Chapter 1: Introduction

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Chapter 1 Introduction

• What Is an Operating System?
• Computer System Components
• What is an Operating System for?
• Different Views
• Different Types of Systems
• Migration of Features

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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.
• Execute user programs and make solving user
  problems easier.
• Make the computer system convenient to use.
• Use the computer hardware in an efficient manner.

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Computer System Components

• 1. Hardware provides basic computing resources
  (CPU, memory, I/O devices).
• 2. Operating system controls and coordinates
  the use of the hardware among various application
  programs for various users.
• 3. Applications programs define the ways in
  which the system resources are used to solve the
  computing problems of the users (compilers,
  database systems, video games, business
  programs).
• 4. Users (people, machines, other computers).

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Abstract View of System Components
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What is an Operating System For?
Beautification Principle

• Goal to make hardware look better than it is
• More regular, uniform
• Instead of lots of idiosyncratic devices
• Easier to program
• E.g. don't have to worry about speeds,
  asynchronous events
• Closer to what's needed for applications
• Named, variable-length files, rather than disk
  blocks
• Multiple CPU's, one for each user (in shared
  system) or activity (in single-user system)
• Multiple large, dynamically-growing memories
  (virtual memory)

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What is an Operating System For?
Resource Principle

• Goal to mediate sharing of scarce resources
• Resource - something costs money
• Why Share?
• Expensive devices
• Need to share data
• Cooperation between people
• Problems
• Getting it to work at all
• Getting it to work efficiently
• Utilization keeping all the devices busy
• Throughput getting a lot of useful work done per
  hour
• Response getting individual things done quickly
• Getting it to work correctly
• Limiting the effects of bugs
• Preventing unauthorized access to data/resources,
  modification of data

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Bottom-Up View
Hardware Components

• One or More CPU's
• Fetches instructions one at a time from location
  specified by PC
• Increments PC after fetching or alters PC for
  branch instructions
• Responds to "interrupts" by jumping to a
  different location
• Main Memory
• Memory responds to "load" and "store" requests
  from the CPU, one at a time.
• I/O Devices - Usually looks like a chunk of
  memory to the CPU.
• CPU sets options and starts I/O by sending
  "store" requests to a particular address.
• CPU gets back status and small amounts of data by
  issuing "load" requests.
• Direct memory access (DMA) device may transfer
  large amounts of data directly to/from memory by
  doing loads and stores just like a CPU.
• Issues an interrupt to the CPU to indicate that
  it is done.
• Bus (or other communication mechanism)

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Bottom-Up View
Timing Problem

• I/O Devices Millions/billions of times slower
  than CPU
• Typical PC gt100 MIPS
• Typical disk takes gt 10 ms to get one byte from
  disk. Ratio 1M1
• Typical typist 60 wpm (1 word or 5 bytes/sec),
  i.e. 200 ms per key-stroke, 20 million
  instructions.
• Solution
• start disk device
• do 100,000 instructions of other useful
  computation
• wait for disk to finish.
• Terrible to write debug. And it would change
  with a faster disk
• Better Solution
• P1 for () start I/O wait for it to finish
• use the data for sth
• P2 for () do some useful computation
• OS takes care of switching back and forth between
  processes as appropriate.
• Question which process should have higher
  priority?

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Bottom-Up View
Space Problem

• Most of the time, a typical program is "wasting"
  most of the memory space allocated to it.
• Looping in one subroutine (wasting space
  allocated to rest of program)
• Fiddling with one data structure (wasting space
  allocated to other data structures)
• Waiting for I/O or user input (wasting all of its
  space)
• Solution virtual memory
• Keep program and data on disk (100-1000 times
  cheaper/byte).
• OS automatically copies to memory pieces needed
  by program on demand.

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Top-Down View
What does it look like to various kinds of users?

• End User
• Wants to get something done
• Doesn't know/care what an OS is
• May not even realize there is a computer there.
• Application Programmer
• Writes software for end users. Uses
  beautified'' virtual machine
• named files of unlimited size
• unlimited memory
• read/write returns immediately
• Calls library routines
• some really are just subroutines written by
  someone else
• sort an array, solve a differential equation,
  search a string for a character
• others call the operating system
• read/write, create process, get more memory
• Systems Programmer
• Creates abstractions for application programmers
• Deals with real devices

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Operating System Definitions

• Resource Allocator
• Decides how to allocate resources for numerous
  and possibly conflicting requests from programs
  and users
• Allocation should be as fair and efficient as
  possible
• Control Program
• Supervision of the execution of user programs to
  prevent errors and improper use of the computer
• Management of operation and control of I/O
  devices .
• Kernel
• The one program running at all times (all else
  being application programs).
• Should web browsers and email programs be
  included?

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Different Types of Systems

• Mainframe Systems
• Batch Systems
• Multi-Programmed Systems
• Time-Sharing Systems
• Desktop Systems
• Multiprocessor Systems
• Distributed Systems
• Clustered System
• Real -Time Systems
• Handheld Systems

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Mainframe Batch Systems

• Early Computers Run from a Front Console
• Maximize resource use at the expense of ease of
  use
• Input/output card readers/punches, tape drives,
  line printers
• User submitted a prepared job (control cards) to
  operator
• Resident monitor
• Initial control in monitor
• Control transfers to job
• When job completes control transfers pack to
  monitor
• Operating System
• Always resident in memory
• Reduce setup time by batching similar jobs as a
  group
• Automatically transfer control from one job to
  another
• CPU is idle when the running job is waiting for
  I/O operation

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Mainframe Multiprogrammed

• Several jobs are kept in main memory
• Selected from job pool - all jobs residing on
  disk awaiting for allocation of main memory
• CPU is multiplexed among them.
• Switch to and run another job if the running job
  needs to wait
• CPU is never idle as long as at least one job
  needs to execute
• OS Concerns
• Job scheduling What if memory is not enough for
  all jobs in the pool?
• CPU scheduling What if several jobs in memory
  are ready to run?
• Memory management
• Allocation of devices

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Time-Sharing SystemsInteractive Computing

• Logical Extension of Multi-programming
• Multi-programmed, batch systems did not provide
  for user interaction with the computer system
• Switch among jobs occurs so frequently that the
  users can interact with each running program
  allows many users to share the computer
  simultaneously
• Each user may have at least one process in memory
• I/O may be interactive
• OS Concerns
• To obtain reasonable response time, jobs may have
  to be swapped in and out of memory to the disk
  virtual memory.
• Users need to access data and code, and file
  system resides on a collection of disks
  disk/file management
• Concurrent execution complex CPU scheduling
• To ensure orderly execution process synch and
  comm.
• Multiprogramming and time-sharing are the central
  themes of modern operating systems

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Desktop Systems

• Personal Computers
• Computer system dedicated to a single user.
• I/O devices keyboards, mice, display screens,
  printers.
• OS Concerns
• Maximize user convenience and responsiveness.
• CPU utilization and protection is no longer a
  prime concern
• Networking changed the protection requirement,
  though
• Can adopt technology developed for larger
  operating system
• Other design decisions still apply, e.g. file
  protection - worm/virus detection
• May run several different types of operating
  systems
• Windows, MacOS, Unix, Linux

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Parallel Systems

• Multiprocessor systems with more than one CPU
• In close communication, sharing bus etc
• Tightly coupled system
• Processors share memory and a clock
• Communication usually takes place through the
  shared memory.
• Advantages
• Increased throughput more work done in less time
• Speed-up ratio with N processors lt N
• Economical
• Share peripherals, mass storage, and power
  supplies
• Increased reliability
• Fault-tolerant/graceful degradation - the ability
  to continue providing service proportional to the
  level of surviving hardware

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Parallel Systems (Cont.)

• Symmetric multiprocessing (SMP)
• All processors are peers no master-slave
  relationship
• Each processor runs an identical copy of the
  operating system.
• The OS copies communicate as needed
• Many processes can run at once without
  performance deterioration.
• Most modern operating systems support SMP
• Asymmetric multiprocessing
• Each processor is assigned a specific task
  master processor schedules and allocated work to
  slave processors.
• More common in extremely large systems
• Some OS Concerns
• I/O control ensure the data reach the
  appropriate processor
• Load balancing avoid the cases where some idle,
  others overloaded

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Distributed Systems

• Distribute computation among several physical
  processors
• Loosely coupled - decentralized
• Each processor has its own local memory
• Communication via network
• Infrastructure LAN, WAN, MAN (metropolitan),
  small-area, etc
• Protocol TCP, ATM, etc
• Connection wire, wireless, dialup
• Computing client-server, peer-to-peer, code
  migration
• Advantages
• Resources sharing
• Computation speed up load sharing
• Reliability
• Communications

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Distributed Systems Client-Server
Compute-server perform actions upon clients
requests and send back results File-server for
clients to create, update, read, and delete files
(ftp) Combination Web server
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Distributed Systems Mobile Agents

• Mobile Agents
• Autonomy
• Code Mobility strong/weak
• Advantages
• Conservation of bandwidth
• Reduction in total completion time and latency
• Dynamic load balancing
• Support disconnected operation in mobile
  environment
• Issues
• Security
• Performance analysis

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Clustered Systems

• What is a Clustered System?
• Computers share storage and closely linked via
  LAN for computation work
• In between multi-processor systems and
  distributed systems
• Asymmetric clustering
• A hot-standby server monitors active servers and
  becomes active when one server fails.
• Symmetric clustering
• All N hosts are running the application and
  monitoring each other.
• Advantage high reliability
• Each node monitors one or more of the others
• When a computer fails, the client would only see
  a brief interruption of service

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Real-Time Systems

• What Is a Real-Time System?
• System behaviors depend on timing constraints.
• Often situated e.g. read sensor data and adjust
  controls
• Quick response is not a mandatory requirement to
  a time-sharing system
• Bounded response time constraints
• If a task is not completed on time, it may cause
  a breakdown or degradation of performance
• Applications
• Control device in a dedicated application
• Controlling scientific experiments, medical
  imaging systems, industrial control systems,
  home-appliance controllers, weapon systems.
• Issues
• Scheduling
• Memory management
• Hard vs Soft Real-Time

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Real-Time Systems (Cont.)

• Hard Real-Time
• Guarantees that critical tasks be completed on
  time (deadline)
• Requires all delays in system be bounded
• Secondary storage limited or absent, data stored
  in short term memory, or ROM
• Conflicts with time-sharing systems
• Not supported by existing general-purpose OSs.
• Intend to separate users from hardware, which
  results in uncertainty about the amount of time
  an operation will take
• Soft Real-Time
• A critical task gets priority over other tasks.
• OS kernel delays need to be bounded
• A task cannot be kept waiting indefinitely
• Lack of deadline support and limited utility in
  industrial control of robotics
• Useful in applications (multimedia, virtual
  reality).

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Handheld Systems

• Handheld Systems
• Examples PDAs, cell phones
• Convenience and portability vs limited
  functionality
• Challenges limited size
• Limited memory 512K-8M
• Memory management return allocated memory when
  not used
• Slow processors
• Faster processor needs more power
• Small display screens lt 3 inches
• Pages must be condensed onto smaller displays
• Wireless connection
• Remote access to e-mail and web browsing

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Migration of Operating-System Concepts and
Features
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Summary

• General Purposes of Operating Systems
• Performance and convenience
• Resource allocator and control
• Different Types of Systems
• Batch Systems
• Multi-Programmed Systems
• Time-Sharing Systems
• Desktop Systems
• Multiprocessor Systems
• Distributed Systems
• Clustered System
• Real -Time Systems
• Handheld Systems
• OS Issues
• Process, CPU, memory, storage, protection