only a minimum of functionality is implemented in the kernel,

such as context switching and MMU management.• Export hardware resources that may be managed by user-level

applications while the kernel implements the protection mechanism

• Apps may optimize to a given hardware platform or create new resource abstractions

References:

• D. R. Engler M. F. Kaashoek J. O'Toole, Jr., “Exokernel: an operating system architecture for application-level resource management”, Proceedings of the fifteenth ACM symposium on Operating systems principles, pp. 251-266, Copper Mountain, Colorado, 1995.

Layered Approach

Layered Approach

• The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

• With modularity, layers are selected such that each uses functions (operations) and services of only lower layers.

General OS Layers

Layered Operating System

Older Windows System Layers

OS/2 Layer Structure

Layered vs. Microkernel Architecture

Virtual Machine Approach

Hardware

Virtual machine software

VM1 VM2 VM3 VM4

• Virtual software layer over hardware• Illusion of multiple instances of hardware• Supports multiple instances of OSs

References:

• Seawright, L., and R. MacKinnon, "VM/370 - A Study of Multiplicity and Usefulness", IBM Systems Journal, 1979, pp. 4-17.

Virtual Machines

• A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware.

• A virtual machine provides an interface identical to the underlying bare hardware.

• The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory.

OS Summary

• Shell -- interface to user• File Manager -- manages mass

memory• Device Drivers -- communicate with

peripherals• Memory Manager -- manages main

memory• Scheduler & Dispatcher -- manage

processes

Different Operating Systems on the Same Machine ?

• It is possible to have more than one operating system available to be used on a machine.

• Only one operating system is run at a time, though.

• Examples: – VAX -- VMS or Ultrix– PCs -- DOS, Windows, or Linux

Utilities

• Operating systems usually come with some associated utility programs

• UNIX usually has the text editors emacs and vi (and sometimes pico)

• UNIX has its own sort utility

• UNIX has its own mail utility