Overview

What does an Operating System do

Silberschatz and Gavin: “An OS is Similar to a government”

Coordinator

  • Manages all resources
  • Settles conflicting requests for resources
  • Prevent errors and improper use of the computer

Facilitator (“useful” abstractions):

  • Provides facilities/services that everyone needs
  • Standard Libraries, Windowing systems
  • Make application programming easier, faster, less error-prone

Some features reflect both tasks:

  • File system is needed by everyone (Facilitator) …
  • … but File system must be protected (Traffic Cop)

What is an Operating System

Most Likely:

  • Memory Management
  • I/O Management
  • CPU Scheduling
  • Synchronization / Mutual exclusion primitives
  • Communications? (Does Email belong in OS?)
  • Multitasking/multiprogramming?

What about?

  • File System?
  • Multimedia Support?
  • User Interface?
  • Internet Browser? 

Sumaary

Operating systems provide a virtual machine abstraction to handle diverse hardware

Operating systems coordinate resources and protect users from each other

Operating systems simplify application development by providing standard services and abstractions

Operating systems can provide an array of fault containment, fault tolerance, and fault recovery

How is the OS implemented

Policy vs. Mechanism

  • Policy: What do you want to do?
  • Mechanism: How are you going to do it?
  • Should be separated, since policies change

Algorithms used

Linear, Tree-based, Log Structured, etc…

Event models used

Threads vs. event loops

Backward compatibility issues

  • Very important for Windows 2000/XP/Vista/…
  • POSIX tries to help here

System generation/configuration

How to make generic OS fit on specific hardware

Virtual Machines

Software emulation of an abstract machine

  • Make it look like hardware has features you want
  • Programs from one hardware & OS on another one

Programming simplicity

  • Each process thinks it has all memory/CPU time
  • Each process thinks it owns all devices
  • Different Devices appear to have same interface

  • Device Interfaces more powerful than raw hardware

    • Bitmapped display  windowing system
    • Ethernet card  reliable, ordered, networking (TCP/IP)

Fault Isolation

  • Processes unable to directly impact other processes
  • Bugs cannot crash whole machine

Protection and Portability

  • Java interface safe and stable across many platforms

Uniprograming and Multiprograming

Uniprogramming: one thread at a time

  • MS/DOS, early Macintosh, Batch processing
  • Easier for operating system builder
  • Get rid of concurrency (only one thread accessing resources!)

Multiprogramming: more than one thread at a time

  • Multics, UNIX/Linux, OS/2, Windows NT – 7, Mac OS X
  • Often called “multitasking”, but multitasking has other meanings (talk about this later)

Challenges of Multiprograming

  • Each applications wants to own the machine  virtual machine abstraction

  • Applications compete with each other for resources

    • Need to arbitrate access to shared resources  concurrency
    • Need to protect applications from each other  protection

  • Applications need to communicate/cooperate with each other  concurrency