Chapter 4: operating systems

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Chapter 4 operating systems
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What is an operating system?

• A program or collection of programs that
  coordinate computer usage among users and handle
  common tasks.

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OS - functions

1. Hide details of input and output
2. Provide an environment for multiple
   users/programs to work concurrently
   multitasking and multi-slacking
3. Allocate memory to different users/programs
4. Share resources (files, printer) among users
5. Provide means for users/programs to communicate
   with each other
6. Provide protection and security for programs

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OS functions - examples

1. You can access disks as file cabinets without the
   knowledge of tracks, sector, etc hide I/O
   details
2. You can work with multiple programs at the same
   time, e.g., using IE to download files and use
   Word to write a report - multitasking
3. In STT475 and 375, we share the same printers
   share resources
4. You can interact with machines via the Internet
   communicate among users/machines
5. When you use password on your machine, no one can
   use your computer without the password -
   protection

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Why are there difference OSs?

• Different types of cars for different purposes
• Vans for family
• Compact cars for gas mileage
• SUV for rough terrains
• Different OSs for different environments and
  applications
• Windows 2000/XP for personal computers
• Windows 2003 Servers for web servers and big
  corporations
• Unix/Linux for personal computers and servers

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What is multitasking?

• The speed of CPUs is much faster than any I/O
  device
• If a computer only execute one program (e.g.,
  Word) from beginning to end, the CPU would stay
  idle most the time since no human can type fast
  enough.
• Multitasking allows multiple programs to reside
  in the memory/virtual memory at the same time and
  switch the CPU between those programs

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Why OSs need to coordinate programs?

• Printing example
• When you print a Word document and a webpage at
  the same time on your home printer, did you ever
  find that one is printed in mid of the other?
• The OS determines which program uses the printer
  first and then next program
• Memory example
• When you use Word and IE at the same time, the OS
  must allocate memory space for both programs

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What cant Windows read Mac or Linux disks

• Different OSs define tracks and sectors
  differently on the disk formatting
• Windows machines do not know how to interpret
  Macs format
• Today, Mac machines can read Windows disks is
  because that Mac OS tries to read disks in
  Windows format after it fails to read it in Macs
  format.

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Portable storage devices

• Floppy disks
• Limited capacity, very slow, not reliable
• Most of todays PCs do not even install floppy
  disk drives
• Jumpdrive, pin drive, USB memory stick, flash
  memory, etc
• High capacity, fast, and reliable
• Use USB connection

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What is booting?

• Start up the computer and allow the OS to set up
  itself
• Then the OS starts behind-the-scenes programs
  (services) for special tasks
• Monitoring email, coordinating printing requests,
  monitoring data on the network.

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What is reboot?

• Stop all current processing and
• Restart the computer
• If everything were perfect in a computer, reboot
  would not be needed
• Reboot sometime is the most efficient way to
  restore the computer back to normal
• Reboot is the only way to restore the system when
  the computer does not respond to any keystroke
  and mouse

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Virtual memory

• A technique for todays computers to run many
  applications beyond the limit of the physical
  memory (RAM)
• Virtual memory is part of the hard disk
• Often it uses the free space of the hard disk
• All programs that are being executed and data are
  being processed are all located in virtual memory.

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Virtual memory
Hard disk
Free space
Used space
Data files And Applications
Running programs and their data
Virtual Memory
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RAM and virtual memory
Workbench .limited space .hand reachable
You
Workshop . Large . Has everything
Your computer
RAM
CPU
Virtual memory
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CPU, RAM, Hard Disks - Program

• You want to use MS WORD to write a letter.
• MS Word and its application data are installed
  onto the hard disk
• When you double-click the Word icon, Word is
  copied from the hard disk to virtual memory along
  with its application data
• When you start using Word, part of Word is copied
  from virtual memory to RAM
• The text you enter from keyboard is saved in RAM
• When you save the letter, the text along with
  format info (bold, italic, etc) is saved into a
  file on the hard disk.
• When Word exits, it is removed from RAM and
  virtual memory along all data related to it.

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CPU, RAM, Hard Disks - Program

• What happens when no free space in RAM?
• As you open more and more applications
  (multitasking) and/or enter more data, RAM may
  become all occupied
• This is like your workbench is all covered by
  tools and materials.
• The operating system moves some part of some
  applications to virtual memory to free up some
  RAM space
• This is like you move some tools and materials
  from workbench to the walls of the workshop.
• Now you can continue entering more data or
  opening more applications.
• This is like you continue your work on workbench
  using newly freed space.
• When you need a part of a program or a program
  that was moved to virtual memory, it may be
  swapped back in RAM.
• This is like you need a tool you moved to the
  walls earlier, you need to clear some space on
  workbench so you can move it back.

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Add More Memory

• Virtual memory allows us to run applications that
  together need more than the available physical
  RAM
• Virtual memory (hard disk) is much slower than
  main memory (RAM).
• The more swapping between RAM and virtual memory,
  the slower of the system
• The more RAM (bigger workbench), the less likely
  swapping occurs
• The more RAM, the faster the system
• Stop applications that no longer needed, this
  frees the RAM space occupied by those
  applications ? remove no longer needed tools from
  your workbench.

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File Management

• Programs and user data are stored as files on the
  hard disk
• Each file always takes multiple sectors
• Folders can be used to organize files like file
  cabinets

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Before DELETE
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After DELETE
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After ERASE
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Deleting a file vs. Erasing it

• Deletion of a file
• Removes the record of the file from the
  corresponding directory
• Marks that the various sectors of the file are
  now free
• Erasure of a file
• Writes over all information in the various
  sectors containing file data
• Removes the record of the file from the
  corresponding directory
• Marks that the various sectors of the file are
  now free

Delete a file throw it into a garbage can
someone may steal it and
read it Erase a file shred it
no one can read it anymore
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I/O Devices - Drivers

• OS needs a special program, called device driver,
  for each I/O device
• The driver is designed for the device and the OS.
  
• A HP printer driver designed for Windows XP
  normally would not work Vista
• Many OSs have device drivers of commonly used
  devices included in their installation
• Device drivers may be updated by the manufacture
  for various reasons
• My advice download drivers only from the
  manufactures website

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Which OS is best?

• Three most common OSs
• Windows by Microsoft
• Aggressively marketed
• Take a large fraction of individual stations
• As a result, users can easily share programs and
  data
• Macintosh OS X by Apple
• Offers a simple, clean, and powerful graphical
  user interface
• Linux by Linus Torvalds and others
• Offer higher reliability and ability to link
  complex tasks

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Summary chapter 4

• Operating systems handle common tasks
• Multitasking allows a computer to split its CPU
  time and resources among multiple programs and
  users
• Booting restarts the computer and the OS sets up
  the environment
• Rebooting involves shutting down the computer and
  starting it up again
• Different OSs are designed for different
  purposes
• Different OSs define different formats on disks
• Virtual memory
• Allow more applications beyond the limit of RAM
• Located on the hard disk
• Deleting vs. erasing files
• Disk fragmentation and defragmentation
• Viruses and anti-virus programs

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Terminology

• Operating systems
• Booting a computer
• Rebooting a computer
• Single-tasking
• Multitasking
• Virtual memory

• Fragmentation
• Defragmentation
• Deleting files
• Erasing files
• MS Windows
• Mac OS X
• Linux