Operating System Structure

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Operating System Structure
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Announcements

• Make sure you are registered for CS 415
• First CS 415 project is up
• Initial design documents due next Friday,
  February 2nd
• Project due following Thursday, February 8th
• Everyone should have access to CMS
  (http//cms3.csuglab.cornell.edu)
• Check and contact me (hweather_at_cs.cornell.edu) or
  Bill Hogan (whh_at_cs.cornell.edu) today if you do
  not have access to CMS
• Also, everyone should have CSUGLab account
• Contact Bill or I if you do not

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Review Protecting Processes from Each Other

• Problem Run multiple applications in such a way
  that they are protected from one another
• Goal
• Keep User Programs from Crashing OS
• Keep User Programs from Crashing each other
• Keep Parts of OS from crashing other parts?
• (Some of the required) Mechanisms
• Dual Mode Operation
• Address Translation (base/limit registers, page
  tables, etc)
• Privileged instructions (set timer, I/O, etc)
• Simple Policy
• Programs are not allowed to read/write memory of
  other Programs or of Operating System

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Review Dual Mode Operation

• Hardware provides at least two modes
• Kernel mode (or supervisor or protected)
• User mode Normal programs executed
• Some instructions/ops prohibited in user mode
• Example cannot modify page tables in user mode
• Attempt to modify ? Exception generated
• Transitions from user mode to kernel mode
• System Calls, Interrupts, Other exceptions

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Todays Lectures

• I/O subsystem and device drivers
• Interrupts and traps
• Protection, system calls and operating mode
• OS structure
• What happens when you boot a computer?

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Operating System Structure

• An OS is just another kind of program running on
  the CPU a process
• It has main() function that gets called only once
  (during boot)
• Like any program, it consumes resources (such as
  memory)
• Can do silly things (like generating an
  exception), etc.
• But it is a very sophisticated program
• Entered from different locations in response to
  external events
• Does not have a single thread of control
• can be invoked simultaneously by two different
  events
• e.g. sys call an interrupt
• It is not supposed to terminate
• It can execute any instruction in the machine

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How do you start the OS?

• Your computer has a very simple program
  pre-loaded in a special read-only memory
• The Basic Input/Output Subsystem, or BIOS
• When the machine boots, the CPU runs the BIOS
• The BIOS, in turn, loads a small OS executable
• From hard disk, CD-ROM, or whatever
• Then transfers control to a standard start
  address in this image
• The small version of the OS loads and starts the
  big version.
• The two stage mechanism is used so that BIOS
  wont need to understand the file system
  implemented by the big OS kernel
• File systems are complex data structures and
  different kernels implement them in different
  ways
• The small version of the OS is stored in a small,
  special-purpose file system that the BIOS does
  understand

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What does the OS do?

• OS runs user programs, if available, else enters
  idle loop
• In the idle loop
• OS executes an infinite loop (UNIX)
• OS performs some system management profiling
• OS halts the processor and enter in low-power
  mode (notebooks)
• OS computes some function (DECs VMS on VAX
  computed Pi)
• OS wakes up on
• interrupts from hardware devices
• traps from user programs
• exceptions from user programs

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OS Control Flow
From boot
main()
System call
Initialization
Interrupt
Exception
Idle Loop
Operating System Modules
RTI
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Operating System Structure

• Simple Structure MS-DOS
• Written to provide the most functionality in the
  least space
• Applications have directcontrol of hardware
• Disadvantages
• Not modular
• Inefficient
• Low protection or security

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General OS Structure
API
Process Manager
Network Support
Service Module
Monolithic Structure
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Layered Structure

• OS divided into number of layers
• bottom layer (layer 0), is the hardware
• highest (layer N) is the user interface
• each uses functions and services of only
  lower-level layers
• Advantages
• Simplicity of construction
• Ease of debugging
• Extensible
• Disadvantages
• Defining the layers
• Each layer adds overhead

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Layered Structure
API
Object Support
Network Support
Process Manager
Machine dependent basic implementations
Hardware Adaptation Layer (HAL)
Boot init
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Microkernel Structure

• Moves as much from kernel into user space
• User modules communicate using message passing
• Benefits
• Easier to extend a microkernel
• Easier to port the operating system to new
  architectures
• More reliable (less code is running in kernel
  mode)
• More secure
• Example Mach, QNX
• Detriments
• Performance overhead of user to kernel space
  communication
• Example Evolution of Windows NT to Windows XP

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Microkernel Structure
Process Manager
Network Support
Basic Message Passing Support
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Modules

• Most modern OSs implement kernel modules
• Uses object-oriented approach
• Each core component is separate
• Each talks to the others over known interfaces
• Each is loadable as needed within the kernel
• Overall, similar to layers but with more flexible
• Examples Solaris, Linux, MAC OS X

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UNIX structure
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Windows Structure
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Modern UNIX Systems
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MAC OS X
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Virtual Machines

• Implements an observation that dates to Turing
• One computer can emulate another computer
• One OS can implement abstraction of a cluster of
  computers, each running its own OS and
  applications
• Incredibly useful!
• System building
• Protection
• Cons
• implementation
• Examples
• VMWare, JVM

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VMWare Structure
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But is it real?

• Can the OS know whether this is a real computer
  as opposed to a virtual machine?
• It can try to perform a protected operation but
  a virtual machine monitor (VMM) could trap those
  requests and emulate them
• It could measure timing very carefully but
  modern hardware runs at variable speeds
• Bottom line you really cant tell!

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Modern version of this question

• Can the spyware removal program tell whether it
  is running on the real computer, or in a virtual
  machine environment created just for it?
• Basically no, it cant!
• Vendors are adding Trusted Computing Base (TCB)
  technologies to help
• Hardware that cant be virtualized
• Well discuss it later in the course