Operating Systems Part 1

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Operating Systems Part 1
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Motivation

• How do we get our applications (programs) to
  easily communicate with the vast array of
  hardware devices?
• How do we communicate/interact with the computer?
• How do we ensure safe and secure access to
  resources?

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Operating Systems

• An Operating System, or OS, is a piece of
  software which resides between the underlying
  hardware and the programs we as users run

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Operating Systems
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Operating Modes

• There are two main operating modes in a
  computer
• User mode
• Real mode (sometimes called kernel mode, or
  privileged mode)

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User Mode

• The mode in which user applications run
• Limited access to resources
• Ex can only access memory which a particular
  application owns
• Only certain operations allowed
• Generally cannot access hardware directly

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Real Mode

• The mode in which most of the OS runs
• Why not all?
• Full access to hardware and memory
• Full access to all instructions
• Can access hardware directly

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OS Architectures

• How do we organize an operating system?
• Monolithic
• Layered approach
• Microkernel

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Monolithic OS's

• A monolithic OS is one where the entire OS is one
  big blob
• Characterized by little or no system organization
• All OS subsystems are interdependent
• Pros? Cons?

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Monolithic OS's

• Pros
• Tend to be fast (very little overhead)
• Cons
• Tend to be extremely difficult to maintain
• Difficult to add new features

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Layered Approach

• The OS is divided into independent subsystems or
  layers each of which is built on top of other
  layers

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Layered Approach
Source Operating System Concepts,
Silberschatz, 6th Ed, p78
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Layered Approach

• Pros?
• Cons?

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Layered Approach

• Pros
• Modularity
• Each layer doesn't need to know about internal
  details of any other (encapsulation)
• Cons
• Can be slower
• Determining what each layer should do

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Microkernel Approach

• Where the portion of the OS which runs in real
  mode (the kernel) is kept extremely small, and
  delegates work to modules which run in user mode.
• Pros? Cons?

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Microkernel Approach

• Pros
• Small kernel, thus less time spent in real mode
• Arguably safer and more secure
• Easy to add new features
• Cons
• Switches between user and real mode are more
  frequent slower performance

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Which is best?

• So which is the best architecture?
• Open to debate
• Most use some aspects of each of the previously
  mentioned architectures
• One of the most famous debates about OS
  architecture is the Torvalds-Tannenbaum debate

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And in this corner....
Andrew Tannenbaum
Linus Torvalds
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The Torvalds-Tannenbaum Debate

• Linus Torvalds is the originator of the Linux
  operating system
• Andrew Tannenbaum is a famous professor and
  researcher in computer science in the Netherlands
• Full text can be found at

http//www.oreilly.com/catalog/opensources/book/ap
pa.html
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Operating System Responsibilities

• The OS has a number of responsibilities
• Providing a User Interface (UI)
• System Security
• Efficient Allocation of Resources
• Safe Use of Resources

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The User Interface

• The portion of a OS which allows a user to
  interact with the system
• Can be command-line or graphical (GUI)
• What is Windows?

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Safe Use of Resources

• Ensuring the safe use of resources can take many
  forms
• We will look at one potential problem deadlock
• But first, lets talk philosophy....

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The Dining Philosophers Problem

• Lets say that there are a number of philosophers
  sitting down at a table to eat dinner.
• Each philosopher has access to two chopsticks to
  eat with, one to their left, and one to their
  right.
• They need both chopsticks to eat.

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Source http//en.wikipedia.org/wiki/ImageDining_
philosophers.png
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Dining Philosophers

• A problem arises if each philosopher is holding
  a chopstick, then neither can eat unless another
  philosopher gives up his/her chopstick.
• Thus we have reached a state of deadlock.

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Deadlock

• When progress in a system cannot continue because
  active programs in the system hold resources that
  other programs need to continue (and vice-versa)
• How can we deal with deadlock?
• Deadlock recovery
• Deadlock prevention

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Deadlock Prevention

• Tries to prevent deadlock from happening by
  eliminating the possibility of it occurring
• E.G. Coffman made the observation that in order
  for deadlock to occur, four conditions must hold.

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Coffman's Four Necessary Conditions of Deadlock

• Mutual exclusion a resource can be owned by at
  most one program
• Hold and wait a program which holds a resource
  may request other resources
• No preemption only a process holding a resource
  may give it up
• Circular wait two or more programs form a
  circular chain where each waits for a resource
  the next program holds

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Deadlock Prevention

• Thus to prevent deadlock we just make one of the
  four necessary conditions impossible to happen
• Easy in theory, very hard in practice!
• Learn more in CSC360 (Bankers algorithm, etc)

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Deadlock Recovery

• Rather than avoiding deadlock, deadlock recovery
  detects when the system has entered a state of
  deadlock and tries to recover
• How do we recover?

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Recovery Techniques

• Many possibilities
• Abort all deadlocked programs
• Abort one program at a time until deadlock ceases
• Forcing a program to give up its owned resources