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Operating Systems

Introduction to Operating System (OS)

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Initial Objectives

  • To describe the basic organization of computer systems and operating systems.
  • To give an overview of the many types of computing environments.
  • To explore varied types of operating systems.
  • To provide a grand tour of the major components of operating systems.
  • To describe the services an operating system provides to users, processes, and other systems.
  • To discuss the various ways of structuring an operating system.

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What is an Operating System (1)?

  • A modern computer consists of:
    • One or more processors
    • Main memory
    • Disks
    • Printers
    • Various input/output devices.
  • Managing all these varied components requires a layer of software – the �Operating System (OS).

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What is an Operating System (2)?

  • An Operating System is a program that acts as an intermediary/interface between a user of a computer and the computer hardware.
  • OS goals:
    • Control/execute user/application programs.
    • Make the computer system convenient to use.
    • Ease the solving of user problems.
    • Use the computer hardware in an efficient manner.

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Where does the OS fit in?

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Services provided by an OS

  • Facilities for program creation
    • editors, compilers, linkers, debuggers, etc.
  • Program execution
    • loading in memory, I/O and file initialization.
  • Access to I/O and files
    • deals with the specifics of I/O and file formats.
  • System access
    • resolves conflicts for resource contention.
    • protection in access to resources and data.

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Why are Operating Systems Important?

  • Important to understand and know how to correctly use when writing user applications.
  • Large and complex systems that have a high economic impact and result in interesting problems of management.
  • Few actually involved in OS design and implementation but nevertheless many general techniques to be learned and applied.
  • Combines concepts from many other areas of Computer Science: Architecture, Languages, �Data Structures, Algorithms, etc.

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Computer Hardware Organization

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Computer System Components

  1. Hardware – provides basic computing resources (CPU, Memory, I/O devices, Communication).
  2. Operating System – controls and coordinates �use of the hardware among various application programs for various users.
  3. System & Application Programs – ways in which the system resources are used to solve computing problems of the users (Word processors, Compilers, Web browsers, Database systems, Video games).
  4. Users – (People, Machines, other computers).

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Hierarchical view of computer system

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Static View of System Components

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Dynamic View of System Components

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Layers of a Computer System

End

User

Programmer

Operating-

System

Designer

Computer Hardware

Operating-System

Utilities

Application

Programs

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What Operating Systems Do

  • Depends on the point of view.
  • Users want convenience, ease of use and good performance
    • Don’t care about resource utilization.
  • But a shared computer such as mainframe or minicomputer must keep all users happy.
  • Users of dedicate systems such as workstations have dedicated resources but frequently use shared resources from servers.
  • Handheld computers are resource poor, optimized for usability and battery life.
  • Some computers have little or no user interface, such as embedded computers in devices and automobiles.

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Views of an Operating System

  • There are three classical views (in literature):
    1. Resource Manager – manages and allocates resources.
    2. Control program – controls the execution of user programs and operations of I/O devices.
    3. Command Executer – Provides an environment for running user commands.
  • But one more modern view: the Operating System as a Virtual Machine.

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1. Resource Manager

  • Resource Manager:
    • Manages and protects multiple computer resources: CPU, Processes, Internal/External memory, Tasks, Applications, Users, Communication channels, etc…
    • Handles and allocates resources to multiple users or multiple programs running at the same time and space (e.g., processor time, memory, I/O devices).
    • Decides between conflicting requests for efficient and fair resource use (e.g., maximize throughput, minimize response time).
  • Sort of a bottom-up view.

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OS as a Resource Manager

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Resource Manager oriented OS names

  • DEC RSX – Resource Sharing eXecutive
  • MIT Multics – MULTiplexed Information and Computing Services
  • IBM MFT/MVT – Multiple Fixed/Variable Tasks
  • IBM MVS – Multiple Virtual Storage
  • DEC VMS – Virtual Memory System
  • MVS TSO – Time Sharing Option
  • CTSS – Compatible Time Sharing System
  • IBM VM – Virtual machine

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2. Control Program

  • Control Program:
    • Manages all the components of a complex computer system in an integrated manner.
    • Controls the execution of user programs and I/O devices to prevent errors and improper use of computer resources.
    • Looks over and protects the computer: Monitor, Supervisor, Executive, Controller, Master, Coordinator ….
  • Sort of a black box view.

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Control program oriented OS names

  • Unisys MCP – Master Control Program
  • DR CP/M – Control Program/Microcomputer
  • IBM VM/CP – VM Control Program
  • IBM AIX – Advanced Interactive eXecutive
  • DEC RSX – Resource Sharing eXecutive

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3. Command Executer

  • Command Executer:
    • Interfaces between the users and machine.
    • Supplies services/utilities to users.
    • Provides the users with a convenient CLI (Command Language Interface), also called a Shell (in UNIX), for entering the user commands.
  • Sort of a top-down view.

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Command Executer oriented OS names

  • IBM AIX – Advanced Interactive Executive
  • IBM VM/CMS – Conversational monitor System

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Modern view: Virtual Machine (1)

  • Operating System as a Virtual Machine:
    • An interface between the user and hardware that hides the details of the hardware (e.g., I/O).
    • Constructs higher-level (virtual) resources out of lower-level (physical) resources (e.g., files).
    • Definition: OS is a collection of software enhancements, executed on the bare hardware, culminating in a high-level virtual machine that serves as an advanced programming environment.
      • virtual machine = software enhancement = extended machine = abstract machine = layer = level = ring.

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Modern view: Virtual Machine (2)

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Definition of Operating System

  • There is no universally accepted definition.
  • “Everything a vendor ships when you order an operating system” is good approximation but varies widely.
  • “The one program running at all times on the computer” is the Kernel.
  • Everything else is either a system program (ships with the operating system) or an application program.

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One Kernel Point of View

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What is the OS/Kernel?

  • Is the Operating System just the Kernel (not the utilities and application programs)?!
  • The Command Line Interface (CLI) (or command layer/interpreter or shell) allows direct command entry by the user.
  • The shell used to be in the kernel but now is a (first between equals) utility outside of it:
    • Easy to change/debug
    • Many of them (sh, bsh, csh, ksh, tcsh, wsh, bash)
    • Possible to switch between them (chsh)

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Utilities

Shell

Kernel

Hardware

User

UNIX Shell and Utilities

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CLI is the User OS Interface

CLI allows direct command entry:

    • Sometimes implemented in kernel, sometimes by systems program.
    • Sometimes multiple flavors implemented – shells.
    • Primarily fetches a command from user and executes it.
    • Sometimes commands built-in, sometimes just names of programs; if the latter, adding new features doesn’t require shell modification.

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Bourne Shell (bsh)

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A very simplified Shell

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UNIX System Layout

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General UNIX Architecture (1)

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Other application programs

cc

Other application programs

Hardware

Kernel

sh

who

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a.out

date

we

grep

ed

vi

ld

as

comp

cpp

nroff

General UNIX Architecture (2)

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