Operating Systems

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Operating Systems
Certificate Program in Software
DevelopmentCSE-TC and CSIM, AITSeptember --
November, 2003
1. Introduction(Chapter 1, SG)

• Objectives
• introduce Operating System (OS) principles

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Contents

• 1. What is an OS?
• 2. Some History
• early systems, batch systems, multiprogramming,
  time-sharing,desktop systems
• 3. Recent Trends
• multiprocessor systems, distributed, clustered,
  real-time, handheld, etc.

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1. What is an OS?
User 1
User 2
User N
..
..
Application Programs
compiler
editor
database
Operating System
Computer Hardware
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Features

• An OS is a program that acts as an intermediary
  between users and hardware
• supports a high-level work environment
• makes hardware convenient to use
• makes efficient use of hardware
• OS design goals are often contradictory

continued
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• The OS manages resources
• e.g. CPU time, memory, files, I/O devices
• avoid bottlenecks
• keeps the hardware busy
• The OS controls
• prevents users/programs adversely affecting the
  hardware or other users
• the system should never crash

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OS Development
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2. Some History

• 2.1. Early Systems
• 2.2. Batch Systems
• 2.3. Multiprogramming Batch Systems
• 2.4. Time-Sharing
• 2.5. Desktop Systems

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2.1. Early Systems

• One person at a contol panel of switches and
  lights
• Load libraries/utilities from tape
• Slow set-up
• Very costly hardware

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2.2. Batch Systems

• Motivation highly utilise the hardware
• dedicated computer operator
• card/tapes handed to the operator
• Similar jobs were run in sequence (batched)
• no machine/user interaction
• errors were recorded as a printed core dump
• very slow turnaround time (for users)

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Resident Monitor

• For automatic job sequencing

Loader
Job Sequencing
Control CardInterpreter
User Program Area
monitor
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Control Cards

• Users cards included control cards to guide the
  monitor.

data cards
program cards
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Off-line I/O Processing
On-line
Card Reader
CPU
Off-line
Line Printer
Card Reader
CPU
tape drives
tape drives
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Spooling

• Job pool on disk, scheduled by OS

disk
I/O
Card Reader
Line Printer
CPU
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Benefit Of Spooling
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2.3. Multiprogrammed Batch Systems

• Have several jobs in memory at a time.
• Multiprogramming allows the OS to switch to
  executing another job when the current one is
  suspended (e.g. waiting for user input).
• The OS may also swap jobs to/from disk.
• Requires CPU scheduling

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2.4. Time-Sharing

• Multiple jobs are executed by the CPU switching
  between them
• as in multiprogramming
• Difference is speed the switching is so fast
  that users feel that their programs are always
  running.
• Executing program is called a process.

continued
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• Virtual memory
• hides the distinction between memory and disk
• Disk management
• Concurrent execution requires fancy scheduling
  schemes
• Job synchronisation, communication, deadlock
  avoidance

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2.5. Desktop Systems

• Personal computers
• computer system dedicated to a single user
• I/O devices
• keyboards, mice, display screens, small printers
• User convenience and responsiveness

continued
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• Can adopt technology developed for larger
  operating system
• sole use means less need for protection features
• May run several different types of operating
  systems
• e.g. Windows, MacOS, UNIX, Linux

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3. Recent Trends

• 3.1. Multiprocessor Systems
• 3.2. Distributed Systems
• 3.3. Clustered Systems
• 3.4. Real-time Systems
• 3.5. Handheld Systems
• 3.6. Computing Environments

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3.1. Multiprocessor Systems

• A system with more than 1 processor, but perhaps
  sharing computer bus, clock, memory and devices
• tightly coupled systems
• n processes n times speed-up

continued
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• Benefits
• save money on hardware
• increased reliability (graceful degradation)
• Two main types
• symmetric multiprocessing systems
• asymmetric multiprocessing systems

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3.1.1. Symmetric Multiprocessing

• Each processor runs an identical copy of the OS
• Often in a master-slave configuration
• Jobs/resources are shared dynamically for better
  load balancing

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3.1.2. Asymmetric Multiprocessing

• Have dedicated processors for specific tasks
• e.g. remote job entry (RJE), I/O terminal
  buffering

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3.2. Distributed Systems

• The system is the network.

continued
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• Multiprocessors, but not sharing memory, clock,
  etc.
• may be geographically separated
• loosely coupled systems
• Benefits
• resource sharing, speed-up, reliability
• matches modern business practise

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3.3. Clustered Systems

• Clustering allows two or more systems to share
  storage.
• provides high reliability
• Asymmetric clustering
• one host runs the application while others
  standby
• Symmetric clustering
• all hosts run the application

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3.4. Real-time Systems

• There are time constraints placed on
  operations/data flows which must be met.
• Uses
• scientific experiments, medicine, industrial
  control systems, home appliances, weapons
• Two types
• hard real-time systems
• soft real-time systems

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3.4.1. Hard Real-time Systems

• Critical tasks are guaranteed to be done on
  time.
• Many useful hardware/OS features are missing
  since they make the time constraints too hard to
  satisfy
• e.g. no secondary storage

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3.4.2. Soft Real-time Systems

• Critical tasks are given priority, but their time
  constraints may not be met.
• Weaker, but can be used alongside typical OS
  features
• Applications VR, multimedia

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3.5. Handheld Systems

• Personal Digital Assistants (PDAs)
• Cellular telephones
• Issues
• limited memory
• slow processors
• small display screens

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3.6. Computing Environments

• Changing environments are leading to changes in
  OS requirements
• traditional
• hand-held, mobile
• Web-based
• embedded, dedicated processors